openharness

import { deflateRawSync } from 'node:zlib'

/**
 * A ZIP file, written by hand.
 *
 * Node ships zlib and nothing that writes a container, and a log bundle has to be a `.zip` — the one
 * archive a person can open by double-clicking on every desktop this app ships to. The format is small:
 * a local header + deflated bytes per entry, a central directory that repeats the headers, and one
 * end-of-directory record. No zip64, no encryption, no streaming: a bundle is a few dozen files under
 * a hundred megabytes, built in memory.
 */
export interface ZipEntry {
  /** Path inside the archive, `/`-separated. */
  name: string
  data: Buffer
  /** Modification time for the entry. Default: now. */
  mtime?: Date
}

const CRC_TABLE = (() => {
  const t = new Uint32Array(256)
  for (let n = 0; n < 256; n++) {
    let c = n
    for (let k = 0; k < 8; k++) c = c & 1 ? 0xedb88320 ^ (c >>> 1) : c >>> 1
    t[n] = c >>> 0
  }
  return t
})()

export function crc32(buf: Buffer): number {
  let c = 0xffffffff
  for (let i = 0; i < buf.length; i++) c = CRC_TABLE[(c ^ buf[i]) & 0xff] ^ (c >>> 8)
  return (c ^ 0xffffffff) >>> 0
}

/** MS-DOS date and time, the only stamp the classic header carries. Local time, two-second resolution. */
function dosStamp(d: Date): { date: number; time: number } {
  const year = Math.max(1980, d.getFullYear())
  return {
    date: ((year - 1980) << 9) | ((d.getMonth() + 1) << 5) | d.getDate(),
    time: (d.getHours() << 11) | (d.getMinutes() << 5) | (d.getSeconds() >> 1),
  }
}

export function writeZip(entries: ZipEntry[]): Buffer {
  const locals: Buffer[] = []
  const centrals: Buffer[] = []
  let offset = 0
  for (const entry of entries) {
    const name = Buffer.from(entry.name, 'utf8')
    const deflated = deflateRawSync(entry.data)
    // Store rather than deflate when compression buys nothing (an already-compressed file).
    const stored = deflated.length >= entry.data.length
    const body = stored ? entry.data : deflated
    const method = stored ? 0 : 8
    const crc = crc32(entry.data)
    const { date, time } = dosStamp(entry.mtime ?? new Date())
    // General purpose bit 11: names are UTF-8.
    const flags = 0x0800

    const local = Buffer.alloc(30)
    local.writeUInt32LE(0x04034b50, 0)
    local.writeUInt16LE(20, 4)          // version needed
    local.writeUInt16LE(flags, 6)
    local.writeUInt16LE(method, 8)
    local.writeUInt16LE(time, 10)
    local.writeUInt16LE(date, 12)
    local.writeUInt32LE(crc, 14)
    local.writeUInt32LE(body.length, 18)
    local.writeUInt32LE(entry.data.length, 22)
    local.writeUInt16LE(name.length, 26)
    local.writeUInt16LE(0, 28)          // extra length
    locals.push(local, name, body)

    const central = Buffer.alloc(46)
    central.writeUInt32LE(0x02014b50, 0)
    central.writeUInt16LE(20, 4)        // version made by
    central.writeUInt16LE(20, 6)        // version needed
    central.writeUInt16LE(flags, 8)
    central.writeUInt16LE(method, 10)
    central.writeUInt16LE(time, 12)
    central.writeUInt16LE(date, 14)
    central.writeUInt32LE(crc, 16)
    central.writeUInt32LE(body.length, 20)
    central.writeUInt32LE(entry.data.length, 24)
    central.writeUInt16LE(name.length, 28)
    central.writeUInt16LE(0, 30)        // extra
    central.writeUInt16LE(0, 32)        // comment
    central.writeUInt16LE(0, 34)        // disk
    central.writeUInt16LE(0, 36)        // internal attrs
    central.writeUInt32LE(0, 38)        // external attrs
    central.writeUInt32LE(offset, 42)
    centrals.push(central, name)

    offset += local.length + name.length + body.length
  }
  const directory = Buffer.concat(centrals)
  const end = Buffer.alloc(22)
  end.writeUInt32LE(0x06054b50, 0)
  end.writeUInt16LE(0, 4)
  end.writeUInt16LE(0, 6)
  end.writeUInt16LE(entries.length, 8)
  end.writeUInt16LE(entries.length, 10)
  end.writeUInt32LE(directory.length, 12)
  end.writeUInt32LE(offset, 16)
  end.writeUInt16LE(0, 20)
  return Buffer.concat([...locals, directory, end])
}