import { FileTypeParser, type Detector } from 'file-type'

const parser = new FileTypeParser({ customDetectors: [this.legacyOfficeDetector] })
const fileType = await parser.fromBuffer(...)

// Custom detector for legacy Microsoft Office files (.doc, .xls, .ppt)
// MS-CFB (Compound File Binary Format) structure:
// - Header: first 512 bytes (we only need first 52 bytes for key fields)
// - Directory stream: contains root entry with CLSID that identifies file type
// Ref: https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-cfb
const legacyOfficeDetector: Detector = {
  id: 'legacy-office',
  async detect(tokenizer) {
    // CFBF signature: D0 CF 11 E0 A1 B1 1A E1
    const cfbfSignature = [0xd0, 0xcf, 0x11, 0xe0, 0xa1, 0xb1, 0x1a, 0xe1]

    // Read minimum header bytes needed (52 bytes covers signature + sector shift + directory sector location)
    const headerBuffer = new Uint8Array(52)
    const headerBytesRead = await tokenizer.peekBuffer(headerBuffer, {
      length: 52,
      mayBeLess: true
    })

    // Need at least 52 bytes to read required header fields
    if (headerBytesRead < 52) {
      return undefined
    }

    // Check CFBF signature
    if (!cfbfSignature.every((value, index) => value === headerBuffer[index])) {
      return undefined
    }

    // Validate sector shift (offset 30): must be 9 (512-byte sectors) or 12 (4096-byte sectors)
    const sectorShift = headerBuffer[30]
    if (sectorShift !== 9 && sectorShift !== 12) {
      return undefined
    }
    const sectorSize = 1 << sectorShift

    // Read _sectDirStart (offset 48-51, little-endian unsigned 32-bit)
    // This is the sector number of the first directory sector
    // Note: JS bitwise ops return signed 32-bit ints, so values >= 0x80000000
    // become negative. Use >>> 0 to reinterpret as unsigned for spec compliance.
    const sectDirStart =
      (headerBuffer[48] |
        (headerBuffer[49] << 8) |
        (headerBuffer[50] << 16) |
        (headerBuffer[51] << 24)) >>>
      0

    // Check for special values per MS-CFB spec:
    // ENDOFCHAIN (0xFFFFFFFE) = end of sector chain
    // FREESECT (0xFFFFFFFF) = unallocated sector
    if (sectDirStart >= 0xfffffffe) {
      return undefined
    }

    // Calculate CLSID location in file:
    // - 512 bytes for header
    // - sectDirStart * sectorSize to reach directory sector
    // - 80 bytes offset to CLSID within root directory entry
    const clsidOffset = 512 + sectDirStart * sectorSize + 80
    const requiredLength = clsidOffset + 16

    // Read enough bytes to reach the CLSID
    const buffer = new Uint8Array(requiredLength)
    const bytesRead = await tokenizer.peekBuffer(buffer, {
      length: requiredLength,
      mayBeLess: true
    })

    // Verify we read enough bytes
    if (bytesRead < requiredLength) {
      return undefined
    }

    // Helper to check CLSID at the calculated offset
    const checkClsid = (clsid: number[]) =>
      clsid.every((value, i) => value === buffer[clsidOffset + i])

    // CLSID for .doc files: {00020906-0000-0000-C000-000000000046}
    const docClsid = [
      0x06, 0x09, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x46
    ]
    if (checkClsid(docClsid)) {
      return { ext: 'doc', mime: 'application/msword' }
    }

    // CLSIDs for .xls files (two variants):
    // {00020810-0000-0000-C000-000000000046}
    // {00020820-0000-0000-C000-000000000046}
    const xlsClsid1 = [
      0x10, 0x08, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x46
    ]
    const xlsClsid2 = [
      0x20, 0x08, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x46
    ]
    if (checkClsid(xlsClsid1) || checkClsid(xlsClsid2)) {
      return { ext: 'xls', mime: 'application/vnd.ms-excel' }
    }

    // CLSID for .ppt files: {64818D10-4F9B-11CF-86EA-00AA00B929E8}
    const pptClsid = [
      0x10, 0x8d, 0x81, 0x64, 0x9b, 0x4f, 0xcf, 0x11, 0x86, 0xea, 0x00, 0xaa, 0x00, 0xb9, 0x29,
      0xe8
    ]
    if (checkClsid(pptClsid)) {
      return { ext: 'ppt', mime: 'application/vnd.ms-powerpoint' }
    }

    return undefined
  }
}