import { createHash } from 'node:crypto'

export interface Par2File {
  /** File ID (hex): MD5 of md5_16k, length and name, as defined by PAR2. */
  fileId: string
  /** MD5 of the whole file (hex). */
  md5: string
  /** MD5 of the first 16 KiB (hex). */
  md5_16k: string
  length: number
  name: string
}

export interface Par2Set {
  /** Recovery set ID (hex), shared by every packet of one par2 set. */
  recoverySetId: string
  files: Par2File[]
}

const MAGIC = Buffer.from('PAR2\0PKT', 'latin1')
const HEADER_SIZE = 64
const FILE_DESC = Buffer.from('PAR 2.0\0FileDesc', 'latin1')

export function isPar2(data: Buffer): boolean {
  return data.subarray(0, MAGIC.length).equals(MAGIC)
}

/**
 * Reads the FileDesc packets of a par2 file (index or recovery volume).
 * Packets with a bad length or checksum are skipped; parsing resumes at the
 * next magic sequence. Returns null when no valid FileDesc packet is found.
 */
export function parsePar2(data: Buffer): Par2Set | null {
  const files = new Map<string, Par2File>()
  let recoverySetId: string | undefined
  let offset = data.indexOf(MAGIC)

  while (offset !== -1 && offset + HEADER_SIZE <= data.length) {
    const length = Number(data.readBigUInt64LE(offset + 8))
    const valid =
      length >= HEADER_SIZE &&
      length % 4 === 0 &&
      offset + length <= data.length &&
      createHash('md5')
        .update(data.subarray(offset + 32, offset + length))
        .digest()
        .equals(data.subarray(offset + 16, offset + 32))

    if (!valid) {
      offset = data.indexOf(MAGIC, offset + 1)
      continue
    }

    const setId = data.subarray(offset + 32, offset + 48).toString('hex')
    const type = data.subarray(offset + 48, offset + 64)
    if (type.equals(FILE_DESC) && (!recoverySetId || recoverySetId === setId)) {
      recoverySetId = setId
      const body = data.subarray(offset + HEADER_SIZE, offset + length)
      const fileId = body.subarray(0, 16).toString('hex')
      files.set(fileId, {
        fileId,
        md5: body.subarray(16, 32).toString('hex'),
        md5_16k: body.subarray(32, 48).toString('hex'),
        length: Number(body.readBigUInt64LE(48)),
        // The name is padded with NULs to a multiple of 4 bytes.
        name: body.subarray(56).toString('utf8').replace(/\0+$/, ''),
      })
    }
    offset += length
  }

  if (!recoverySetId || files.size === 0) return null
  return { recoverySetId, files: [...files.values()] }
}