/**
 * Minimal `.tbl` writer, shared by the fixture generator and its checker.
 *
 * Tooling only: the engine reads TBL files, it never writes them. It lives here so
 * the fixture generator and the construction check encode a table the same way,
 * instead of two copies drifting apart.
 */

/** One entry: a string, or null for an unused index. */
export type TblEntry = string | null

/**
 * Encode a classic `.tbl`: crc, entry count, index table, then
 * `u16 characterCount` + UTF-16LE strings.
 *
 * @param entries - the entries, in index order.
 * @returns the encoded bytes.
 */
export function encodeTbl(entries: readonly TblEntry[]): Uint8Array {
  const headerBytes = 4
  const indexBytes = entries.length * 2
  const blocks: { index: number; bytes: Uint8Array }[] = []
  let cursor = headerBytes + indexBytes
  for (const [index, entry] of entries.entries()) {
    if (entry === null) continue
    // The count is in characters; an astral character is two UTF-16 units.
    let characters = 0
    for (const character of entry) characters += character.codePointAt(0)! > 0xffff ? 2 : 1
    const body = new Uint8Array(characters * 2)
    const view = new DataView(body.buffer)
    let units = 0
    for (const character of entry) {
      const code = character.codePointAt(0)!
      if (code > 0xffff) {
        const adjusted = code - 0x10000
        view.setUint16(units * 2, 0xd800 + (adjusted >> 10), true)
        view.setUint16((units + 1) * 2, 0xdc00 + (adjusted & 0x3ff), true)
        units += 2
      } else {
        view.setUint16(units * 2, code, true)
        units += 1
      }
    }
    const bytes = new Uint8Array([characters & 0xff, (characters >> 8) & 0xff, ...body])
    blocks.push({ index, bytes })
    cursor += bytes.byteLength
  }
  const out = new Uint8Array(cursor)
  const view = new DataView(out.buffer)
  view.setUint16(0, 0x1234, true)
  view.setUint16(2, entries.length, true)
  let at = headerBytes + indexBytes
  for (const block of blocks) {
    view.setUint16(headerBytes + block.index * 2, at, true)
    out.set(block.bytes, at)
    at += block.bytes.byteLength
  }
  return out
}