/**
 * Zstandard frame primitives for the JSONL persistence backend. The backend
 * owns a concatenated-frame container so it can append and recover batches
 * without exposing compression mechanics through the persistence seam.
 * @module dsh-session-persistence-jsonl/zstd
 */

import { constants, zstdCompress, zstdDecompress, type ZstdOptions } from 'node:zlib'
import { promisify } from 'node:util'

const ZSTD_MAGIC = 0xFD2FB528
const zstdCompressAsync = promisify(zstdCompress)
const zstdDecompressAsync = promisify(zstdDecompress)
const CHECKSUM_OPTIONS: ZstdOptions = {
  params: { [constants.ZSTD_c_checksumFlag]: 1 },
}

/** Byte range occupied by one structurally complete Zstandard frame. */
export interface ZstdFrameRange {
  /** Inclusive frame start. */
  start: number
  /** Exclusive frame end. */
  end: number
}

/** Structural scan result for a concatenated Zstandard stream. */
export interface ZstdFrameScan {
  /** Complete frames in file order. */
  frames: ZstdFrameRange[]
  /** Start of an incomplete final frame, when EOF interrupts one. */
  tornStart?: number
}

/**
 * Locate complete frames without decompressing their blocks. Invalid complete
 * structure rejects; EOF inside the final frame returns its start for repair.
 * @param buffer - complete bytes currently present in the session artifact.
 * @param maxFrames - optional complete-frame limit for metadata-only readers.
 * @returns complete frame ranges and an optional incomplete-final-frame start.
 */
export function scanZstdFrames(buffer: Buffer, maxFrames = Number.POSITIVE_INFINITY): ZstdFrameScan {
  const frames: ZstdFrameRange[] = []
  let offset = 0

  while (offset < buffer.length) {
    const start = offset
    if (buffer.length - offset < 4) return { frames, tornStart: start }
    if (buffer.readUInt32LE(offset) !== ZSTD_MAGIC) {
      throw new Error(`corrupt Zstandard session log: invalid frame magic at byte ${offset}`)
    }
    offset += 4

    if (offset === buffer.length) return { frames, tornStart: start }
    const descriptor = buffer.readUInt8(offset)
    offset += 1
    if ((descriptor & 0x18) !== 0) {
      throw new Error(`corrupt Zstandard session log: reserved frame-header bit at byte ${offset - 1}`)
    }

    const contentSizeFlag = descriptor >>> 6
    const singleSegment = (descriptor & 0x20) !== 0
    const checksum = (descriptor & 0x04) !== 0
    const dictionaryFlag = descriptor & 0x03
    const dictionaryBytes = dictionaryFlag === 3 ? 4 : dictionaryFlag
    const contentSizeBytes = contentSizeFlag === 0
      ? (singleSegment ? 1 : 0)
      : 1 << contentSizeFlag
    const remainingHeaderBytes = (singleSegment ? 0 : 1) + dictionaryBytes + contentSizeBytes
    if (buffer.length - offset < remainingHeaderBytes) return { frames, tornStart: start }
    offset += remainingHeaderBytes

    for (;;) {
      if (buffer.length - offset < 3) return { frames, tornStart: start }
      const blockHeader = buffer.readUIntLE(offset, 3)
      offset += 3
      const lastBlock = (blockHeader & 1) !== 0
      const blockType = (blockHeader >>> 1) & 0x03
      const blockSize = blockHeader >>> 3
      if (blockType === 0x03) {
        throw new Error(`corrupt Zstandard session log: reserved block type at byte ${offset - 3}`)
      }
      const payloadBytes = blockType === 0x01 ? 1 : blockSize
      if (buffer.length - offset < payloadBytes) return { frames, tornStart: start }
      offset += payloadBytes
      if (lastBlock) break
    }

    if (checksum) {
      if (buffer.length - offset < 4) return { frames, tornStart: start }
      offset += 4
    }
    frames.push({ start, end: offset })
    if (frames.length === maxFrames) return { frames }
  }

  return { frames }
}

/**
 * Compress one independently decodable, checksummed Zstandard frame.
 * @param input - JSONL bytes for a header or durable event batch.
 * @returns the complete encoded frame.
 */
export async function compressZstdFrame(input: Buffer | string): Promise<Buffer> {
  return zstdCompressAsync(input, CHECKSUM_OPTIONS)
}

/**
 * Decompress one complete frame or the available prefix of a torn final frame.
 * Complete-frame checksums are validated by Node's decoder.
 * @param input - bytes beginning at a Zstandard frame boundary.
 * @returns plaintext produced from the available input.
 */
export async function decompressZstdFrame(input: Buffer): Promise<Buffer> {
  return zstdDecompressAsync(input)
}