/**
 * The entries a ZIP's start shows: its local file headers, one before each entry's data, as far as the data
 * holds them (the first segment of a .zip; REBUILD.md, phase 2). The full list is in the central directory at
 * the end; the start names the first entries and says whether they're encrypted, as a RAR's first volume does.
 */
export interface ZipStart {
  files: { name: string; size: number }[]
  encrypted: boolean
}

const LOCAL = 0x04034b50
/** Traditional PKWARE encryption (bit 0 of the flags), or WinZip AES (method 99). */
const ENCRYPTED_FLAG = 0x1
const DATA_DESCRIPTOR_FLAG = 0x8
const AES_METHOD = 99
const MAX_ENTRIES = 10_000

export function parseZipStart(data: Buffer): ZipStart | null {
  if (data.length < 30 || data.readUInt32LE(0) !== LOCAL) return null
  const result: ZipStart = { files: [], encrypted: false }
  let pos = 0
  while (pos + 30 <= data.length && data.readUInt32LE(pos) === LOCAL && result.files.length < MAX_ENTRIES) {
    const flags = data.readUInt16LE(pos + 6)
    const method = data.readUInt16LE(pos + 8)
    let compressed = data.readUInt32LE(pos + 18)
    let size = data.readUInt32LE(pos + 22)
    const nameLength = data.readUInt16LE(pos + 26)
    const extraLength = data.readUInt16LE(pos + 28)
    const nameEnd = pos + 30 + nameLength
    if (nameEnd > data.length) break
    // Bit 11: UTF-8 names; others are mostly CP437, read as latin1.
    const name = data.toString(flags & 0x800 ? 'utf8' : 'latin1', pos + 30, nameEnd)
    // Zip64: the real sizes in extra field 0x0001.
    for (let e = nameEnd; e + 4 <= Math.min(nameEnd + extraLength, data.length); ) {
      const id = data.readUInt16LE(e)
      const length = data.readUInt16LE(e + 2)
      if (id === 0x0001 && e + 4 + length <= data.length && length >= 16) {
        size = Number(data.readBigUInt64LE(e + 4))
        compressed = Number(data.readBigUInt64LE(e + 12))
      }
      e += 4 + length
    }
    if (flags & ENCRYPTED_FLAG || method === AES_METHOD) result.encrypted = true
    if (!name.endsWith('/')) result.files.push({ name, size })
    // Sizes after the data (a streamed zip): where the next header starts isn't known.
    if (flags & DATA_DESCRIPTOR_FLAG) break
    pos = nameEnd + extraLength + compressed
  }
  return result.files.length > 0 || result.encrypted ? result : null
}