import yencode from 'yencode'
import { YencCrcError, YencError } from './errors.js'
import { parseYencMeta, type YencMeta } from './meta.js'

export interface DecodedSegment {
  meta: YencMeta
  /** Decoded bytes of this segment. */
  data: Buffer
  /** 0-based offset of `data` within the complete file. */
  offset: number
}

export interface DecodeOptions {
  /**
   * Undo NNTP dot-stuffing (`..` at line start). nntp-js hands back raw
   * bodies, so this defaults to true.
   */
  stripDots?: boolean
  /** Verify the part (or whole-file) CRC32 from the `=yend` line. Default true. */
  verifyCrc?: boolean
  /**
   * Accept a decoded length that differs from the declared size (the CRC is
   * still checked). Some posting tools declare a size that leaves out a
   * header, e.g. the 20-byte header of `nzbe` containers.
   */
  lenientSize?: boolean
}

export function crc32Hex(data: Buffer): string {
  return yencode.crc32(data).toString('hex')
}

/**
 * Whole-file CRC32 from the CRCs and decoded lengths of its parts, in order,
 * without the data itself.
 */
export function combineCrc32(parts: { crc: string; length: number }[]): string {
  let combined: Buffer | undefined
  for (const part of parts) {
    const crc = Buffer.from(part.crc.padStart(8, '0'), 'hex')
    combined = combined ? yencode.crc32_combine(combined, crc, part.length) : crc
  }
  return (combined ?? Buffer.alloc(4)).toString('hex')
}

/**
 * Decodes one yEnc article body and checks its size and CRC32 against the
 * `=yend` trailer.
 */
export function decodeSegment(body: Buffer, options: DecodeOptions = {}): DecodedSegment {
  const { stripDots = true, verifyCrc = true, lenientSize = false } = options
  const meta = parseYencMeta(body)
  const data = yencode.decode(body.subarray(meta.dataStart, meta.dataEnd), stripDots)

  if (lenientSize) {
    // Only the CRC below decides.
  } else if (data.length !== meta.end.size) {
    throw new YencError(`Decoded size ${data.length} does not match =yend size ${meta.end.size}`)
  }
  if (!lenientSize && meta.part && meta.part.end - meta.part.begin + 1 !== data.length) {
    throw new YencError(
      `Decoded size ${data.length} does not match =ypart range ${meta.part.begin}-${meta.part.end}`
    )
  }

  if (verifyCrc) {
    // Multi-part posts carry pcrc32 per part; single-part posts only crc32.
    const expected = meta.end.pcrc32 ?? (meta.part ? undefined : meta.end.crc32)
    if (expected) {
      const actual = crc32Hex(data)
      if (actual !== expected) throw new YencCrcError(expected, actual)
    }
  }

  return { meta, data, offset: meta.part ? meta.part.begin - 1 : 0 }
}