import { zstdCompressSync, zstdDecompressSync, constants } from 'node:zlib'

/** One article of a posted file: its number, message-id (without <>) and size in bytes. */
export interface Segment {
  number: number
  id: string
  size: number
}

const VERSION = 1

/**
 * Segment lists as stored in `files.segments` (CONCEPT.md D31): a version
 * byte, then zstd (level 1) of a JSON line with the sizes (and the numbers,
 * unless they are 1..N) followed by one message-id per line. About 20 bytes
 * per segment, against ~38 for the pglz-compressed JSON object it replaces:
 * message-ids are mostly random, so this is close to what's possible.
 */
export function packSegments(segments: Segment[]): Buffer {
  const sorted = [...segments].sort((a, b) => a.number - b.number)
  const sequential = sorted.every((s, i) => s.number === i + 1)
  const head = JSON.stringify(sequential ? { s: sorted.map((s) => s.size) } : { s: sorted.map((s) => s.size), n: sorted.map((s) => s.number) })
  const text = Buffer.from([head, ...sorted.map((s) => s.id)].join('\n'), 'utf8')
  const packed = zstdCompressSync(text, { params: { [constants.ZSTD_c_compressionLevel]: 1 } })
  return Buffer.concat([Buffer.from([VERSION]), packed])
}

export function unpackSegments(data: Buffer): Segment[] {
  if (data[0] !== VERSION) throw new Error(`Unknown segment list version ${data[0]}`)
  const [head, ...ids] = zstdDecompressSync(data.subarray(1)).toString('utf8').split('\n')
  const { s: sizes, n: numbers } = JSON.parse(head) as { s: number[]; n?: number[] }
  return sizes.map((size, i) => ({ number: numbers?.[i] ?? i + 1, id: ids[i], size }))
}