const { readSync } = require('node:fs')

const localFileHeaderSignature = 0x04034b50
const centralDirectoryHeaderSignature = 0x02014b50
const endOfCentralDirectorySignature = 0x06054b50

function readExactZipBytes(handle, length, position, label) {
  const output = Buffer.allocUnsafe(length)
  let offset = 0
  while (offset < length) {
    const bytesRead = readSync(
      handle,
      output,
      offset,
      length - offset,
      position + offset
    )
    if (bytesRead <= 0) {
      throw new Error(`${label} is truncated`)
    }
    offset += bytesRead
  }
  return output
}

function readZipCentralDirectory(handle, archiveSize, limits) {
  if (archiveSize < 22) {
    throw new Error(`${limits.label} central directory is missing`)
  }
  const endSize = Math.min(archiveSize, 65_557)
  const endStart = archiveSize - endSize
  const end = readExactZipBytes(
    handle,
    endSize,
    endStart,
    `${limits.label} end record`
  )
  let endOffset = -1
  for (let index = end.length - 22; index >= 0; index -= 1) {
    if (
      end.readUInt32LE(index) === endOfCentralDirectorySignature &&
      index + 22 + end.readUInt16LE(index + 20) === end.length
    ) {
      endOffset = index
      break
    }
  }
  if (endOffset < 0) {
    throw new Error(`${limits.label} central directory is missing`)
  }
  const diskNumber = end.readUInt16LE(endOffset + 4)
  const centralDisk = end.readUInt16LE(endOffset + 6)
  const diskEntryCount = end.readUInt16LE(endOffset + 8)
  const entryCount = end.readUInt16LE(endOffset + 10)
  const centralSize = end.readUInt32LE(endOffset + 12)
  const centralOffset = end.readUInt32LE(endOffset + 16)
  const absoluteEndOffset = endStart + endOffset
  if (
    diskNumber !== 0 ||
    centralDisk !== 0 ||
    diskEntryCount !== entryCount ||
    entryCount === 0 ||
    entryCount === 0xffff ||
    entryCount < limits.minimumEntries ||
    entryCount > limits.maximumEntries ||
    centralSize === 0xffffffff ||
    centralSize < 46 ||
    centralSize > limits.maximumCentralDirectoryBytes ||
    centralOffset === 0xffffffff ||
    centralOffset + centralSize !== absoluteEndOffset
  ) {
    throw new Error(`${limits.label} central directory is invalid`)
  }
  const central = readExactZipBytes(
    handle,
    centralSize,
    centralOffset,
    `${limits.label} central directory`
  )
  const entries = []
  let offset = 0
  for (let index = 0; index < entryCount; index += 1) {
    if (
      offset + 46 > central.length ||
      central.readUInt32LE(offset) !==
        centralDirectoryHeaderSignature
    ) {
      throw new Error(
        `${limits.label} central directory entry is invalid`
      )
    }
    const nameLength = central.readUInt16LE(offset + 28)
    const extraLength = central.readUInt16LE(offset + 30)
    const commentLength = central.readUInt16LE(offset + 32)
    const entryLength =
      46 + nameLength + extraLength + commentLength
    if (offset + entryLength > central.length) {
      throw new Error(
        `${limits.label} central directory entry exceeds its bounds`
      )
    }
    const flags = central.readUInt16LE(offset + 8)
    const nameBytes = central.subarray(
      offset + 46,
      offset + 46 + nameLength
    )
    const name = new TextDecoder(
      flags & 0x0800 ? 'utf-8' : 'latin1',
      { fatal: true }
    ).decode(nameBytes)
    entries.push({
      name,
      nameBytes: Buffer.from(nameBytes),
      flags,
      compression: central.readUInt16LE(offset + 10),
      checksum: central.readUInt32LE(offset + 16),
      compressedSize: central.readUInt32LE(offset + 20),
      size: central.readUInt32LE(offset + 24),
      diskStart: central.readUInt16LE(offset + 34),
      localOffset: central.readUInt32LE(offset + 42)
    })
    offset += entryLength
  }
  if (offset !== central.length) {
    throw new Error(
      `${limits.label} central directory count is invalid`
    )
  }
  return {
    centralOffset,
    entries
  }
}

module.exports = {
  centralDirectoryHeaderSignature,
  endOfCentralDirectorySignature,
  localFileHeaderSignature,
  readExactZipBytes,
  readZipCentralDirectory
}