diablo2-web/src/game/maze.ts

/**
 * Diablo II's random-maze level generator (`Levels.txt.DrlgType == 1`).
 *
 * 70 of the game's 136 levels have no fixed layout: their shape is produced at
 * load time from `LvlMaze.txt` (how many sections, how big each section is, and
 * how eagerly neighbouring sections are joined) plus the level type's
 * `LvlPrest.txt` *piece* rows — each piece a DS1 that knows which of its four
 * sides has a doorway. This module rebuilds that generator and emits a single
 * synthesized {@link Ds1} the existing isometric renderer can draw unmodified.
 *
 * ## Provenance
 *
 * This is an **independent TypeScript port** of the level-generation logic in
 * **ThePhrozenKeep/D2MOO** ("Diablo II Method and Ordinal Overhaul", a C++
 * re-implementation of Diablo II 1.10f). D2MOO ships no licence file, so no code
 * was copied: every algorithm below was read from that project and re-expressed
 * here in this repository's own structure and naming, with the source function
 * cited at each step. The algorithms themselves are Blizzard's, and D2MOO's
 * contribution is having recovered them from the shipped binary.
 *
 * Primary sources, all under `source/D2Common/` in D2MOO:
 *
 * - `src/Drlg/DrlgMaze.cpp` — `DRLGMAZE_GenerateLevel` (the per-level-type
 *   dispatch), `DRLGMAZE_AddAdjacentMazeRoom` + `DRLGMAZE_BuildBasicMaze` +
 *   `DRLGMAZE_MergeMazeRooms` (placement and the `Merge` roll),
 *   `DRLGMAZE_InitBasicMazeLayout` (the ring layout), `DRLGMAZE_PickRoomPreset`
 *   (side bitmask → piece), `DRLGMAZE_RollAct_1_2_3_BasicPresets` (the `Theme`
 *   pass), `DRLGMAZE_ScanReplaceSpecialPreset` (`Prev`/`Next`/quest replacement),
 *   `DRLGMAZE_PlaceArcaneSanctuary`, `DRLGMAZE_PlaceAct5LavaPresets`,
 *   `DRLGMAZE_FillBlankMazeSpaces`, `DRLGMAZE_PlaceAct2TombPrev_Act5BaalPrev`.
 * - `src/Drlg/DrlgDrlgRoom.cpp` — `DRLG_ComputeManhattanDistance`,
 *   `DRLG_GetRectanglesManhattanDistanceAndCheckNotOverlapping`,
 *   `DRLGMAZE_CheckRoomNotOverlaping`, `DRLGROOM_AllocDrlgOrthsForRooms`.
 * - `src/Drlg/DrlgDrlg.cpp` — `DRLG_GetDirectionFromCoordinates`,
 *   `DRLG_UpdateRoomExCoordinates`.
 * - `src/Drlg/DrlgPreset.cpp` — `DRLGPRESET_InitPresetRoomGrids` and
 *   `DRLGPRESET_AddPresetUnitToDrlgMap`, which fix the stamping geometry and the
 *   unit of a DS1 object's coordinates.
 * - `doc/Coordinates.md` — the coordinate systems.
 *
 * ## What D2MOO settles that this repository did not know
 *
 * **`LvlMaze.Merge` is a per-mille probability, not a flag.** Both
 * `DRLGMAZE_AddAdjacentMazeRoom` (inline) and `DRLGMAZE_MergeMazeRooms` contain
 * `if (SEED_RollRandomNumber(&i->pSeed) % 1000 < pLevel->pMaze->dwMerge)` before
 * adding an extra doorway. So while a level grows, every *accidental* adjacency
 * between the room just placed and some other already-placed room (orthogonally
 * touching, not already connected) is joined with probability `Merge / 1000`.
 * `Merge = 0` yields a tree (Act 1's cave treasures), `Merge = 500` joins about
 * half of those contacts, and `Merge = 1000` (Act 2's Harem, Basement and the
 * Arcane Sanctuary) joins all of them. The shipped values are 0, 500, 800 and
 * 1000, which is why a per-mille reading is the only one that fits. The knowledge
 * base's "possibly related to how adjacent ds1s are connected, but what the
 * different values are for is unknown" is answered: it controls how many *loops*
 * the maze has, i.e. how many alternate routes exist between rooms.
 *
 * **Connectivity is structural, not luck.** Growth only ever attaches a new room
 * to an existing one and immediately records a doorway between them
 * (`DRLGROOM_AllocDrlgOrthsForRooms`), so the doorway graph always contains a
 * spanning tree rooted at the first room. `Merge` only ever *adds* doorways.
 * {@link generateMaze} re-derives that with a flood fill and throws if it ever
 * fails, so the guarantee is checked rather than asserted.
 *
 * **A maze piece is exactly one cell larger than its section.** `LvlMaze.SizeX`
 * is the placement stride (24 in Act 1) while the piece DS1 is 25×25 — verified
 * against the shipped archives: `Act1/Caves/CaveW.ds1` is 25×25 with
 * `LvlPrest.SizeX = 24`. `DRLGPRESET_InitPresetRoomGrids` confirms the intent: it
 * fills a room grid of `nTileWidth + 1` by `nTileHeight + 1` cells from the
 * piece, so adjacent pieces overlap by exactly one cell and the later-stamped
 * piece owns the shared border. Both halves of that border are authored to match,
 * so the overwrite is harmless. The repository's assumption was correct.
 *
 * **Objects are in sub-tiles, cells are in cells.**
 * `DRLGPRESET_AddPresetUnitToDrlgMap` converts the room's *tile* origin with
 * `DUNGEON_GameTileToSubtileCoords` (×5) and *then* adds the DS1 object's own
 * coordinates, which are therefore already sub-tiles relative to the piece. A
 * stamp offset of `N` cells shifts an object by `N * 5` sub-tiles, exactly as
 * this repository assumed.
 */
import type { Ds1, Ds1Cell, Ds1Floor, Ds1Object, Ds1Wall } from '../formats/ds1.ts'
import type { D2Table } from './acts.ts'
import { Rng } from './rng.ts'
import { SUB_TILES_PER_TILE } from './map.ts'
export { SUB_TILES_PER_TILE }
import { type SpecialPassName, runMazeSpecialPasses } from './maze-special-passes.ts'
import {
  populateDungeonObjects,
  type PlacedMazeObject,
  type MazeObjectPopulationOptions,
  type MazeRoomDescriptor,
} from './maze-objects.ts'

export type {
  PlacedMazeObject,
  MazeObjectPopulationOptions,
  MazeObjectCategory,
  ShrineSubType,
  ChestSubType,
  RackSubType,
  ContainerSubType,
  WallAlignment,
} from './maze-objects.ts'

/* ------------------------------------------------------------------------- *
 * Directions
 * ------------------------------------------------------------------------- */

/**
 * Direction indices, in D2MOO's `D2AltDirections` order.
 *
 * `DRLG_GetDirectionFromCoordinates` returns the `D2Directions` *names* but the
 * same numbers, and D2MOO relies on that coincidence: `DIRECTION_SOUTHWEST` is 0
 * and `ALTDIR_WEST` is 0, `DIRECTION_NORTHWEST`/`ALTDIR_NORTH` are 1,
 * `DIRECTION_SOUTHEAST`/`ALTDIR_EAST` are 2 and `DIRECTION_NORTHEAST`/`ALTDIR_SOUTH`
 * are 3. Both are reproduced here as one space, since a maze direction is always
 * an offset on the cell grid.
 *
 * The signs are the ones `doc/Coordinates.md` states: East is +X, West is -X,
 * South is +Y, North is -Y. Note that D2's *names* for the quarter directions do
 * not mean screen directions — `DIRECTION_NORTHEAST` is +Y, i.e. towards the
 * bottom-left of the isometric view. Only the numbers are meaningful.
 */
export const DIRECTION_WEST = 0
/** North is -Y in game coordinates. */
export const DIRECTION_NORTH = 1
/** East is +X in game coordinates. */
export const DIRECTION_EAST = 2
/** South is +Y in game coordinates. */
export const DIRECTION_SOUTH = 3

/**
 * The side bit `DRLGMAZE_PickRoomPreset` sets for each direction.
 *
 * It ORs 1 for direction 0, 8 for 1, 2 for 2 and 4 for 3, so the resulting 4-bit
 * value is **W=1, E=2, S=4, N=8**. That looks arbitrary until it is compared with
 * `LvlPrest.txt`: side tokens are written with their letters in N,S,E,W order and
 * read as a bitfield most-significant-bit first, so `NS` = 8+4 = 12, `EW` = 2+1 =
 * 3 and `NSEW` = 15. Verified against the real table — `Act 1 - Cave
 * W/E/EW/S/SW/SE/SEW/N/NW/NE/NEW/NS/NSW/NSE/NSEW` are fifteen consecutive rows
 * following the level type's base row, so a piece's row number is `base + mask`.
 */
const DIRECTION_SIDE_BIT = [1, 8, 2, 4] as const

/**
 * Cell offset for one step in a direction.
 *
 * Directions 4..7 are the diagonals, which only `DRLGMAZE_FillBlankMazeSpaces`
 * uses (it tries all eight). Quarters and diagonals were cross-checked against
 * `DRLGMAZE_LinkMazeRooms`' eight `case`s.
 *
 * @param direction - 0..7.
 * @param width - one section's width in cells.
 * @param height - one section's height in cells.
 * @returns the offset.
 */
function step(direction: number, width: number, height: number): { dx: number; dy: number } {
  const dx = (direction === DIRECTION_WEST || direction === 4 || direction === 7 ? -width
    : direction === DIRECTION_EAST || direction === 5 || direction === 6 ? width : 0)
  const dy = (direction === DIRECTION_NORTH || direction === 4 || direction === 5 ? -height
    : direction === DIRECTION_SOUTH || direction === 6 || direction === 7 ? height : 0)
  return { dx, dy }
}

/* ------------------------------------------------------------------------- *
 * Side tokens
 * ------------------------------------------------------------------------- */

/** The four side letters, in the order `LvlPrest.txt` writes them. */
const SIDE_LETTERS = ['N', 'S', 'E', 'W'] as const

/**
 * Decode a side token such as `NSEW`, `EW` or `N` into a bitmask.
 *
 * @param token - the token; characters other than `NSEW` are ignored, so a whole
 *   piece name may be passed.
 * @returns the mask, 0..15.
 */
export function sideMaskFromToken(token: string): number {
  let mask = 0
  for (const letter of token.toUpperCase()) {
    const index = SIDE_LETTERS.indexOf(letter as (typeof SIDE_LETTERS)[number])
    if (index !== -1) mask |= 1 << (3 - index)
  }
  return mask
}

/**
 * Encode a bitmask the way `LvlPrest.txt` spells it.
 *
 * @param mask - the mask, 0..15.
 * @returns the token, e.g. `NSEW`; an empty string for 0.
 */
export function sideTokenFromMask(mask: number): string {
  let token = ''
  SIDE_LETTERS.forEach((letter, index) => {
    if ((mask & (1 << (3 - index))) !== 0) token += letter
  })
  return token
}

/* ------------------------------------------------------------------------- *
 * Public request / response shapes
 * ------------------------------------------------------------------------- */

/** How a maze piece participates in generation. */
export type MazePieceKind =
  | 'entrance'
  | 'room'
  | 'theme'
  | 'prev'
  | 'next'
  | 'down'
  | 'quest'
  | 'treasure'
  | 'waypoint'
/**
 * A waypoint room, as reported in `stats.waypoints`.
 */
export interface MazeWaypoint {
  /** The room the piece went into, matching `specialsApplied`. */
  readonly room: number
  /** The room's top-left corner in the final map, in cells. */
  readonly x: number
  readonly y: number
  /** The room's extent, in cells. */
  readonly width: number
  readonly height: number
  /** The room's centre, where the player lands or clicks. */
  readonly centreX: number
  readonly centreY: number
  /** Sub-tile coordinates of the waypoint entity within the synthesized map. */
  readonly entityX: number
  readonly entityY: number
}

/**
 * One `LvlPrest.txt` row that can be stamped into a maze level.
 *
 * The caller owns the classification, because it is the only party that knows
 * which DS1 files sit behind each row. {@link classifyMazePieceName} does that
 * classification for callers that want it, since the trailing-token grammar is
 * subtle enough to be worth centralising.
 */
export interface MazePiece {
  /** `LvlPrest.txt` `Name`, e.g. `Act 1 - Cave NSEW`. */
  readonly name: string
  /** Which generator pass may place this piece. */
  readonly kind: MazePieceKind
  /**
   * The side token decoded from the name — `NSEW`, `EW`, `N`, or `''` for a piece
   * with no doorways (`Entrance`).
   */
  readonly sides: string
  /**
   * Decoded variants, in `File1..File6` order.
   *
   * A piece usually has several: `Act 1 - Cave W` ships `CaveW.ds1` and
   * `CaveW2.ds1`. D2MOO walks them round-robin per piece so consecutive rooms of
   * the same shape do not look identical (`DRLGMAZE_RollBasicPresets`), and
   * {@link generateMaze} does the same.
   */
  readonly levels: readonly Ds1[]
  /**
   * `LvlPrest.txt` `KillEdge`.
   *
   * 1.13c reads this off the maze map's `LvlPrest` record and, when set, trims
   * the one-cell overhang off whichever room sits on the map's outer edge — see
   * {@link stamp} and `DRLGPRESET_InitPresetRoomGrids`
   * (`DrlgPreset.cpp:1105`). The shipped table sets it per *family*, not per
   * row: `Act 1 - Cave`, `Act 2 - Sewer`, `Act 2 - Arcane`, `Act 4 - Mesa`,
   * `Act 5 - Temple` and `Act 5 - Ice` are all `1`, while `Act 1 - Crypt`,
   * `Barracks`, `Jail`, `Catacombs`, `Act 2 - Basement`, `Act 2 - Tomb` and
   * `Act 3 - Sewer` are all `0`.
   *
   * Optional so that callers which do not read the column keep the pre-1.13c
   * behaviour of never trimming.
   */
  readonly killEdge?: boolean
  /**
   * Animated tile playback speed from `LvlPrest.txt` `Animate` column.
   * In 1.13c, 0 defaults to 80 (128ms per frame at 25Hz).
   */
  readonly animSpeed?: number
}

/** Everything {@link generateMaze} needs. */
export interface MazeRequest {
  /** `Levels.txt` `Id`; used in diagnostics and for a few per-level specials. */
  readonly levelId: number
  /** `Levels.txt` `Name`, e.g. `Act 1 - Cave 1`. */
  readonly levelName: string
  /** `LvlMaze.txt` `SizeX`: the section stride in cells. */
  readonly sectionSize: number
  /**
   * `LvlMaze.txt` `SizeY`.
   *
   * Defaults to {@link sectionSize}. It is a separate field because the two
   * genuinely differ for Act 1's Barracks (`SizeX = 10`, `SizeY = 14`), and a
   * single number would silently produce a wrong layout there.
   */
  readonly sectionHeight?: number
  /** `LvlMaze.txt` `Rooms`: the target number of sections. */
  readonly minRooms: number
  /**
   * `LvlMaze.txt` `Merge`, in per-mille.
   *
   * Values in the shipped table are 0, 500, 800 and 1000.
   */
  readonly merge: number
  /** Seed for the whole generator; reported back in `stats`. */
  readonly seed: number
  /** The level type's pieces, classified by kind and side token. */
  readonly pieces: readonly MazePiece[]
  /**
   * Overrides the `KillEdge` value derived from {@link pieces}.
   *
   * 1.13c takes the flag from the maze map's own `LvlPrest` record
   * (`pMazeMap->pLvlPrestTxtRecord->dwKillEdge`), which in the shipped table
   * always agrees with the level type's piece rows. When this is omitted the
   * value is derived from the pieces instead, so a caller that already parsed
   * `LvlPrest.txt` needs to do nothing extra.
   */
  readonly killEdge?: boolean

  /**
   * `LvlTypes.txt` name, e.g. `Act 1 - Cave`.
   *
   * Optional because it is derivable from {@link pieces} — every piece name
   * starts with it — and the derivation is attempted when it is absent.
   */
  readonly levelTypeName?: string
  /**
   * `Levels.txt` `Id` of the tomb holding the Horadric Staff, when the caller
   * knows it. In the shipped game this is one of the seven Tal Rasha tombs and
   * that tomb's `LvlMaze.Rooms` is tripled.
   */
  readonly staffTombLevelId?: number
  /** Boss tomb (`LvlMaze.Rooms` doubled); see {@link staffTombLevelId}. */
  readonly bossTombLevelId?: number
  /**
   * Explicit override for waypoint placement.
   * If true, forces a waypoint room to be placed (using waypoint pieces).
   * If false, skips waypoint room placement even if the level profile includes it.
   * If omitted, defaults to whether the level id has a waypoint in the profile.
   */
  readonly hasWaypoint?: boolean
  /**
   * Maximum allowed width of the entire maze in cells (`Levels.txt` `SizeX`).
   * When specified, any room placement that would expand the overall maze bounding box
   * beyond this width will be rejected.
   */
  readonly maxCellsX?: number | undefined
  /**
   * Maximum allowed height of the entire maze in cells (`Levels.txt` `SizeY`).
   * When specified, any room placement that would expand the overall maze bounding box
   * beyond this height will be rejected.
   */
  readonly maxCellsY?: number | undefined
  /**
   * Maximum allowed sections along X axis.
   * If provided and `maxCellsX` is not specified, `maxCellsX` is computed as `maxSectionsX * sectionSize`.
   */
  readonly maxSectionsX?: number | undefined
  /**
   * Maximum allowed sections along Y axis.
   * If provided and `maxCellsY` is not specified, `maxCellsY` is computed as `maxSectionsY * (sectionHeight ?? sectionSize)`.
   */
  readonly maxSectionsY?: number | undefined
  /** Dynamic dungeon object seeding options. */
  readonly objectOptions?: MazeObjectPopulationOptions
  /** Configurable object density multiplier or options. */
  readonly objectDensity?: number | MazeObjectPopulationOptions
}

/** The generated level. */
export interface MazeResult {
  /** A single synthesized map covering every placed section. */
  readonly level: Ds1
  /** Dynamically seeded interactive objects (shrines, chests, racks, breakables). */
  readonly objects: readonly PlacedMazeObject[]
  /** What the generator did, for reporting and tests. */
  readonly stats: Record<string, unknown>
  /** Animated tile playback speed from `LvlPrest.txt` `Animate` column. */
  readonly animSpeed?: number
}

/* ------------------------------------------------------------------------- *
 * Classification
 * ------------------------------------------------------------------------- */

/** Roles whose names are spelled `<role> <sides>` after the level type. */
const KIND_PREFIXES: readonly (readonly [string, MazePieceKind])[] = [
  ['Den Of Evil', 'quest'],
  ['Coldcrow', 'quest'],
  ['Pitspawn', 'quest'],
  ['Bonebreak', 'quest'],
  ['Chest', 'treasure'],
  ['Treasure', 'treasure'],
  ['Cath', 'down'],
  ['Drain', 'next'],
  ['Warp', 'prev'],
  ['Down', 'down'],
  ['Next', 'next'],
  ['Prev', 'prev'],
  ['Theme', 'theme'],
  ['Waypoint', 'waypoint'],
  ['waypoint', 'waypoint'],
  ['Talrasha', 'quest'],
  ["Radament's Lair", 'quest'],
  ['Radament', 'quest'],
  ['Tight Spot', 'quest'],
  ['Forge', 'quest'],
  ['Court Connect', 'next'],
  ['Bridge', 'next'],
  ['Complex', 'quest'],
  ['Final Room', 'quest'],
  ['Cube', 'treasure'],
  ['Leatherarm', 'quest'],
  ['Kaa', 'quest'],
]

/**
 * Relationship between Levels.txt, LvlTypes.txt, and LvlPrest.txt:
 *
 * In Diablo II's data architecture, map pieces and environments are resolved
 * across three interconnected tables:
 *
 * 1. `Levels.txt` contains level declarations. Each row has an `Id` (1..136) and
 *    a `LevelType` foreign key pointing to `LvlTypes.txt` (or 0 for preset-only
 *    levels like towns).
 * 2. `LvlTypes.txt` defines visual and audio themes (e.g. `Act 1 - Cave`,
 *    `Act 1 - Catacombs`, `Act 2 - Harem`, `Act 3 - Kurast`). It specifies up to
 *    32 DT1 tile libraries (`File 1` .. `File 32`) containing the graphic art.
 * 3. `LvlPrest.txt` catalogs the authored DS1 map presets and maze room pieces.
 *    Each row contains `Def` (the preset ID), `Name`, `LevelId`, `Dt1Mask`,
 *    `SizeX`, `SizeY`, and up to 6 DS1 file paths (`File1` .. `File6`).
 *
 * ### Why Catacombs / Sewers index pieces via `Def` / `LevelId = 0` / first `LevelId`:
 *
 * - Preset levels (like Towns, Tristram, Countess Tower) have fixed layouts and
 *   their rows in `LvlPrest.txt` set `LevelId` to their exact `Levels.txt.Id`
 *   (e.g., Rogue Encampment has `LevelId = 1`, Lut Gholein has `LevelId = 40`).
 * - Maze levels (Catacombs 1-4 [Levels 32-35], Sewers Act 2 [Levels 41-43],
 *   Sewers Act 3 [Levels 92-93], Caves 1-2 [Levels 2-3]), in contrast, are generated
 *   dynamically by stitching dozens of modular 25x25 cell DS1 rooms (with 1..4 doors,
 *   stairs up/down, waypoints, quest rooms, themed variants).
 * - Blizzard North did not duplicate 30+ room rows for each level layer in
 *   `LvlPrest.txt`. Instead:
 *   - Room pieces are shared across all levels sharing the theme, stored with
 *     `LevelId = 0` or set to the first `LevelId` where that theme appears (e.g. 32
 *     for Catacombs).
 *   - In D2MOO and the native Blizzard engine (`DRLGMAZE_PickRoomPreset`,
 *     `DRLGMAZE_RollBasicPresets`, `DRLGMAZE_ScanReplaceSpecialPreset`), pieces
 *     are indexed either by hardcoded `Def` base offsets (`base + door_bitmask`)
 *     or discovered by prefix matching against the piece's `Name` column.
 *
 * ### Why composite themes have sub-families:
 *
 * Certain levels share a primary tile theme (`LevelType`) but introduce distinct
 * visual zones, boss chambers, or quest rooms authored under different `Name`
 * prefixes in `LvlPrest.txt`:
 * - `Act 2 - Harem`: Combines normal palace rooms (`Act 2 - Harem ...`) with
 *   corrupted harem cellar and basement pieces (`Act 2 - Corrupt Harem ...`).
 * - `Act 3 - Kurast`: Standard Kurast dungeon rooms (`Act 3 - Kurast ...`) are
 *   supplemented in Durance of Hate Level 3 with Mephisto's sanctum pieces
 *   (`Act 3 - Mephisto ...`).
 * - `Act 3 - Dungeon`: Jungle dungeon rooms (`Act 3 - Dungeon ...`) incorporate
 *   temple quest chambers (`Act 3 - Temple ...`).
 * - `Act 4 - Lava`: River of Flame and Chaos Sanctuary lava island pieces
 *   (`Act 4 - Lava ...`) are linked by preset bridge pieces (`Act 4 - Bridge ...`).
 * - `Act 5 - Ice Caves`: Contains both full-name (`Act 5 - Ice Caves ...`) and
 *   abbreviated (`Act 5 - Ice ...`) piece definitions in `LvlPrest.txt`.
 * - `Act 5 - Baal`: Worldstone Keep pieces (`Act 5 - Baal ...`) terminate in the
 *   Throne Room of Destruction (`Act 5 - ThroneRoom ...`).
 *
 * Because `LvlPrest.txt` lacks a `LevelType` column, these sub-families cannot be
 * joined by database foreign key alone; they reflect Blizzard's C++ DRLG engine
 * conventions and are captured canonically in {@link MAZE_PIECE_FAMILIES}.
 */
export const MAZE_PIECE_FAMILIES: Readonly<Record<string, readonly string[]>> = {
  'Act 1 - Cave': ['Act 1 - Cave'],
  'Act 1 - Crypt': ['Act 1 - Crypt'],
  'Act 1 - Courtyard': ['Act 1 - Courtyard'],
  'Act 1 - Barracks': ['Act 1 - Barracks'],
  'Act 1 - Jail': ['Act 1 - Jail'],
  'Act 1 - Catacombs': ['Act 1 - Catacombs'],
  'Act 2 - Sewer': ['Act 2 - Sewer'],
  'Act 2 - Harem': ['Act 2 - Corrupt Harem', 'Act 2 - Harem'],
  'Act 2 - Basement': ['Act 2 - Basement'],
  'Act 2 - Tomb': ['Act 2 - Tomb'],
  'Act 2 - Lair': ['Act 2 - Lair'],
  'Act 2 - Sanctuary': ['Act 2 - Sanctuary'],
  'Act 3 - Sewer': ['Act 3 - Sewer'],
  'Act 3 - Kurast': ['Act 3 - Mephisto', 'Act 3 - Kurast'],
  'Act 3 - Dungeon': ['Act 3 - Dungeon', 'Act 3 - Temple'],
  'Act 4 - Lava': ['Act 4 - Lava', 'Act 4 - Bridge'],
  'Act 5 - Ice Caves': ['Act 5 - Ice Caves', 'Act 5 - Ice'],
  'Act 5 - Temple': ['Act 5 - Temple'],
  'Act 5 - Baal': ['Act 5 - Baal', 'Act 5 - ThroneRoom'],
}

/**
 * Discover the `LvlPrest` name families belonging to a maze level type.
 *
 * If `lvlPrestTable` is supplied, validates and discovers family prefixes present
 * in the table that correspond to the given `levelTypeName`.
 * If `lvlPrestTable` is omitted or contains no matching family rows, falls back to
 * {@link MAZE_PIECE_FAMILIES}.
 *
 * @param levelTypeName - the owning `LvlTypes.txt` name (e.g. `'Act 2 - Harem'`).
 * @param lvlPrestTable - optional loaded `LvlPrest.txt` table.
 * @returns list of family name prefixes in precedence order (longer/more specific prefixes first).
 */
export function discoverMazePieceFamilies(
  levelTypeName: string,
  lvlPrestTable?: D2Table,
): readonly string[] {
  const staticFamilies = MAZE_PIECE_FAMILIES[levelTypeName] ?? [levelTypeName]
  if (lvlPrestTable === undefined) {
    return staticFamilies
  }

  const nameCol = lvlPrestTable.header.indexOf('Name')
  if (nameCol === -1) {
    return staticFamilies
  }

  const discovered = new Set<string>()
  for (const fam of staticFamilies) {
    const lowerFam = fam.toLowerCase()
    for (const row of lvlPrestTable.rows) {
      const rowName = (row[nameCol] ?? '').trim().toLowerCase()
      if (rowName.startsWith(lowerFam)) {
        discovered.add(fam)
        break
      }
    }
  }

  if (discovered.size === 0) {
    return staticFamilies
  }

  const result = staticFamilies.filter(fam => discovered.has(fam))
  return result.length > 0 ? result : staticFamilies
}

/**
 * Classify one `LvlPrest.txt` piece name.
 *
 * The grammar is `<level type name> [<role>] [<sides>]`: `Act 1 - Cave NSEW` is a
 * plain room, `Act 1 - Cave Theme NS` a themed variant, `Act 1 - Cave Prev W` a
 * staircase, `Act 1 - Cave Treasure 3` a numbered treasure room. A name that does
 * not start with the level type's name is not a piece of that type and yields
 * `null`, which is how `Act 1 - DOE Entrance` stays out of the `Act 1 - Cave`
 * group even though D2MOO treats its row as that group's base.
 *
 * @param name - the `LvlPrest.txt` `Name`.
 * @param levelTypeName - the owning `LvlTypes.txt` name.
 * @param lvlPrestTable - optional loaded `LvlPrest.txt` table for dynamic family discovery.
 * @returns the kind and side token, or `null` when the name is not a piece.
 */
export function classifyMazePieceName(
  name: string,
  levelTypeName: string,
  lvlPrestTable?: D2Table,
): { kind: MazePieceKind; sides: string } | null {
  const families = discoverMazePieceFamilies(levelTypeName, lvlPrestTable)
  const matchedFamily = families.find(family => name.toLowerCase().startsWith(family.toLowerCase()))
  if (matchedFamily === undefined) return null
  const rest = name.slice(matchedFamily.length).trim()
  if (rest === '' || rest.toLowerCase() === 'entrance') {
    if (matchedFamily.toLowerCase().endsWith('throneroom')) return { kind: 'quest', sides: '' }
    return { kind: 'entrance', sides: '' }
  }
  for (const [prefix, kind] of KIND_PREFIXES) {
    if (rest.toLowerCase().startsWith(prefix.toLowerCase())) {
      const tail = rest.slice(prefix.length).trim()
      // `Treasure 1`, `Temple 1`, `Bridge 1` are numbered rather than sided; everything else uses sides.
      if ((kind === 'treasure' || kind === 'quest' || kind === 'next') && /^\d+$/.test(tail)) {
        return { kind, sides: '' }
      }
      return { kind, sides: tail }
    }
  }
  if (/^\d+$/.test(rest)) {
    if (matchedFamily.toLowerCase().endsWith('bridge')) return { kind: 'next', sides: '' }
    if (matchedFamily.toLowerCase().endsWith('temple')) return { kind: 'quest', sides: '' }
    if (matchedFamily.toLowerCase().endsWith('treasure')) return { kind: 'treasure', sides: '' }
  }
  const reversedMatch = /^([NSEW]+)\s+(up|down|waypoint)$/i.exec(rest)
  if (reversedMatch !== null) {
    const sides = reversedMatch[1]!.toUpperCase()
    const role = reversedMatch[2]!.toLowerCase()
    return { kind: role === 'up' ? 'prev' : role === 'down' ? 'next' : 'waypoint', sides }
  }
  // Anything left is a bare side token: `W`, `EW`, `NSEW`, or `X` (blank filler)
  if (/^([NSEW]+|X)$/i.test(rest)) return { kind: 'room', sides: rest.toUpperCase() === 'X' ? '' : rest.toUpperCase() }
  if (/^(River|Pool|Tainted Sun)/i.test(rest)) return { kind: 'room', sides: '' }
  return null
}

/**
 * Derive a level type's name from its pieces.
 *
 * Used when {@link MazeRequest.levelTypeName} is omitted: the name is the longest
 * prefix shared by every piece name, trimmed back to a word boundary, because
 * `Act 1 - Cave NSEW` and `Act 1 - Cave Theme W` agree on `Act 1 - Cave ` and the
 * trailing space is the boundary.
 *
 * @param pieces - the level's pieces.
 * @returns the inferred name, or `null` when the pieces disagree.
 */
export function inferLevelTypeName(pieces: readonly MazePiece[]): string | null {
  const first = pieces[0]
  if (first === undefined) return null
  let prefix = first.name
  for (const piece of pieces) {
    while (prefix.length > 0 && !piece.name.startsWith(prefix)) prefix = prefix.slice(0, -1)
  }
  const trimmed = prefix.replace(/[\s-]+$/, '')
  if (trimmed === '') return null
  if (trimmed === 'Act 5 - Ice') return 'Act 5 - Ice Caves'
  return trimmed
}

/* ------------------------------------------------------------------------- *
 * Level-type profiles
 * ------------------------------------------------------------------------- */

/**
 * One `DRLGMAZE_ScanReplaceSpecialPreset` call site.
 *
 * That function has two modes and both are reproduced: it first looks for the
 * earliest room still holding the *plain* piece for this side pattern and swaps
 * the piece in place; only if no such room exists does it grow a brand-new room
 * off the first available room in `direction` and put the special piece there.
 *
 * `DRLGMAZE_ReplaceRoomPreset`, used by Act 3's Sewer 1, is the in-place-only
 * variant and is modelled with {@link inPlaceOnly}.
 */
interface SpecialStep {
  /** Which kind of special piece to place. */
  readonly kind: 'prev' | 'next' | 'down' | 'quest' | 'treasure' | 'waypoint'
  /** Side token of the plain room this step replaces, e.g. `N`. */
  readonly plainSides: string
  /** Side token of the special piece. */
  readonly sides: string
  /** Direction used when a new room must be created (`nDirection`). */
  readonly direction: number
  /** Only replace an existing room; never grow one (D2MOO's `ReplaceRoomPreset`). */
  readonly inPlaceOnly?: boolean
}

/**
 * A group of four candidate steps selected by one rotating counter.
 *
 * D2MOO draws `nRand = SEED_RollRandomNumber(&pLevel->pSeed) & 3` once and then
 * indexes each hardcoded table with it, incrementing the counter after every
 * call that actually happens. So the *n*-th group applied to a level uses entry
 * `(nRand + n) % 4`, not the first entry. Modelling the tables as groups with a
 * shared cursor reproduces that exactly; flattening them into one ordered list
 * would not.
 */
interface SpecialGroup {
  readonly kind: SpecialStep['kind']
  /** Four entries, in D2MOO's table order (N, E, S, W). */
  readonly entries: readonly Omit<SpecialStep, 'kind'>[]
  /** Restrict the group to these `Levels.txt` ids; absent means every level. */
  readonly levelIds?: readonly number[]
  /** Apply to every level *except* these ids (D2MOO's `else` arms). */
  readonly exceptLevelIds?: readonly number[]
  /** Only replace; never grow (D2MOO's `ReplaceRoomPreset`). */
  readonly inPlaceOnly?: boolean
}

/**
 * How one maze level type is laid out.
 *
 * D2MOO's `DRLGMAZE_GenerateLevel` `switch (pLevel->nLevelType)` turned into
 * data. Where that function has both a `while (pLevel->nRooms < nRooms)` loop and
 * a `DRLGMAZE_BuildBasicMaze` call, the two are the same growth primitive spelled
 * twice — see {@link growMaze} — so both are represented by `grow: true`.
 */
export interface MazeLevelProfile {
  /** `LvlTypes.txt` name this profile applies to. */
  readonly levelTypeName: string
  /**
   * `DRLGMAZE_InitBasicMazeLayout(nRoomsPerDirection)`: pre-place a closed ring of
   * `4n - 4` rooms, then grow from it. Act 3's Sewer 1 overrides this with 5.
   */
  readonly ring?: number
  /** Grow to `LvlMaze.Rooms` sections with the random-adjacency loop. */
  readonly grow: boolean
  /**
   * `DRLGMAZE_RollAct_1_2_3_BasicPresets`: convert up to
   * `max(rooms / 5 + 1, 2)` plain rooms to their `Theme` variant.
   */
  readonly theme: boolean
  /** `'catacombs'`: the Act 1 Catacombs opening moves. */
  readonly opening?: 'catacombs'
  /** Chain of three rooms plus a `Prev` piece on the entrance. */
  readonly prevChain?: 'tomb' | 'baal'
  /** `DRLGMAZE_PlaceArcaneSanctuary`: four 15-room spiral branches. */
  readonly arcane?: boolean
  /** `DRLGMAZE_PlaceAct5LavaPresets`: two fixed pieces plus blank filling. */
  readonly lavaPairs?: boolean
  /**
   * Levels that are a single fixed `LvlPrest.txt` row rather than a maze, with
   * the exact names to try, in order. Used for the three Act 5 ice set pieces and
   * Claw Viper Temple level 2.
   */
  readonly singleRoom?: Readonly<Record<number, readonly string[]>>
  /** `Prev`/`Next`/quest/treasure replacement groups, in call order. */
  readonly specials: readonly SpecialGroup[]
  /** Optional official dungeon special pass from D2MOO. */
  readonly specialPass?: SpecialPassName
  /** Animated tile speed override from LvlPrest.txt (default 80 if 0 or omitted). */
  readonly animSpeed?: number
}

/** The four single-side layouts, in the order the D2MOO tables list them. */
const NESW = ['N', 'E', 'S', 'W'] as const

/** Direction paired with each of {@link NESW} in the D2MOO tables. */
const NESW_DIRECTIONS = [DIRECTION_SOUTH, DIRECTION_WEST, DIRECTION_NORTH, DIRECTION_EAST] as const

/**
 * Build the four side entries of a D2MOO table.
 *
 * @param plainSides - the four plain side tokens, in table order.
 * @returns the entries.
 */
function entries(plainSides: readonly string[] = NESW): Omit<SpecialStep, 'kind'>[] {
  return plainSides.map((sides, index) => ({ plainSides: sides, sides, direction: NESW_DIRECTIONS[index]! }))
}

/**
 * The per-level-type layout table, transcribed from `DRLGMAZE_GenerateLevel`.
 *
 * `specials` covers the replacement tables that live inline in that function.
 * The per-act decoration passes that follow it in D2MOO — `DRLGMAZE_PlaceAct2TombStuff`,
 * `DRLGMAZE_PlaceAct3DungeonStuff`, `DRLGMAZE_PlaceAct5IceStuff` and the rest —
 * live in `maze-special-passes.ts` and are named by a row's `specialPass`; each
 * one that runs is listed in `stats.specialPassesApplied`. A row with an empty
 * `specials` table has no transcribed staircase pass at all, and
 * {@link unimplementedPassesFor} reports that gap in `stats.unimplementedPasses`
 * so the omission is visible rather than silent.
 */
export const MAZE_LEVEL_TYPE_PROFILES: readonly MazeLevelProfile[] = [
  {
    levelTypeName: 'Act 1 - Cave',
    grow: true,
    theme: true,
    specials: [
      { kind: 'prev', entries: entries() },
      { kind: 'quest', entries: entries(), levelIds: [8] },
      { kind: 'down', entries: entries(), exceptLevelIds: [8] },
      { kind: 'quest', entries: entries(), levelIds: [9] },
      { kind: 'next', entries: entries(), levelIds: [10] },
    ],
  },
  {
    levelTypeName: 'Act 1 - Crypt',
    grow: true,
    theme: true,
    specials: [
      { kind: 'prev', entries: entries() },
      { kind: 'quest', entries: entries(), levelIds: [18] },
      { kind: 'treasure', entries: entries(), levelIds: [19] },
      { kind: 'next', entries: entries(), levelIds: [21, 22, 23, 24] },
    ],
  },
  {
    levelTypeName: 'Act 1 - Barracks',
    ring: 2,
    grow: true,
    theme: true,
    specials: [
      { kind: 'next', entries: entries() },
    ],
    specialPass: 'DRLGMAZE_PlaceAct1Barracks',
  },
  {
    levelTypeName: 'Act 1 - Jail',
    ring: 2,
    grow: true,
    theme: true,
    specials: [
      { kind: 'prev', entries: entries() },
      { kind: 'waypoint', entries: entries(), levelIds: [29] },
      { kind: 'quest', entries: entries(), levelIds: [30] },
      // D2MOO's `else` hangs off the `JAILLEV3` test, so levels 1 and 2 get a
      // `Next` staircase and level 3 gets the Cathedral descent instead.
      { kind: 'next', entries: entries(), levelIds: [29, 30] },
      { kind: 'down', entries: entries(), levelIds: [31] },
    ],
  },
  {
    levelTypeName: 'Act 1 - Catacombs',
    grow: true,
    theme: true,
    opening: 'catacombs',
    specials: [
      { kind: 'next', entries: entries() },
      { kind: 'waypoint', entries: entries(), levelIds: [35] },
    ],
  },
  {
    levelTypeName: 'Act 2 - Sewer',
    ring: 2,
    grow: true,
    theme: true,
    specials: [
      { kind: 'prev', entries: entries() },
      { kind: 'next', entries: entries(), levelIds: [47, 48] },
    ],
    specialPass: 'DRLGMAZE_ScanReplaceSpecialAct2SewersPresets',
  },
  {
    levelTypeName: 'Act 2 - Tomb',
    grow: true,
    theme: true,
    prevChain: 'tomb',
    singleRoom: { 61: ['Act 2 - Tomb Tainted Sun X'] },
    specials: [
      { kind: 'next', entries: entries(), levelIds: [55, 56, 57, 58] },
    ],
    specialPass: 'DRLGMAZE_PlaceAct2TombStuff',
  },
  {
    levelTypeName: 'Act 2 - Lair',
    ring: 2,
    grow: true,
    theme: false,
    specials: [
      { kind: 'prev', entries: entries() },
      { kind: 'next', entries: entries(), levelIds: [62, 63] },
    ],
    specialPass: 'DRLGMAZE_PlaceAct2LairStuff',
  },
  {
    levelTypeName: 'Act 2 - Arcane',
    grow: false,
    theme: false,
    arcane: true,
    specials: [{ kind: 'quest', entries: entries() }],
  },
  {
    levelTypeName: 'Act 2 - Harem',
    ring: 2,
    grow: false,
    theme: false,
    specials: [
      { kind: 'prev', entries: entries(), levelIds: [51] },
      { kind: 'next', entries: entries(), levelIds: [51] },
    ],
  },
  {
    levelTypeName: 'Act 2 - Basement',
    ring: 2,
    grow: false,
    theme: false,
    specials: [
      { kind: 'prev', entries: entries(), levelIds: [52, 53, 54] },
      { kind: 'waypoint', entries: entries(), levelIds: [52] },
      { kind: 'next', entries: entries(), levelIds: [52, 53] },
    ],
  },
  {
    levelTypeName: 'Act 3 - Spider',
    ring: 2,
    grow: false,
    theme: false,
    specials: [
      { kind: 'prev', entries: entries(), levelIds: [84, 85] },
      { kind: 'treasure', entries: entries(), levelIds: [84, 85] },
    ],
  },
  {
    levelTypeName: 'Act 3 - Kurast',
    ring: 2,
    grow: true,
    theme: true,
    specials: [
      { kind: 'prev', entries: entries(), levelIds: [100, 101] },
      { kind: 'next', entries: entries(), levelIds: [100, 101] },
    ],
    specialPass: 'DRLGMAZE_PlaceAct3MephistoStuff',
  },
  {
    levelTypeName: 'Act 3 - Dungeon',
    ring: 2,
    grow: true,
    theme: true,
    specials: [
      { kind: 'prev', entries: entries(), levelIds: [86, 87, 88, 89] },
      { kind: 'next', entries: entries(), levelIds: [86, 87, 88, 89] },
    ],
    specialPass: 'DRLGMAZE_PlaceAct3DungeonStuff',
  },
  {
    levelTypeName: 'Act 3 - Sewer',
    ring: 2,
    grow: true,
    theme: true,
    specials: [
      // Sewer 1 runs `InitBasicMazeLayout(5)` and places corner staircases for
      // both surface exits (Kurast Bazaar via NW/NE and Upper Kurast via SW/SE),
      // plus the drain descent to Sewer Level 2.
      {
        kind: 'prev',
        entries: [
          { plainSides: 'NW', sides: 'NW', direction: DIRECTION_NORTH },
          { plainSides: 'NE', sides: 'NE', direction: DIRECTION_NORTH },
          { plainSides: 'NW', sides: 'NW', direction: DIRECTION_NORTH },
          { plainSides: 'NE', sides: 'NE', direction: DIRECTION_NORTH },
        ],
        levelIds: [92],
        inPlaceOnly: true,
      },
      {
        kind: 'prev',
        entries: [
          { plainSides: 'SW', sides: 'SW', direction: DIRECTION_SOUTH },
          { plainSides: 'SE', sides: 'SE', direction: DIRECTION_SOUTH },
          { plainSides: 'SW', sides: 'SW', direction: DIRECTION_SOUTH },
          { plainSides: 'SE', sides: 'SE', direction: DIRECTION_SOUTH },
        ],
        levelIds: [92],
        inPlaceOnly: true,
      },
      {
        kind: 'next',
        entries: entries(),
        levelIds: [92],
      },
    ],
    specialPass: 'DRLGMAZE_PlaceAct3SewerStuff',
  },
  {
    levelTypeName: 'Act 4 - Lava',
    grow: true,
    theme: false,
    animSpeed: 1,
    specials: [],
    specialPass: 'DRLGMAZE_PlaceAct4Lava',
  },
  {
    levelTypeName: 'Act 5 - Ice Caves',
    ring: 2,
    grow: true,
    theme: false,
    singleRoom: {
      114: ['Act 5 - Ice River A', 'Act 5 - Ice River B'],
      116: ['Act 5 - Ice Pool A'],
      119: ['Act 5 - Ice Pool B'],
    },
    specials: [
      { kind: 'prev', entries: entries(), levelIds: [113, 115, 118] },
      { kind: 'next', entries: entries(), levelIds: [113, 115, 118] },
      { kind: 'down', entries: entries(), levelIds: [113, 115, 118] },
    ],
    specialPass: 'DRLGMAZE_PlaceAct5IceStuff',
  },
  {
    levelTypeName: 'Act 5 - Temple',
    ring: 2,
    grow: false,
    theme: false,
    specials: [
      {
        kind: 'prev',
        entries: [
          { plainSides: 'SE', sides: 'SE', direction: DIRECTION_SOUTH },
          { plainSides: 'SE', sides: 'SE', direction: DIRECTION_SOUTH },
          { plainSides: 'SE', sides: 'SE', direction: DIRECTION_SOUTH },
          { plainSides: 'SE', sides: 'SE', direction: DIRECTION_SOUTH },
        ],
        levelIds: [122, 123],
        inPlaceOnly: true,
      },
      {
        kind: 'next',
        entries: [
          { plainSides: 'NE', sides: 'NE', direction: DIRECTION_NORTH },
          { plainSides: 'NW', sides: 'NW', direction: DIRECTION_NORTH },
          { plainSides: 'SW', sides: 'SW', direction: DIRECTION_SOUTH },
          { plainSides: 'NE', sides: 'NE', direction: DIRECTION_NORTH },
        ],
        levelIds: [122, 123],
        inPlaceOnly: true,
      },
    ],
    specialPass: 'DRLGMAZE_PlaceAct5TempleStuff',
  },
  {
    levelTypeName: 'Act 5 - Baal',
    grow: true,
    theme: false,
    prevChain: 'baal',
    specials: [
      { kind: 'next', entries: entries(), levelIds: [128, 129, 130] },
    ],
    specialPass: 'DRLGMAZE_PlaceAct5BaalStuff',
  },
  {
    levelTypeName: 'Act 5 - Lava',
    grow: false,
    theme: false,
    lavaPairs: true,
    specials: [
      { kind: 'prev', entries: entries(), levelIds: [125, 126, 127] },
      { kind: 'treasure', entries: entries(), levelIds: [125, 126, 127] },
    ],
  },
]

/**
 * `LvlPrest.txt` names a level type needs from outside its own name prefix.
 *
 * Only Act 5's Lava levels do this: `DRLGMAZE_PlaceAct5LavaPresets` fills the
 * blank space around the Infernal Pit's two set pieces with the *Act 4* lava
 * filler row, so a caller that groups pieces strictly by name prefix must fetch
 * these extra rows too.
 */
export const MAZE_SHARED_PRESET_NAMES: readonly string[] = ['Act 4 - Lava X']

/**
 * The gaps one level type still has, derived from its own profile row.
 *
 * This used to be a module constant hardcoded to `[]` with a comment claiming
 * every pass was implemented, which made `stats.unimplementedPasses` report
 * "no gaps" for a level that stamps no staircase at all. The list is now
 * *measured* from {@link MAZE_LEVEL_TYPE_PROFILES}, so it cannot drift away
 * from the table it describes:
 *
 * - **no profile row** — `generateMaze` cannot build the type at all.
 * - **an empty `specials` table** — the `Prev`/`Next`/quest replacement rows
 *   that `DRLGMAZE_GenerateLevel` runs inline have not been transcribed, so the
 *   level stamps no staircase room and the packer has to invent a fallback warp
 *   (see `scripts/pack-act-assets.ts`). A `singleRoom` type is exempt: it is one
 *   fixed `LvlPrest.txt` row whose stairs are part of the artwork.
 *
 * The 11 per-act decoration passes named in D2MOO (`DRLGMAZE_PlaceAct2TombStuff`
 * and the rest) *are* reproduced, in `maze-special-passes.ts`; a type that runs
 * one names it in `stats.specialPassesApplied` instead.
 *
 * @param levelTypeName - the `LvlTypes.txt` name.
 * @returns the gaps, empty when the type is fully transcribed.
 */
export function unimplementedPassesFor(levelTypeName: string): readonly string[] {
  const profile = MAZE_LEVEL_TYPE_PROFILES.find(candidate => candidate.levelTypeName === levelTypeName)
  if (profile === undefined) return [`DRLGMAZE_GenerateLevel profile (${levelTypeName})`]
  const gaps: string[] = []
  if (profile.specials.length === 0 && profile.singleRoom === undefined && profile.specialPass === undefined) {
    gaps.push(`DRLGMAZE_GenerateLevel specials table (${levelTypeName})`)
  }
  return gaps
}

/**
 * Every gap {@link unimplementedPassesFor} can name, across all level types.
 *
 * Non-empty while any level type is still missing its replacement table. Prefer
 * the per-level-type function: a caller that reports this constant for one level
 * is reporting other levels' gaps as its own.
 */
export const UNIMPLEMENTED_PASSES: readonly string[] = MAZE_LEVEL_TYPE_PROFILES
  .flatMap(profile => unimplementedPassesFor(profile.levelTypeName))

/* ------------------------------------------------------------------------- *
 * Geometry
 * ------------------------------------------------------------------------- */

/** A rectangle in cells. Mirrors the `nTileXPos`/`nTileYPos`/`nTileWidth`/
 * `nTileHeight` view of `D2DrlgRoomStrc`, which D2MOO also reads as
 * `D2DrlgCoordStrc` through a union. */
interface Rect {
  x: number
  y: number
  width: number
  height: number
}

/**
 * Signed gap between two cell rectangles, per axis.
 *
 * `DRLG_ComputeManhattanDistance` verbatim, including its one-axis-at-a-time
 * formulation: a negative value means the rectangles overlap on that axis, and
 * two rooms sharing a face give one gap of 0 and one of `-height`, which is the
 * signature every caller tests for.
 *
 * @param a - first rectangle.
 * @param b - second rectangle.
 * @returns the per-axis gaps.
 */
function manhattan(a: Rect, b: Rect): { dx: number; dy: number } {
  const dx = a.x >= b.x ? a.x - b.width - b.x : b.x - a.width - a.x
  const dy = a.y >= b.y ? a.y - b.height - b.y : b.y - a.height - a.y
  return { dx, dy }
}

/**
 * Whether two rectangles are clear of each other by at least `margin` cells.
 *
 * `DRLG_GetRectanglesManhattanDistanceAndCheckNotOverlapping`: true when either
 * axis is separated by `margin` or more. With `margin = 0` a pair of face-to-face
 * rooms has `dx = 0` and therefore *passes*, so sharing a face counts as "not
 * overlapping" — which is what lets rooms tile a grid.
 *
 * @param a - first rectangle.
 * @param b - second rectangle.
 * @param margin - required separation on at least one axis.
 * @returns true when they are clear.
 */
function notOverlapping(a: Rect, b: Rect, margin: number): boolean {
  const { dx, dy } = manhattan(a, b)
  return dx >= margin || dy >= margin
}

/**
 * The quarter direction from `from` to `to`, or `-1` when they are not
 * face-to-face neighbours.
 *
 * `DRLG_GetDirectionFromCoordinates`. Only the four quarters are produced; the
 * diagonals exist solely as placement offsets in `FillBlankMazeSpaces`.
 *
 * @param from - the origin rectangle.
 * @param to - the candidate neighbour.
 * @returns a direction index 0..3, or -1.
 */
function directionFrom(from: Rect, to: Rect): number {
  if (from.x <= to.x) {
    if (to.x === from.x + from.width) return DIRECTION_EAST
  } else if (from.x === to.x + to.width) {
    return DIRECTION_WEST
  }
  if (from.y <= to.y) {
    if (to.y === from.y + from.height) return DIRECTION_SOUTH
  } else if (from.y === to.y + to.height) {
    return DIRECTION_NORTH
  }
  return -1
}

/* ------------------------------------------------------------------------- *
 * Rooms
 * ------------------------------------------------------------------------- */

/** One maze section. */
export interface Room extends Rect {
  /** Doorways to neighbouring rooms, in insertion order. */
  readonly orths: { room: Room; direction: number }[]
  /** Creation index, so diagnostics can name a room stably. */
  readonly id: number
  /** The chosen piece; `null` until a preset pass runs. */
  piece: MazePiece | null
  /** Which variant of `piece` to stamp. */
  variant: number
  /**
   * `DRLGPRESETROOMFLAG_HAS_MAP_DS1`.
   *
   * Set by the passes that call `DRLGMAZE_SetPickedFileAndPresetId` with
   * `bResetFlag = FALSE`; cleared by every `DRLGMAZE_PickRoomPreset` call, which
   * passes 1. A set flag means "this room's artwork is final", so the merge pass
   * skips it and no later preset pass may overwrite it.
   */
  hasDs1: boolean
  /** Per-room random stream, seeded from the level's (`DRLGROOM_AllocRoomEx`). */
  readonly rng: Rng
}

/** The level under construction. */
export interface MazeLevel {
  /** Rooms newest-first, matching `DRLGROOM_AddRoomExToLevel`'s prepend. */
  readonly rooms: Room[]
  /** The first room created; D2MOO keeps this pointer across prepends. */
  readonly first: Room
  readonly width: number
  readonly height: number
  readonly rng: Rng
  readonly mergePerMille: number
  /** Next room id. */
  nextId: number
  /** Doorways added by the merge pass, for reporting. */
  merges: number
  readonly maxCellsX?: number | undefined
  readonly maxCellsY?: number | undefined
  readonly stats?: MazeStats | undefined
}

/**
 * Report collector shared by the passes.
 *
 * Dense on purpose: the generator's job is to be inspectable, and the interesting
 * facts are which piece answered which side pattern and where the exact pattern
 * was missing.
 */
export interface MazeStats {
  readonly patterns: Record<string, Record<string, number>>
  readonly fallbacks: { room: number; requested: string; used: string }[]
  readonly specialsApplied: { room: number; kind: string; sides: string }[]
  readonly specialPassesApplied: string[]
  readonly unresolvedRoles: string[]
  readonly notes: string[]
  ringRooms: number
  placementAttempts: number
  boundsRejected?: number
  /** Stair rooms, with the coordinates only `stamp` can work out. */
  warps: MazeWarp[]
  /** Waypoint rooms, with the coordinates and entity positions only `stamp` can work out. */
  waypoints: MazeWaypoint[]
}

/**
 * A staircase, as reported in `stats.warps`.
 *
 * `specialsApplied` already records which room got a stair piece, but a room id
 * is not a place: rooms are positioned in an unbounded coordinate space that
 * `stamp` shifts by `minX`/`minY` to produce the final map. The coordinates
 * below are in that final map's cells, so they can be baked straight into the
 * pack.
 */
export interface MazeWarp {
  /** The room the piece went into, matching `specialsApplied`. */
  readonly room: number
  /**
   * Which way it goes:
   * - `up` — `Prev`, back towards the level above.
   * - `down` — `Next` or `Down`, deeper in. `Down` is the Act 1 cave descent,
   *   which uses a different piece but means the same thing here.
   */
  readonly direction: 'up' | 'down'
  /** The piece kind it came from, for tracing back to the artwork. */
  readonly kind: 'prev' | 'next' | 'down'
  /** The room's top-left corner in the final map, in cells. */
  readonly x: number
  readonly y: number
  /** The room's extent, in cells. */
  readonly width: number
  readonly height: number
  /** The room's centre, where the player lands or the warp is clicked. */
  readonly centreX: number
  readonly centreY: number
}

/**
 * Create a room with no position or piece.
 *
 * @param level - the level.
 * @returns the room.
 */
function makeRoom(level: MazeLevel): Room {
  const room: Room = {
    x: 0, y: 0, width: level.width, height: level.height,
    orths: [], id: level.nextId, piece: null, variant: 0, hasDs1: false,
    rng: new Rng(level.rng.int(0, 0x7fffffff)),
  }
  level.nextId += 1
  return room
}

/**
 * Add a doorway between two rooms.
 *
 * `DRLGROOM_AllocDrlgOrthsForRooms`: idempotent per room pair, and the reciprocal
 * orth points the opposite way, `(direction - 2) & 3`.
 *
 * @param a - first room.
 * @param b - second room.
 * @param direction - direction from `a` to `b`.
 */
function linkRooms(a: Room, b: Room, direction: number): void {
  if (!a.orths.some(orth => orth.room === b)) a.orths.push({ room: b, direction })
  const opposite = (direction - 2) & 3
  if (!b.orths.some(orth => orth.room === a)) b.orths.push({ room: a, direction: opposite })
}

/**
 * The 4-bit side mask a room's doorways produce.
 *
 * `DRLGMAZE_PickRoomPreset` ORs {@link DIRECTION_SIDE_BIT} for every doorway. A
 * room with no doorways gets 0, which in D2MOO selects the level type's base row
 * and here selects the `entrance` piece.
 *
 * @param room - the room.
 * @returns the mask, 0..15.
 */
export function roomSideMask(room: Room): number {
  let mask = 0
  for (const orth of room.orths) mask |= DIRECTION_SIDE_BIT[orth.direction]!
  return mask
}

/**
 * Whether placing `room` keeps the overall maze within `maxCellsX` and `maxCellsY`.
 *
 * In Diablo II, maze room growth is constrained by the level's global SizeX and SizeY limits
 * from `Levels.txt`. If growing a candidate room would expand the bounding box of all placed
 * rooms beyond the allowed dimensions, the placement attempt is rejected, forcing the maze
 * to grow in other available directions and cluster compactly.
 *
 * @param level - the level.
 * @param room - the candidate room with proposed coordinates and dimensions.
 * @returns true if within bounds or bounds are unconstrained.
 */
function checkRoomWithinBounds(level: MazeLevel, room: Rect): boolean {
  if (level.maxCellsX === undefined && level.maxCellsY === undefined) return true
  if (level.rooms.length === 0) return true

  if (level.maxCellsX !== undefined && level.maxCellsX > 0) {
    let minX = room.x
    let maxX = room.x + room.width
    for (const other of level.rooms) {
      if (other.x < minX) minX = other.x
      if (other.x + other.width > maxX) maxX = other.x + other.width
    }
    if (maxX - minX > level.maxCellsX) {
      if (level.stats) {
        level.stats.boundsRejected = (level.stats.boundsRejected ?? 0) + 1
      }
      return false
    }
  }

  if (level.maxCellsY !== undefined && level.maxCellsY > 0) {
    let minY = room.y
    let maxY = room.y + room.height
    for (const other of level.rooms) {
      if (other.y < minY) minY = other.y
      if (other.y + other.height > maxY) maxY = other.y + other.height
    }
    if (maxY - minY > level.maxCellsY) {
      if (level.stats) {
        level.stats.boundsRejected = (level.stats.boundsRejected ?? 0) + 1
      }
      return false
    }
  }

  return true
}

/**
 * Whether a room overlaps anything other than itself and one ignored room, and fits within level bounds.
 *
 * `DRLGMAZE_CheckRoomNotOverlaping`. The margin is 0, so face-to-face neighbours
 * are fine and only genuine overlap fails.
 *
 * @param level - the level.
 * @param room - the candidate.
 * @param ignored - a room to skip, usually its parent.
 * @returns true when the room is clear and within global level bounds.
 */
function checkRoomNotOverlapping(level: MazeLevel, room: Rect, ignored: Room | null): boolean {
  if (!checkRoomWithinBounds(level, room)) return false
  for (const other of level.rooms) {
    if (other === room || other === ignored) continue
    if (!notOverlapping(room, other, 0)) return false
  }
  return true
}

/**
 * Position `room` against `anchor` in `direction` and validate the result.
 *
 * `DRLGMAZE_LinkMazeRooms`: the coordinate assignment happens first, so a failed
 * link leaves the caller to discard the room.
 *
 * @param level - the level.
 * @param room - the room to move.
 * @param anchor - the room to sit against.
 * @param direction - which side of the anchor to occupy.
 * @returns true when the position is legal.
 */
function linkMazeRooms(level: MazeLevel, room: Room, anchor: Room, direction: number): boolean {
  const { dx, dy } = step(direction, anchor.width, anchor.height)
  room.x = anchor.x + dx
  room.y = anchor.y + dy
  for (const orth of anchor.orths) {
    // Anything already touching the anchor must not be run into.
    if (!notOverlapping(room, orth.room, 0)) return false
  }
  return checkRoomNotOverlapping(level, room, anchor)
}

/**
 * The earliest room that still needs a piece, optionally of a given shape.
 *
 * @param level - the level, newest-first.
 * @param mask - required side mask, or `null` for any.
 * @returns the room, or null.
 */
function findUnfinishedRoom(level: MazeLevel, mask: number | null): Room | null {
  for (const room of level.rooms) {
    if (room.hasDs1) continue
    if (mask === null || roomSideMask(room) === mask) return room
  }
  return null
}

/**
 * Grow one room off `parent`, or return null when the spot is taken.
 *
 * `DRLGMAZE_AddAdjacentMazeRoom`, with its inline copy of the merge loop factored
 * into {@link mergeMazeRooms}. The fold is safe because in D2MOO the new room has
 * not been added to the roster when either spelling runs, so both iterate exactly
 * the same candidates in the same order; the only difference is that
 * `DRLGMAZE_BuildBasicMaze` calls `MergeMazeRooms` unconditionally while
 * `AddAdjacentMazeRoom` guards it with `bMergeRooms`, and every caller that
 * passes `FALSE` has already established that there is nothing to merge.
 *
 * @param level - the level.
 * @param parent - the room to grow from.
 * @param direction - which side to grow towards, 0..7.
 * @param merge - whether the accidental-adjacency join pass runs.
 * @param repickParent - whether to re-derive the parent's piece afterwards.
 *   `DRLGMAZE_FillBlankMazeSpaces` deliberately does not.
 * @returns the new room, or null.
 */
export function addAdjacentMazeRoom(
  level: MazeLevel,
  parent: Room,
  direction: number,
  merge: boolean,
  repickParent: boolean,
  index: PieceIndex,
  stats: MazeStats,
): Room | null {
  const room = makeRoom(level)
  const { dx, dy } = step(direction, parent.width, parent.height)
  room.x = parent.x + dx
  room.y = parent.y + dy
  for (const orth of parent.orths) {
    if (!notOverlapping(room, orth.room, 0)) return null
  }
  if (!checkRoomNotOverlapping(level, room, parent)) return null

  linkRooms(parent, room, direction)
  if (merge) mergeMazeRooms(level, room, index, stats)
  level.rooms.unshift(room)
  if (repickParent) pickRoomPreset(parent, index, stats)
  pickRoomPreset(room, index, stats)
  return room
}

/**
 * Join a newly placed room to every already-placed room it happens to touch.
 *
 * **This is what `LvlMaze.Merge` does.** `DRLGMAZE_MergeMazeRooms` (and the
 * inline copy inside `DRLGMAZE_AddAdjacentMazeRoom`) walks the whole roster for
 * rooms no more than one cell away where `dx !== dy` — which, given that overlap
 * has already been ruled out, can only mean a face-to-face neighbour — and, when
 * the two are not already connected, rolls
 * `SEED_RollRandomNumber(&i->pSeed) % 1000 < dwMerge`, on the *candidate's* seed
 * rather than the pivot's. A success adds a doorway and re-picks that room's
 * piece, because its side pattern just changed.
 *
 * The consequence is why `Merge` matters: the growth loop attaches a room on one
 * side only, but the section grid is coarse enough that a new room frequently
 * lands flush against an unrelated older room. With `Merge = 0` those contacts
 * stay walls and the level is a bare tree; with `Merge = 1000` every one of them
 * becomes a door and the level acquires loops.
 *
 * @param level - the level.
 * @param pivot - the room just placed.
 * @param index - the piece index, for re-picking.
 * @param stats - report collector.
 */
function mergeMazeRooms(level: MazeLevel, pivot: Room, index: PieceIndex, stats: MazeStats): void {
  if (pivot.hasDs1) return
  for (const other of level.rooms) {
    if (other === pivot || other.hasDs1) continue
    const { dx, dy } = manhattan(pivot, other)
    // `!(dx >= 1 || dy >= 1)`, then `dx !== dy`.
    if (dx >= 1 || dy >= 1) continue
    if (dx === dy) continue
    if (pivot.orths.some(orth => orth.room === other)) continue
    if (other.rng.int(0, 999) >= level.mergePerMille) continue
    const direction = directionFrom(other, pivot)
    if (direction === -1) continue
    linkRooms(other, pivot, direction)
    level.merges += 1
    pickRoomPreset(other, index, stats)
  }
}

/* ------------------------------------------------------------------------- *
 * Piece lookup
 * ------------------------------------------------------------------------- */

/** Indexed pieces, so a side mask is one map lookup. */
export interface PieceIndex {
  /**
   * The piece for a room's doorway mask.
   *
   * @param mask - the room's doorway mask.
   * @param stats - report collector for fallbacks.
   * @param roomId - the room being served, for the report.
   * @returns the piece to stamp.
   */
  choose(mask: number, stats: MazeStats, roomId: number): MazePiece
}

/**
 * Count set bits.
 *
 * @param value - the value.
 * @returns the population count.
 */
function popcount(value: number): number {
  let count = 0
  let rest = value
  while (rest !== 0) { count += rest & 1; rest >>>= 1 }
  return count
}

/**
 * Index a level type's pieces by kind and side mask.
 *
 * `DRLGMAZE_PickRoomPreset` resolves a mask by reading the level type's
 * `LvlPrest` row at `base + mask`, where `base` is a hardcoded per-type preset
 * id. This port has no preset ids, so it resolves by *kind and mask* instead —
 * the same thing expressed in the data the caller supplies.
 *
 * A level type may genuinely lack a piece for a side pattern (Act 1's Catacombs
 * ships no four-sided room). D2MOO would read whatever row happens to sit at
 * `base + mask` — a neighbouring level type's artwork — and produce a map whose
 * doors do not line up. Here the closest piece is substituted and the mismatch is
 * recorded in `stats.fallbacks`, because a map that cannot be walked is worse for
 * this project than a map with one wrong room. This is a deliberate divergence.
 *
 * @param pieces - the level type's pieces.
 * @returns the index.
 */
function buildPieceIndex(pieces: readonly MazePiece[]): PieceIndex {
  const rooms = new Map<number, MazePiece>()
  let entrance: MazePiece | undefined
  for (const piece of pieces) {
    if (piece.kind === 'entrance' && entrance === undefined) entrance = piece
    if (piece.kind !== 'room') continue
    const mask = sideMaskFromToken(piece.sides)
    if (!rooms.has(mask)) rooms.set(mask, piece)
  }
  for (const piece of pieces) {
    if (piece.kind !== 'prev') continue
    const mask = sideMaskFromToken(piece.sides)
    if (mask !== 0 && !rooms.has(mask)) rooms.set(mask, piece)
  }
  const entrancePiece = entrance

  /**
   * Pick the closest available room piece to a mask.
   *
   * @param mask - the wanted mask.
   * @returns the piece and whether it is exact.
   */
  const closest = (mask: number): { piece: MazePiece; exact: boolean } | null => {
    const exact = rooms.get(mask)
    if (exact !== undefined) return { piece: exact, exact: true }
    let best: MazePiece | null = null
    let bestCost = Number.POSITIVE_INFINITY
    for (const [candidateMask, piece] of rooms) {
      // A missing doorway seals the room off completely; an extra one merely
      // adds a dead end. Weight the two accordingly.
      const cost = popcount(mask & ~candidateMask) * 100 + popcount(candidateMask & ~mask)
      if (cost < bestCost) { bestCost = cost; best = piece }
    }
    return best === null ? null : { piece: best, exact: false }
  }

  return {
    choose: (mask: number, stats: MazeStats, roomId: number): MazePiece => {
      const result = closest(mask)
      if (result === null) {
        if (entrancePiece !== undefined) return entrancePiece
        const any = pieces[0]
        if (any === undefined) throw new Error('maze piece list is empty')
        return any
      }
      if (!result.exact) {
        stats.fallbacks.push({
          room: roomId,
          requested: sideTokenFromMask(mask),
          used: `${result.piece.name} (${sideTokenFromMask(sideMaskFromToken(result.piece.sides)) || 'no side'})`,
        })
      }
      return result.piece
    },
  }
}

/**
 * Look up a piece by kind and side token.
 *
 * @param pieces - the level's pieces.
 * @param kind - the kind wanted.
 * @param sides - the side token; `''` accepts any piece of that kind.
 * @returns the piece, or undefined.
 */
export function findSpecial(pieces: readonly MazePiece[], kind: MazePieceKind, sides: string): MazePiece | undefined {
  return pieces.find(piece => piece.kind === kind
    && (sides === '' || sideMaskFromToken(piece.sides) === sideMaskFromToken(sides)))
}

/**
 * Give a room the piece its doorways call for.
 *
 * `DRLGMAZE_PickRoomPreset`. Rooms whose artwork is already final (`hasDs1`) are
 * left alone, mirroring the passes that call it with `bResetFlag = FALSE`; every
 * other call clears the flag, which is why the flag cannot be used as a "was
 * visited" marker.
 *
 * @param room - the room to give a piece.
 * @param index - the piece lookup.
 * @param stats - the report collector.
 */
function pickRoomPreset(room: Room, index: PieceIndex, stats: MazeStats): void {
  if (room.hasDs1) return
  const mask = roomSideMask(room)
  const chosen = index.choose(mask, stats, room.id)
  room.piece = chosen
  room.variant = 0
  room.hasDs1 = false
  const token = sideTokenFromMask(mask) === '' ? 'no side' : sideTokenFromMask(mask)
  stats.patterns[chosen.kind] ??= {}
  const bucket = stats.patterns[chosen.kind]!
  bucket[token] = (bucket[token] ?? 0) + 1
}

/**
 * Record that a special piece was placed.
 *
 * @param room - the room it went into.
 * @param piece - the piece.
 * @param token - the side token it answers.
 * @param stats - report collector.
 */
export function noteSpecial(room: Room, piece: MazePiece, token: string, stats: MazeStats): void {
  stats.specialsApplied.push({ room: room.id, kind: piece.kind, sides: token })
  stats.patterns[piece.kind] ??= {}
  stats.patterns[piece.kind]![token] = (stats.patterns[piece.kind]![token] ?? 0) + 1
}

/* ------------------------------------------------------------------------- *
 * Layout passes
 * ------------------------------------------------------------------------- */

/**
 * Pre-place a closed ring of `4n - 4` rooms.
 *
 * `DRLGMAZE_InitBasicMazeLayout`. Four legs walk north, west, south and east, and
 * the last room is then linked back to the first — so `n = 2` gives a 2×2 square
 * and `n = 5` (Act 3's Sewer 1) a 4×4 ring of 16 rooms. The east leg runs one
 * iteration short because the first room already occupies that corner. Each leg
 * calls `DRLGMAZE_MergeMazeRooms` just like the growth loop does, so the
 * accidental contacts a ring makes are subject to `Merge` too.
 *
 * @param level - the level.
 * @param roomsPerDirection - `n`; `4n - 4` rooms are created.
 * @param index - the piece index.
 * @param stats - report collector.
 */
function ringLayout(level: MazeLevel, roomsPerDirection: number, index: PieceIndex, stats: MazeStats): void {
  const legs: readonly number[] = [DIRECTION_NORTH, DIRECTION_WEST, DIRECTION_SOUTH, DIRECTION_EAST]
  let cursor = level.first
  for (const direction of legs) {
    const count = direction === DIRECTION_EAST ? roomsPerDirection - 2 : roomsPerDirection - 1
    for (let remaining = count; remaining > 0; remaining -= 1) {
      const room = addAdjacentMazeRoom(level, cursor, direction, true, true, index, stats)
      if (room !== null) stats.ringRooms += 1
      cursor = room ?? cursor
    }
  }
  linkRooms(cursor, level.first, DIRECTION_EAST)
  pickRoomPreset(cursor, index, stats)
  pickRoomPreset(level.first, index, stats)
}

/**
 * Grow the level to its target room count.
 *
 * The shared body of `DRLGMAZE_BuildBasicMaze` and of the
 * `while (pLevel->nRooms < nRooms)` loop `DRLGMAZE_GenerateLevel` inlines for the
 * Act 1 dungeons: pick a room from the roster at a uniformly random index (the
 * roster is newest-first, so recent rooms are favoured exactly as in the game),
 * roll a direction from *that room's* seed, and try to attach a neighbour.
 * Failure is routine — the target square may already be occupied — so the loop
 * simply tries again. That also means it can only be bounded by a retry cap
 * rather than by construction, and D2MOO would spin forever where this throws.
 *
 * @param level - the level.
 * @param target - rooms wanted.
 * @param index - the piece index.
 * @param stats - report collector.
 * @returns the number of attempts consumed.
 */
function growMaze(level: MazeLevel, target: number, index: PieceIndex, stats: MazeStats): number {
  const cap = target * 400 + 20_000
  let attempts = 0
  while (level.rooms.length < target) {
    attempts += 1
    if (attempts > cap) {
      throw new Error(`layout stalled at ${String(level.rooms.length)}/${String(target)} rooms after ${String(attempts)} attempts`)
    }
    const room = level.rooms[level.rng.int(0, level.rooms.length - 1)]!
    const direction = room.rng.int(0, 3)
    if (room.hasDs1) continue
    addAdjacentMazeRoom(level, room, direction, true, true, index, stats)
  }
  return attempts
}

/**
 * The opening moves Act 1's Catacombs get before growth.
 *
 * `DRLGMAZE_GenerateLevel`'s `LVLTYPE_ACT1_CATACOMBS` arm. Level 1 opens in all
 * four directions and pins the entrance to the `NSEW` staircase piece; the rest
 * pick an axis at random and open both ways, pinning `EW` or `NS`. These are the
 * only places the entrance room is finalised with `bResetFlag = FALSE`, i.e. the
 * only rooms `DRLGMAZE_MergeMazeRooms` will later refuse to touch.
 *
 * @param level - the level.
 * @param horizontal - whether the non-first levels open west/east rather than
 *   north/south.
 * @param pieces - the level's pieces.
 * @param index - the piece index.
 * @param stats - report collector.
 */
function catacombsOpening(level: MazeLevel, horizontal: boolean, pieces: readonly MazePiece[], index: PieceIndex, stats: MazeStats): void {
  const directions = horizontal ? [DIRECTION_WEST, DIRECTION_EAST] : [DIRECTION_NORTH, DIRECTION_SOUTH]
  for (const direction of directions) addAdjacentMazeRoom(level, level.first, direction, true, true, index, stats)
  const token = horizontal ? 'EW' : 'NS'
  const piece = findSpecial(pieces, 'prev', token)
  if (piece === undefined) {
    stats.unresolvedRoles.push(`Act 1 - Catacombs Prev ${token}`)
    return
  }
  level.first.piece = piece
  level.first.hasDs1 = true
  noteSpecial(level.first, piece, token, stats)
}

/**
 * The three-room prologue Act 2's tombs and Act 5's Baal levels share.
 *
 * `DRLGMAZE_PlaceAct2TombPrev_Act5BaalPrev`: three rooms are chained off the
 * entrance, one per direction starting from a random one, each merging as it
 * lands; the entrance then takes the `Prev` piece whose three free sides face the
 * rooms that actually got built. That piece is chosen with the direction counter
 * *after* the loop, which is why it is the successor of the last direction tried
 * rather than the last direction used.
 *
 * @param level - the level.
 * @param kind - which act's `Prev` set to use.
 * @param pieces - the level's pieces.
 * @param index - the piece index.
 * @param stats - report collector.
 */
function prevChain(level: MazeLevel, kind: 'tomb' | 'baal', pieces: readonly MazePiece[], index: PieceIndex, stats: MazeStats): void {
  let direction = level.rng.int(0, 3)
  for (let attempt = 0; attempt < 3; attempt += 1) {
    const room = makeRoom(level)
    if (linkMazeRooms(level, room, level.first, direction)) {
      linkRooms(level.first, room, direction)
      mergeMazeRooms(level, room, index, stats)
      level.rooms.unshift(room)
      pickRoomPreset(level.first, index, stats)
      pickRoomPreset(room, index, stats)
    }
    direction = (direction + 1) % 4
  }
  const tokens = ['NSE', 'SEW', 'NSW', 'NEW']
  const token = tokens[direction]!
  const piece = findSpecial(pieces, 'prev', token)
  if (piece === undefined) {
    stats.unresolvedRoles.push(`${kind === 'tomb' ? 'Act 2 - Tomb' : 'Act 5 - Baal'} Prev ${token}`)
    return
  }
  level.first.piece = piece
  level.first.hasDs1 = true
  noteSpecial(level.first, piece, token, stats)
}

/**
 * `DRLGMAZE_PlaceArcaneSanctuary`: four 15-room spiral branches.
 *
 * Each branch starts at the level's first room and walks the pattern D2MOO
 * documents in a comment, encoded in
 * `DRLGMAZE_ArcaneSanctuaryDirectionFromRoomIdx`: one room forward, then a side
 * step, then a long run, then a side step back. Rooms 8 and 12 are placed but
 * deliberately *not* recorded and do not advance the parent, which D2MOO flags
 * with "Is this a mistake?" and then reproduces anyway; so does this port, since
 * the object is fidelity to the shipped maze rather than to the diagram.
 *
 * The four branches then take four different variants of the same pieces,
 * `(nRand + branch) % 4`, which is what gives the Sanctuary its noticeably varied
 * arms; the entry room takes variant 4.
 *
 * @param level - the level.
 * @param pieces - the level's pieces.
 * @param index - the piece index.
 * @param stats - report collector.
 */
function arcaneSanctuary(level: MazeLevel, pieces: readonly MazePiece[], index: PieceIndex, stats: MazeStats): void {
  const rand = level.rng.int(0, 3)
  const branches: Room[] = []
  for (let branch = 0; branch < 4; branch += 1) {
    let parent = level.first
    for (let slot = 0; slot < 15; slot += 1) {
      let direction = branch
      if (slot === 2 || slot === 12) direction = branch + 3
      else if (slot === 7 || slot === 9) direction = branch + 1
      else if (slot === 10 || slot === 11 || slot === 13 || slot === 14) direction = branch + 2
      direction %= 4
      const room = addAdjacentMazeRoom(level, parent, direction, true, true, index, stats)
      if (room === null) continue
      if (slot !== 8 && slot !== 12) {
        branches.push(room)
        parent = room
      }
    }
  }
  branches.forEach((room, position) => {
    room.variant = (rand + Math.floor(position / 15)) % 4
  })
  level.first.variant = 4
  void pieces
}

/**
 * `DRLGMAZE_PlaceAct5LavaPresets`: the Infernal Pit's two fixed pieces.
 *
 * The level is not a maze at all. It is an entrance plus exactly two lava pieces,
 * chosen as a pair by a single random draw, after which every still-empty
 * adjacent square is filled with the shared `Act 4 - Lava X` filler — see
 * {@link fillBlankSpaces}.
 *
 * @param level - the level.
 * @param pieces - the level's pieces.
 * @param index - the piece index.
 * @param stats - report collector.
 */
function lavaPairs(level: MazeLevel, pieces: readonly MazePiece[], index: PieceIndex, stats: MazeStats): void {
  // `dword_6FDCE850`, transcribed as (piece name, direction, variant).
  const table: readonly (readonly [string, number, number])[] = [
    ['Act 5 - Lava S', DIRECTION_SOUTH, 0],
    ['Act 5 - Lava N', DIRECTION_NORTH, 1],
    ['Act 5 - Lava S', DIRECTION_SOUTH, 1],
    ['Act 5 - Lava N', DIRECTION_NORTH, 0],
    ['Act 5 - Lava E', DIRECTION_EAST, 1],
    ['Act 5 - Lava W', DIRECTION_WEST, 0],
    ['Act 5 - Lava E', DIRECTION_EAST, 0],
    ['Act 5 - Lava W', DIRECTION_WEST, 1],
  ]
  const set = 2 * level.first.rng.int(0, 3)
  for (const [name, direction, variant] of [table[set]!, table[set + 1]!]) {
    const room = makeRoom(level)
    room.variant = variant
    // The table's direction is the direction *from the new room to the entrance*,
    // so the new room sits on the opposite side.
    const { dx, dy } = step(direction, level.first.width, level.first.height)
    room.x = level.first.x + dx
    room.y = level.first.y + dy
    if (!checkRoomNotOverlapping(level, room, level.first)) continue
    linkRooms(level.first, room, direction)
    level.rooms.unshift(room)
    pickRoomPreset(level.first, index, stats)
    const piece = pieces.find(candidate => candidate.name === name)
    if (piece === undefined) {
      stats.unresolvedRoles.push(name)
      pickRoomPreset(room, index, stats)
      continue
    }
    room.piece = piece
    room.hasDs1 = true
    noteSpecial(room, piece, 'no side', stats)
  }
  fillBlankSpaces(level, pieces, index, stats)
}

/**
 * `DRLGMAZE_FillBlankMazeSpaces`: wall a fixed-tile level in with filler rooms.
 *
 * The roster is snapshotted first, then every room in it tries all eight compass
 * directions in order and keeps whatever fits. The filler never takes a role, so
 * this only ever adds area.
 *
 * D2MOO does not re-derive the parent's piece here, which would leave a wall
 * where the filler attaches and make the filler unreachable; this port does
 * re-derive it, because an unreachable room is a defect rather than a detail.
 *
 * @param level - the level.
 * @param pieces - the level's pieces.
 * @param index - the piece index.
 * @param stats - report collector.
 */
export function fillBlankSpaces(
  level: MazeLevel,
  pieces: readonly MazePiece[],
  index: PieceIndex,
  stats: MazeStats,
  ignoreRoom?: Room | null,
): void {
  const filler = pieces.find(piece => piece.name === 'Act 4 - Lava X')
  if (filler === undefined) {
    stats.unresolvedRoles.push('Act 4 - Lava X')
    return
  }
  for (const room of [...level.rooms]) {
    if (ignoreRoom !== null && ignoreRoom !== undefined && room === ignoreRoom) continue
    for (let direction = 0; direction < 8; direction += 1) {
      const created = addAdjacentMazeRoom(level, room, direction, false, true, index, stats)
      if (created === null) continue
      created.piece = filler
      created.hasDs1 = true
      noteSpecial(created, filler, 'filler', stats)
    }
  }
}

/**
 * `DRLGMAZE_RollAct_1_2_3_BasicPresets`: the `Theme` decoration pass.
 *
 * A shuffled list of the fifteen side patterns is walked cyclically and the first
 * plain room matching each pattern is converted to that pattern's `Theme`
 * variant, until `max(rooms / 5 + 1, 2)` rooms have been converted or the attempt
 * budget of `2 × rooms` is spent. D2MOO shuffles the offset list with fifteen
 * Fisher-Yates swaps drawn from the level seed, which is reproduced here.
 *
 * A level type with no `Theme` pieces for a pattern is skipped rather than given
 * a neighbouring type's artwork, and the shortfall is reported.
 *
 * @param level - the level.
 * @param pieces - the level's pieces.
 * @param stats - report collector.
 */
function themeRoll(level: MazeLevel, pieces: readonly MazePiece[], stats: MazeStats): void {
  const offsets = Array.from({ length: 15 }, (_, index) => index)
  let cursor = level.rng.int(0, 14)
  for (let swap = 0; swap < 15; swap += 1) {
    const a = level.rng.int(0, 14)
    const b = level.rng.int(0, 14)
    const temporary = offsets[a]!
    offsets[a] = offsets[b]!
    offsets[b] = temporary
  }
  let wanted = Math.max(Math.floor(level.rooms.length / 5) + 1, 2)
  let budget = 2 * level.rooms.length
  while (wanted > 0 && budget > 0) {
    budget -= 1
    const mask = offsets[cursor]! + 1
    cursor = (cursor + 1) % 15
    const token = sideTokenFromMask(mask)
    const room = findUnfinishedRoom(level, mask)
    if (room === null) continue
    const themed = findSpecial(pieces, 'theme', token)
    if (themed === undefined) {
      // D2MOO would write a neighbouring level type's row; here the room keeps
      // its plain piece and the gap is reported.
      if (!stats.unresolvedRoles.includes(`Theme ${token}`)) stats.unresolvedRoles.push(`Theme ${token}`)
      continue
    }
    room.piece = themed
    room.variant = 0
    room.hasDs1 = true
    noteSpecial(room, themed, token, stats)
    wanted -= 1
  }
}

/**
 * Compute BFS topological distance of every room from the level's entrance room
 * (the room holding a 'prev' or 'entrance' piece, or `level.first`).
 */
export function bfsDistances(level: MazeLevel): Map<Room, number> {
  const start = level.rooms.find(r => r.piece?.kind === 'prev' || r.piece?.kind === 'entrance') ?? level.first
  const dist = new Map<Room, number>([[start, 0]])
  const queue: Room[] = [start]
  while (queue.length > 0) {
    const current = queue.shift()!
    const d = dist.get(current)!
    for (const orth of current.orths) {
      if (!dist.has(orth.room)) {
        dist.set(orth.room, d + 1)
        queue.push(orth.room)
      }
    }
  }
  return dist
}

/**
 * Place a special room group following standard D2 scanning rules:
 * scans room candidates against valid wall connections, LvlPrest file definitions,
 * and room flags, rather than imposing an artificial global maximum BFS distance.
 *
 * @param level - the level.
 * @param pieces - the level's pieces.
 * @param group - which special group to place.
 * @param startCursor - initial table offset.
 * @param index - the piece index.
 * @param stats - report collector.
 */
function placeSpecialGroup(
  level: MazeLevel,
  pieces: readonly MazePiece[],
  group: SpecialGroup,
  startCursor: number,
  index: PieceIndex,
  stats: MazeStats,
): void {
  // Phase 1: Try to replace an existing unfinished room in place, scanning
  // room candidates against valid wall connections, LvlPrest definitions,
  // and room flags in standard D2 roster order.
  for (let offset = 0; offset < group.entries.length; offset += 1) {
    const entry = group.entries[(startCursor + offset) % group.entries.length]!
    const wanted = sideMaskFromToken(entry.plainSides)
    const token = sideTokenFromMask(wanted)
    const special = findSpecial(pieces, group.kind, entry.sides)
    if (special === undefined) continue

    const chosen = level.rooms.find(r => {
      if (roomSideMask(r) !== wanted) return false
      if (!r.hasDs1) return true
      if (group.inPlaceOnly === true && (r.piece === special || r.piece?.kind === 'theme')) return true
      return false
    })

    if (chosen !== undefined) {
      chosen.piece = special
      chosen.variant = 0
      chosen.hasDs1 = true
      noteSpecial(chosen, special, token, stats)
      return
    }
  }

  if (group.inPlaceOnly === true) {
    const firstEntry = group.entries[startCursor]!
    stats.notes.push(`no ${firstEntry.plainSides} room left for a ${group.kind} ${firstEntry.sides} replacement`)
    return
  }

  // Phase 2: Grow a new dead-end room off an available parent room in D2
  // roster order (plain rooms without DS1 scanned first, followed by theme rooms).
  const parentCandidates = [
    ...level.rooms.filter(r => !r.hasDs1),
    ...level.rooms.filter(r => r.hasDs1 && r.piece?.kind === 'theme'),
  ]

  for (const parent of parentCandidates) {
    for (let offset = 0; offset < group.entries.length; offset += 1) {
      const entry = group.entries[(startCursor + offset) % group.entries.length]!
      const wanted = sideMaskFromToken(entry.plainSides)
      const token = sideTokenFromMask(wanted)
      const special = findSpecial(pieces, group.kind, entry.sides)
      if (special === undefined) continue

      const wasHasDs1 = parent.hasDs1
      parent.hasDs1 = false
      const created = addAdjacentMazeRoom(level, parent, entry.direction, false, true, index, stats)
      if (created !== null) {
        created.piece = special
        created.variant = 0
        created.hasDs1 = true
        noteSpecial(created, special, token, stats)
        return
      }
      parent.hasDs1 = wasHasDs1
    }
  }

  const fallbackEntry = group.entries[startCursor]!
  stats.notes.push(`could not place ${group.kind} ${fallbackEntry.sides}: no room could host it`)
}

/* ------------------------------------------------------------------------- *
 * Stamping
 * ------------------------------------------------------------------------- */

/** A blank wall cell, matching the decoder's placeholder. */
function emptyWall(): Ds1Wall {
  return { prop1: 0, sequence: 0, style: 0, type: 0, unknown1: 0, unknown2: 0, hidden: false }
}

/** A blank floor/shadow cell. */
function emptyFloor(): Ds1Floor {
  return { prop1: 0, sequence: 0, style: 0, unknown1: 0, unknown2: 0, hidden: false }
}

/* ------------------------------------------------------------------------- *
 * Edge suppression and doorway thresholds (1.13c `DrlgRoomTile.cpp`)
 * ------------------------------------------------------------------------- */

/** `TILETYPE_WALL_LEFT_DOOR` — a wall cell that a door unit occupies. */
const TILETYPE_WALL_LEFT_DOOR = 8
/** `TILETYPE_WALL_RIGHT_DOOR` — the mirrored door wall. */
const TILETYPE_WALL_RIGHT_DOOR = 9
/** `TILETYPE_WALL_LEFT_EXIT` — a warp marker, not artwork. */
const TILETYPE_WALL_LEFT_EXIT = 10
/** `TILETYPE_WALL_RIGHT_EXIT` — the mirrored warp marker. */
const TILETYPE_WALL_RIGHT_EXIT = 11

/**
 * Highest `style` an exit tile may carry before 1.13c ignores it outright.
 *
 * `DRLGROOMTILE_LoadInitRoomTiles` opens with
 * `if ((nTileType == TILETYPE_WALL_LEFT_EXIT || nTileType == TILETYPE_WALL_RIGHT_EXIT)
 * && nTileStyle >= 8) continue;` — the style is the `Levels.txt` `Warp0..7` slot,
 * so anything at 8 or above is not a warp and the cell is dropped.
 */
const MAX_EXIT_WARP_SLOT = 7

/**
 * The 2×2 offsets `DRLGROOMTILE_LoadFloorWarpTiles` writes its threshold under.
 *
 * Verbatim `gWarpTileOffsets_6FDD1320`; each is applied to `(nX - 1, nY - 1)`, so
 * the patch covers the doorway cell and the three cells up-left of it.
 */
const WARP_TILE_OFFSETS: readonly (readonly [number, number])[] = [
  [0, 0], [1, 0], [0, 1], [1, 1],
]

/**
 * Decide whether a wall cell survives 1.13c's load pass.
 *
 * Three rules from `DRLGROOMTILE_LoadInitRoomTiles`, in the order it applies
 * them:
 *
 * 1. An exit tile whose `style` is above {@link MAX_EXIT_WARP_SLOT} is skipped
 *    before anything else looks at it (line 466).
 * 2. A *hidden* door tile spawns a door unit and `continue`s, so its wall
 *    artwork is never emitted (line 485).
 * 3. A *hidden* exit tile registers a warp and `continue`s likewise (line 500).
 *
 * Everything else is ordinary artwork and is kept.
 *
 * @param wall - the wall cell as the piece authored it.
 * @returns whether the wall should be drawn.
 */
function wallSurvivesLoad(wall: Ds1Wall): boolean {
  const isExit = wall.type === TILETYPE_WALL_LEFT_EXIT || wall.type === TILETYPE_WALL_RIGHT_EXIT
  if (isExit && wall.style > MAX_EXIT_WARP_SLOT) return false
  if (!wall.hidden) return true
  const isDoor = wall.type === TILETYPE_WALL_LEFT_DOOR || wall.type === TILETYPE_WALL_RIGHT_DOOR
  return !(isDoor || isExit)
}

/**
 * Whether a dropped wall is the kind that gets a threshold patch.
 *
 * 1.13c calls `DRLGROOMTILE_LoadFloorWarpTiles` from exactly one place
 * (`DrlgRoomTile.cpp:505`): a *hidden exit* tile whose style is a real
 * `Warp0..7` slot. A hidden *door* takes the branch above it and only spawns a
 * door unit, and an out-of-range exit is skipped before either runs — neither
 * lays down floor, so neither may widen the patch here.
 *
 * @param wall - the wall that `wallSurvivesLoad` rejected.
 * @returns whether its cell needs the 2×2 floor patch.
 */
function needsThresholdBackfill(wall: Ds1Wall): boolean {
  const isExit = wall.type === TILETYPE_WALL_LEFT_EXIT || wall.type === TILETYPE_WALL_RIGHT_EXIT
  return isExit && wall.hidden && wall.style <= MAX_EXIT_WARP_SLOT
}

/**
 * Whether a floor cell actually draws something.
 *
 * A slot left at style 0 / sequence 0 is the decoder's placeholder, which is the
 * hole the threshold backfill exists to cover.
 *
 * @param floor - the floor cell.
 * @returns whether it references a tile.
 */
function floorIsBlank(floor: Ds1Floor | undefined): boolean {
  return floor === undefined || (floor.style === 0 && floor.sequence === 0 && floor.prop1 === 0)
}

/**
 * Backfill the threshold floor under a doorway, the way 1.13c does.
 *
 * `DRLGROOMTILE_LoadFloorWarpTiles` reserves six floor slots per hidden exit
 * (`DRLGROOMTILE_CountWallWarpTiles` does `pTiles.nFloors += 6`) and fills a 2×2
 * block anchored one cell up-left of the doorway, flagging each tile hidden so
 * it renders under the warp artwork rather than over it. Without it the cells a
 * doorway punches through the wall line keep the piece's blank floor slot and
 * the ground shows a hole at the threshold.
 *
 * The donor tile is the nearest non-blank floor around the doorway, which is the
 * practical stand-in for `DRLGROOMTILE_GetTileCache(TILETYPE_FLOOR, ...)`: the
 * room's own floor style is exactly what the tile cache would have returned.
 *
 * @param cells - the composite grid, mutated in place.
 * @param doorX - the doorway cell's x in the composite grid.
 * @param doorY - the doorway cell's y in the composite grid.
 * @returns how many blank cells were filled.
 */
function backfillThresholdFloor(cells: Ds1Cell[][], doorX: number, doorY: number): number {
  const donor = findDonorFloor(cells, doorX, doorY)
  if (donor === null) return 0
  let filled = 0
  for (const [dx, dy] of WARP_TILE_OFFSETS) {
    const x = doorX - 1 + dx
    const y = doorY - 1 + dy
    const row = cells[y]
    if (row === undefined) continue
    const cell = row[x]
    if (cell === undefined) continue
    if (!floorIsBlank(cell.floors[0])) continue
    const floors = [...cell.floors]
    // 1.13c marks these `MAPTILE_HIDDEN` so they sit beneath the warp artwork.
    floors[0] = { ...donor, hidden: true }
    row[x] = { ...cell, floors }
    filled += 1
  }
  return filled
}

/**
 * Find a floor tile to copy into a doorway threshold.
 *
 * Searches the doorway cell first and then rings outwards, so the patch always
 * picks up the floor of the room the doorway belongs to rather than a default.
 *
 * @param cells - the composite grid.
 * @param atX - the doorway cell's x.
 * @param atY - the doorway cell's y.
 * @returns the donor floor, or `null` when nothing nearby draws a floor.
 */
function findDonorFloor(cells: readonly (readonly Ds1Cell[])[], atX: number, atY: number): Ds1Floor | null {
  for (let radius = 0; radius <= 2; radius += 1) {
    for (let dy = -radius; dy <= radius; dy += 1) {
      for (let dx = -radius; dx <= radius; dx += 1) {
        if (Math.max(Math.abs(dx), Math.abs(dy)) !== radius) continue
        const floor = cells[atY + dy]?.[atX + dx]?.floors[0]
        if (!floorIsBlank(floor)) return floor!
      }
    }
  }
  return null
}

/**
 * Choose each room's variant the way `DRLGMAZE_RollBasicPresets` does.
 *
 * For the plain side pieces D2MOO keeps a per-preset counter initialised to a
 * random file index and then increments it modulo the file count, so two
 * consecutive rooms of the same shape never get the same artwork. Specials and
 * themed pieces keep whatever index the earlier pass assigned — for the Arcane
 * Sanctuary that is the branch number, and for the Infernal Pit an explicit 0/1.
 *
 * @param level - the level.
 * @param levelId - `Levels.txt` ID.
 * @param stats - report collector.
 */
function assignVariants(level: MazeLevel, levelId: number, stats: MazeStats): void {
  const counters = new Map<string, number>()
  const usage = new Map<string, number>()
  // D2MOO walks the roster from its head, i.e. newest room first.
  for (const room of level.rooms) {
    const piece = room.piece
    if (piece === null) continue
    usage.set(piece.name, (usage.get(piece.name) ?? 0) + 1)
    if (piece.kind !== 'room' || piece.levels.length <= 1) continue
    // On Palace Cellar Levels 1 & 2 (52, 53), SE and NW variants 0..2 carry the
    // down-staircase warp tiles while variant 3 (CelSE3/CelNW3) is reserved for
    // Level 54 (Palace Cellar Level 3) which has no descent to a fourth cellar.
    const count = piece.name.startsWith('Act 2 - Basement') && levelId !== 54
      ? Math.min(piece.levels.length, 3)
      : piece.levels.length
    let counter = counters.get(piece.name)
    if (counter === undefined) counter = level.rng.int(0, count - 1)
    counters.set(piece.name, (counter + 1) % count)
    room.variant = counter
  }
  for (const [name, count] of usage) stats.notes.push(`piece "${name}" used ${String(count)} time(s)`)
}

/**
 * Flatten the room list into one synthesized map.
 *
 * The size is the union of the stamped piece extents — `(span of the room
 * origins) + (piece size)`. Because a piece is one cell larger than the stride,
 * consecutive rooms overlap by exactly one cell and the piece stamped later owns
 * that border, reproducing `DRLGPRESET_InitPresetRoomGrids`, which fills
 * `nTileWidth + 1` cells per room. D2MOO stamps in roster order (newest first),
 * so the level's *first* room is stamped last and wins every overlap it takes
 * part in; the same order is used here.
 *
 * Objects keep their per-piece sub-tile coordinates and gain the room origin
 * converted from cells at 5 sub-tiles per cell.
 *
 * Two 1.13c load-time behaviours are reproduced here rather than at draw time,
 * because this is the only pass that sees both the piece and its place:
 *
 * - **`bKillEdge`** (`DrlgPreset.cpp:1105`, `DrlgRoomTile.cpp:449`). When the
 *   level type's `LvlPrest.KillEdge` is set, the room flush against the map's
 *   outer edge stamps `nTileWidth` cells instead of `nTileWidth + 1`, so the
 *   shared-border overhang that has no neighbour to be overwritten by is
 *   dropped instead of duplicating the wall line one cell past the map.
 * - **Hidden door and exit walls** (`DrlgRoomTile.cpp:466`, `479`). A hidden
 *   door or exit tile is a marker, not artwork: 1.13c spawns a door unit or
 *   registers a warp and `continue`s, then backfills the threshold with the 2×2
 *   floor patch `DRLGROOMTILE_LoadFloorWarpTiles` writes. Emitting the wall
 *   instead leaves the doorway blocked and the threshold floor blank.
 *
 * @param level - the level.
 * @param stats - report collector.
 * @param killEdge - the level type's `LvlPrest.KillEdge`.
 * @returns the synthesized map.
 */
function stamp(level: MazeLevel, stats: MazeStats, killEdge: boolean): Ds1 {
  const minX = Math.min(...level.rooms.map(room => room.x))
  const minY = Math.min(...level.rooms.map(room => room.y))
  const maxX = Math.max(...level.rooms.map(room => room.x))
  const maxY = Math.max(...level.rooms.map(room => room.y))

  let wallLayers = 1
  let floorLayers = 1
  let substitutionType = 0
  let version = 0
  let act = 1
  let widest = 1
  let highest = 1
  for (const room of level.rooms) {
    const piece = room.piece
    if (piece === null) continue
    const variant = piece.levels[room.variant % piece.levels.length]
    if (variant === undefined) throw new Error(`piece "${piece.name}" has no decoded map to stamp`)
    wallLayers = Math.max(wallLayers, variant.wallLayers)
    floorLayers = Math.max(floorLayers, variant.floorLayers)
    substitutionType = Math.max(substitutionType, variant.substitutionType)
    version = Math.max(version, variant.version)
    act = variant.act
    widest = Math.max(widest, variant.width)
    highest = Math.max(highest, variant.height)
  }

  // `KillEdge` shortens the map by the overhang column and row that the
  // right-most and bottom-most rooms would otherwise contribute. A piece is one
  // cell wider than its section so that neighbours share a border; at the outer
  // edge there is no neighbour to share with, so 1.13c drops it rather than
  // letting the piece spill past `pDrlgCoord.nWidth`
  // (`DrlgPreset.cpp:1109`, `DrlgRoomTile.cpp:449`).
  const edgeTrim = killEdge ? 1 : 0
  const width = (maxX - minX) + Math.max(1, widest - edgeTrim)
  const height = (maxY - minY) + Math.max(1, highest - edgeTrim)
  if (width <= 0 || height <= 0 || width > MAX_CELLS_PER_SIDE || height > MAX_CELLS_PER_SIDE
    || width * height > MAX_CELLS) {
    throw new Error(`synthesized map is ${String(width)}x${String(height)} cells, outside the supported bound`)
  }

  const substitutionLayers = substitutionType === 1 || substitutionType === 2 ? 1 : 0
  const cells: Ds1Cell[][] = []
  for (let y = 0; y < height; y += 1) {
    const row: Ds1Cell[] = []
    for (let x = 0; x < width; x += 1) {
      row.push({
        walls: Array.from({ length: wallLayers }, emptyWall),
        floors: Array.from({ length: floorLayers }, emptyFloor),
        shadows: [emptyFloor()],
        substitutions: Array.from({ length: substitutionLayers }, () => ({ value: 0 })),
      })
    }
    cells.push(row)
  }

  const objects: Ds1Object[] = []
  // Doorway cells discovered while blitting; their thresholds are backfilled
  // after every room is down, so a neighbour's floor can serve as the donor.
  const thresholds: { x: number; y: number }[] = []
  let stampedRooms = 0
  let killedEdgeX = 0
  let killedEdgeY = 0
  let hiddenWalls = 0
  for (const room of level.rooms) {
    const piece = room.piece
    if (piece === null) continue
    const variant = piece.levels[room.variant % piece.levels.length]!
    const originX = room.x - minX
    const originY = room.y - minY

    // `bKillEdgeX = pDrlgRoom->nTileXPos + pDrlgRoom->nTileWidth == pDrlgCoord.nPosX + nWidth`
    // — true only for the room flush against the map's outer edge, which is the
    // one whose overhang has no neighbour to be overwritten by.
    const killEdgeX = killEdge && originX + room.width >= width
    const killEdgeY = killEdge && originY + room.height >= height
    if (killEdgeX) killedEdgeX += 1
    if (killEdgeY) killedEdgeY += 1

    // `nTileCountX = pDrlgRoom->nTileWidth + (bKillEdgeX == 0)`. The clamp keeps
    // an oddly-sized piece from running off the grid rather than trusting that
    // every piece is exactly one cell wider than its section.
    const spanX = Math.min(variant.width - (killEdgeX ? 1 : 0), width - originX)
    const spanY = Math.min(variant.height - (killEdgeY ? 1 : 0), height - originY)
    if (originX + spanX > width || originY + spanY > height) {
      throw new Error(`piece "${piece.name}" overflows the ${String(width)}x${String(height)} map`)
    }
    for (let y = 0; y < spanY; y += 1) {
      const source = variant.cells[y]
      if (source === undefined) continue
      const target = cells[originY + y]!
      for (let x = 0; x < spanX; x += 1) {
        const cell = source[x]
        if (cell === undefined) continue
        // 1.13c filters the wall list while loading rather than while drawing:
        // hidden door and exit tiles are markers that spawn a unit or register a
        // warp, and their artwork is never emitted.
        const kept = cell.walls.filter(wallSurvivesLoad)
        if (kept.length === cell.walls.length) {
          target[originX + x] = cell
        } else {
          hiddenWalls += cell.walls.length - kept.length
          while (kept.length < cell.walls.length) kept.push(emptyWall())
          target[originX + x] = { ...cell, walls: kept }
          if (cell.walls.some(needsThresholdBackfill)) {
            thresholds.push({ x: originX + x, y: originY + y })
          }
        }
      }
    }

    for (const object of variant.objects) {
      objects.push({
        type: object.type,
        id: object.id,
        x: originX * SUB_TILES_PER_TILE + object.x,
        y: originY * SUB_TILES_PER_TILE + object.y,
        flags: object.flags,
      })
    }
    // This is the only pass that knows both which piece a room got and where
    // that room ends up, so it is also the only place the stairs can be
    // located. `Down` is the Act 1 cave descent and `Next` the ordinary one;
    // they differ in artwork, not in meaning.
    if (piece.kind === 'prev' || piece.kind === 'next' || piece.kind === 'down') {
      stats.warps.push({
        room: room.id,
        direction: piece.kind === 'prev' ? 'up' : 'down',
        kind: piece.kind,
        x: originX,
        y: originY,
        width: variant.width,
        height: variant.height,
        centreX: originX + Math.floor(variant.width / 2),
        centreY: originY + Math.floor(variant.height / 2),
      })
    }
    if (piece.kind === 'waypoint') {
      const wpObj = variant.objects.find(obj => obj.type === 2 && (obj.id === 119 || obj.id === 145 || obj.id === 156 || obj.id === 157 || obj.id === 237 || obj.id === 238 || obj.id === 288 || obj.id === 323 || obj.id === 324 || obj.id === 398 || obj.id === 402 || obj.id === 429)) ?? variant.objects.find(obj => obj.type === 2)
      let entityX: number
      let entityY: number
      if (wpObj !== undefined) {
        entityX = originX * SUB_TILES_PER_TILE + wpObj.x
        entityY = originY * SUB_TILES_PER_TILE + wpObj.y
      } else {
        entityX = (originX + Math.floor(variant.width / 2)) * SUB_TILES_PER_TILE + 2
        entityY = (originY + Math.floor(variant.height / 2)) * SUB_TILES_PER_TILE + 2
        objects.push({
          type: 2,
          id: 119,
          x: entityX,
          y: entityY,
          flags: 0,
        })
      }
      stats.waypoints.push({
        room: room.id,
        x: originX,
        y: originY,
        width: variant.width,
        height: variant.height,
        centreX: originX + Math.floor(variant.width / 2),
        centreY: originY + Math.floor(variant.height / 2),
        entityX,
        entityY,
      })
    }
    stampedRooms += 1
  }

  // Thresholds are filled only after every room is down: a doorway sits on the
  // border between two rooms, so the donor floor may belong to the neighbour
  // that had not been stamped yet when the doorway was found.
  let thresholdCells = 0
  for (const at of thresholds) thresholdCells += backfillThresholdFloor(cells, at.x, at.y)

  stats.notes.push(`stamped ${String(stampedRooms)} of ${String(level.rooms.length)} rooms`)
  if (killEdge) {
    stats.notes.push(`killEdge trimmed ${String(killedEdgeX)} east and ${String(killedEdgeY)} south room edges`)
  }
  if (hiddenWalls > 0) {
    stats.notes.push(`hid ${String(hiddenWalls)} door/exit walls, backfilled ${String(thresholdCells)} threshold floor cells`)
  }

  return {
    version: version === 0 ? 18 : version,
    width,
    height,
    act,
    substitutionType,
    wallLayers,
    floorLayers,
    cells,
    objects,
    // The synthesized map is written into an asset pack rather than re-encoded,
    // so there is no NPC-path section to point at.
    npcPathOffset: null,
  }
}

/** Largest synthesized map side, in cells. Well above the widest shipped maze. */
const MAX_CELLS_PER_SIDE = 4096
/** Largest synthesized map area, matching the decoder's own sanity limit. */
const MAX_CELLS = 1 << 22

/* ------------------------------------------------------------------------- *
 * Connectivity
 * ------------------------------------------------------------------------- */

/**
 * Flood fill the doorway graph from the first room.
 *
 * The generator's correctness claim is "every placed room is reachable", so it is
 * verified rather than asserted: growth attaches each room to an existing one and
 * `Merge` only adds doorways, so this must always reach everything.
 *
 * @param level - the level.
 * @returns the number of rooms reached.
 */
function reachableRooms(level: MazeLevel): number {
  const seen = new Set<Room>([level.first])
  const queue: Room[] = [level.first]
  while (queue.length > 0) {
    const room = queue.pop()!
    for (const orth of room.orths) {
      if (seen.has(orth.room)) continue
      seen.add(orth.room)
      queue.push(orth.room)
    }
  }
  return seen.size
}

/* ------------------------------------------------------------------------- *
 * Entry point
 * ------------------------------------------------------------------------- */

/**
 * Generate one random-maze level.
 *
 * The pipeline is D2MOO's `DRLGMAZE_GenerateLevel`: an opening (ring and/or the
 * Catacombs arms and/or the tomb prologue), optional growth to `LvlMaze.Rooms`
 * sections, a preset for every room from its side pattern, the `Theme` pass, the
 * `Prev`/`Next`/quest replacement passes, and finally one synthesized map.
 *
 * @param request - the real table data for one `DrlgType == 1` level.
 * @returns the synthesized map plus a report of what was done.
 * @throws when the level cannot be built — an unknown level type, a placement
 *   stall, or rooms that ended up unreachable. The message always names the level,
 *   because an empty map silently baked into an asset pack is worse than a crash.
 */
export function generateMaze(request: MazeRequest): MazeResult {
  const width = request.sectionSize
  const height = request.sectionHeight ?? request.sectionSize
  const where = `level ${String(request.levelId)} (${request.levelName})`
  if (!Number.isFinite(width) || width <= 0) throw new Error(`${where}: LvlMaze.SizeX is ${String(width)}`)
  if (!Number.isFinite(height) || height <= 0) throw new Error(`${where}: LvlMaze.SizeY is ${String(height)}`)
  if (request.pieces.length === 0) throw new Error(`${where}: no usable LvlPrest pieces`)
  if (!Number.isFinite(request.minRooms) || request.minRooms < 1) {
    throw new Error(`${where}: LvlMaze.Rooms is ${String(request.minRooms)}`)
  }

  const levelTypeName = request.levelTypeName ?? inferLevelTypeName(request.pieces)
  if (levelTypeName === null) throw new Error(`${where}: cannot infer the level type from the piece names`)
  const profile = MAZE_LEVEL_TYPE_PROFILES.find(candidate => candidate.levelTypeName === levelTypeName)
  const animSpeed = request.pieces.find(p => p.animSpeed && p.animSpeed > 0)?.animSpeed ?? profile?.animSpeed
  if (profile === undefined) throw new Error(`${where}: no maze profile for level type "${levelTypeName}"`)

  const stats: MazeStats = {
    patterns: {}, fallbacks: [], specialsApplied: [], specialPassesApplied: [], unresolvedRoles: [], notes: [],
    ringRooms: 0, placementAttempts: 0, warps: [], waypoints: [],
  }
  const rng = new Rng(request.seed)
  const mergePerMille = Math.max(0, Math.min(1000, Math.floor(request.merge)))
  const index = buildPieceIndex(request.pieces)
  const maxCellsX = request.maxCellsX !== undefined
    ? request.maxCellsX
    : (request.maxSectionsX !== undefined ? request.maxSectionsX * width : undefined)
  const maxCellsY = request.maxCellsY !== undefined
    ? request.maxCellsY
    : (request.maxSectionsY !== undefined ? request.maxSectionsY * height : undefined)

  const level: MazeLevel = {
    rooms: [], first: null as unknown as Room, width, height, rng, mergePerMille, nextId: 0, merges: 0,
    maxCellsX, maxCellsY, stats,
  }
  const first = makeRoom(level)
  const levelRef: MazeLevel = { ...level, rooms: [first], first }
  // `first` was built against `level`, whose `rooms` array is the live one; copy
  // the identity over rather than duplicating state.
  Object.assign(level, levelRef)

  // `DRLGMAZE_GetRandomRoomExFromLevel` draws uniformly over the roster and the
  // level seed advances on every call, so the seed must be consumed in the same
  // order as the passes below.
  let target = request.minRooms
  if (request.staffTombLevelId === request.levelId) target *= 3
  else if (request.bossTombLevelId === request.levelId) target *= 2

  const singleNames = profile.singleRoom?.[request.levelId]
  if (singleNames !== undefined) {
    const piece = singleNames.map(name => request.pieces.find(candidate => candidate.name === name))
      .find(found => found !== undefined)
    if (piece === undefined) {
      for (const name of singleNames) stats.unresolvedRoles.push(name)
      stats.notes.push('single-room level: no named preset resolved, using the first piece')
      const fallback = request.pieces[0]!
      first.piece = fallback
      first.hasDs1 = true
      noteSpecial(first, fallback, 'single room', stats)
    } else {
      first.piece = piece
      first.hasDs1 = true
      noteSpecial(first, piece, 'single room', stats)
    }
  } else {
    if (profile.ring !== undefined) {
      const roomsPerDirection = levelTypeName === 'Act 3 - Sewer' && request.levelId === 92 ? 5 : profile.ring
      ringLayout(level, roomsPerDirection, index, stats)
    }
    if (profile.opening === 'catacombs') {
      // D2MOO branches on the level id before rolling, so level 1 consumes no
      // random number here.
      const horizontal = request.levelId === 34 ? false : rng.int(0, 1) === 1
      catacombsOpening(level, horizontal, request.pieces, index, stats)
    }
    if (profile.prevChain !== undefined) prevChain(level, profile.prevChain, request.pieces, index, stats)
    if (profile.arcane === true) arcaneSanctuary(level, request.pieces, index, stats)
    if (profile.lavaPairs === true) lavaPairs(level, request.pieces, index, stats)
    if (profile.grow) stats.placementAttempts = growMaze(level, target, index, stats)

    // Every room needs a piece before the replacement passes can look for the
    // plain ones to convert.
    for (const room of level.rooms) {
      if (room.piece === null) pickRoomPreset(room, index, stats)
    }

    if (profile.theme) themeRoll(level, request.pieces, stats)

    // One counter shared by every table, advanced once per call that happens.
    let cursor = rng.int(0, 3)
    for (const group of profile.specials) {
      if (group.kind === 'waypoint') {
        if (request.hasWaypoint === false) continue
        if (request.hasWaypoint !== true) {
          if (group.levelIds !== undefined && !group.levelIds.includes(request.levelId)) continue
          if (group.exceptLevelIds !== undefined && group.exceptLevelIds.includes(request.levelId)) continue
        }
      } else {
        if (group.levelIds !== undefined && !group.levelIds.includes(request.levelId)) continue
        if (group.exceptLevelIds !== undefined && group.exceptLevelIds.includes(request.levelId)) continue
      }
      placeSpecialGroup(level, request.pieces, group, cursor, index, stats)
      cursor = (cursor + 1) % 4
    }
  }

  if (profile.specialPass !== undefined) {
    runMazeSpecialPasses(profile.specialPass, {
      level,
      request,
      pieces: request.pieces,
      index,
      stats,
      rng,
    })
  }

  const unreachable = level.rooms.length - reachableRooms(level)
  if (unreachable !== 0) {
    throw new Error(`${where}: ${String(unreachable)} of ${String(level.rooms.length)} rooms are unreachable`)
  }
  const orphans = level.rooms.filter(room => room.piece === null).length
  if (orphans !== 0) throw new Error(`${where}: ${String(orphans)} rooms ended up with no piece`)

  assignVariants(level, request.levelId, stats)
  // 1.13c reads `dwKillEdge` off the maze map's own `LvlPrest` record. The
  // shipped table sets the column uniformly across a level type's piece rows, so
  // deriving it from the pieces gives the same answer without asking callers to
  // look the maze map up separately.
  const killEdge = request.killEdge ?? request.pieces.some(piece => piece.killEdge === true)
  const stamped = stamp(level, stats, killEdge)

  const minX = Math.min(...level.rooms.map(room => room.x))
  const minY = Math.min(...level.rooms.map(room => room.y))
  const roomDescriptors: MazeRoomDescriptor[] = level.rooms.map(room => {
    const variant = room.piece ? room.piece.levels[room.variant % room.piece.levels.length] : undefined
    return {
      id: room.id,
      x: room.x,
      y: room.y,
      width: variant?.width ?? width,
      height: variant?.height ?? height,
      orths: room.orths,
      piece: room.piece,
    }
  })

  const popOptions: MazeObjectPopulationOptions | undefined =
    typeof request.objectDensity === 'number'
      ? { density: request.objectDensity, ...request.objectOptions }
      : { ...request.objectDensity, ...request.objectOptions }

  const objectRng = popOptions?.objectSeed !== undefined
    ? new Rng(popOptions.objectSeed)
    : rng.fork('dungeon-objects')

  const popResult = populateDungeonObjects(
    roomDescriptors,
    minX,
    minY,
    stamped,
    levelTypeName,
    request.levelId,
    popOptions,
    objectRng,
  )

  const finalLevel: Ds1 = {
    ...stamped,
    objects: [...stamped.objects, ...popResult.placedObjects],
  }

  return {
    level: finalLevel,
    objects: popResult.placedObjects,
    stats: {
      levelId: request.levelId,
      levelName: request.levelName,
      levelTypeName,
      seed: request.seed,
      sectionX: width,
      sectionY: height,
      targetRooms: target,
      roomsPlaced: level.rooms.length,
      ringRooms: stats.ringRooms,
      placementAttempts: stats.placementAttempts,
      doorways: level.rooms.reduce((sum, room) => sum + room.orths.length, 0) / 2,
      mergePerMille,
      mergedDoorways: level.merges,
      gridSize: { width: stamped.width, height: stamped.height },
      gridSizeSubtiles: { width: stamped.width * SUB_TILES_PER_TILE, height: stamped.height * SUB_TILES_PER_TILE },
      objects: finalLevel.objects.length,
      dynamicObjects: popResult.placedObjects,
      dynamicObjectsCount: popResult.stats.dynamicObjectsCount,
      shrinesSpawned: popResult.stats.shrinesSpawned,
      chestsSpawned: popResult.stats.chestsSpawned,
      superChestsSpawned: popResult.stats.superChestsSpawned,
      racksSpawned: popResult.stats.racksSpawned,
      containersSpawned: popResult.stats.containersSpawned,
      cells: stamped.width * stamped.height,
      sidePatterns: stats.patterns,
      fallbacks: stats.fallbacks,
      specialsApplied: stats.specialsApplied,
      specialPassesApplied: stats.specialPassesApplied,
      warps: stats.warps,
      waypoints: stats.waypoints,
      waypoint: stats.waypoints[0] ?? null,
      waypointTile: stats.waypoints[0]
        ? {
            x: Math.floor(stats.waypoints[0].entityX / SUB_TILES_PER_TILE),
            y: Math.floor(stats.waypoints[0].entityY / SUB_TILES_PER_TILE),
          }
        : null,
      unresolvedRoles: stats.unresolvedRoles,
      unimplementedPasses: unimplementedPassesFor(levelTypeName),
      maxCellsX,
      maxCellsY,
      boundsRejected: stats.boundsRejected ?? 0,
      notes: stats.notes,
      ...(animSpeed !== undefined ? { animSpeed } : {}),
    },
    ...(animSpeed !== undefined ? { animSpeed } : {}),
  }
}