wbc.js

const path = require('path');

//each part of header length
const HEADER_LENGTH = 4;
const HEADER_CHUNK_TYPE = 4;
const HEADER_COMPRESSION_METHOD = 1;
const HEADER_COMPILE_LEVEL = 1;
const HEADER_BYTECODE_VERSION = 1;
const HEADER_ADDITIONAL_DATA = 3;
const HEADER_CRC32 = 4;

//each part of body length
const BODY_LENGTH = 4;
const BODY_CHUNK_TYPE = 4;
const BODY_CRC32 = 4;

//each part of end length
const END_LENGTH = 4;
const END_CHUNK_TYPE = 4;

const HEADER_FIELDS = [
  HEADER_LENGTH,
  HEADER_CHUNK_TYPE,
  HEADER_COMPRESSION_METHOD,
  HEADER_COMPILE_LEVEL,
  HEADER_BYTECODE_VERSION,
  HEADER_ADDITIONAL_DATA,
  HEADER_CRC32
];

class Wbc {
  constructor(options = {}) {
    const rootDir = path.resolve(__dirname, '.');
    this._bindings = require('node-gyp-build')(rootDir);
  }

  getAdler32(buffer) {
    return this._bindings.getAdler32(buffer);
  }

  generateWbcBytecode(oriBody) {
    let signatureBuffer = this.generateSignature();
    let headerBuffer = this.generateHeader();
    let bodyBuffer = this.generateBody(oriBody);
    let endBuffer = this.generateEnd();

    let totalLength = signatureBuffer.length + headerBuffer.length + bodyBuffer.length + endBuffer.length;
    let bytecodeBuffer = Buffer.concat([signatureBuffer, headerBuffer, bodyBuffer, endBuffer], totalLength);
    return bytecodeBuffer;
  }

  //0x89 0x57 0x42 0x43 0x31 0x0D 0x0A 0x1A 0x0A
  generateSignature() {
    const buffer = Buffer.alloc(9);
    buffer.writeUInt8(0x89, 0);
    buffer.writeUInt8(0x57, 1);
    buffer.writeUInt8(0x42, 2);
    buffer.writeUInt8(0x43, 3);
    buffer.writeUInt8(0x31, 4);
    buffer.writeUInt8(0x0D, 5);
    buffer.writeUInt8(0x0A, 6);
    buffer.writeUInt8(0x1A, 7);
    buffer.writeUInt8(0x0A, 8);
    return buffer;
  }

  generateHeader() {
    let pointer = 0;
    let length = this.calculateHeaderLength();
    const headerBuffer = Buffer.alloc(length);

    // Length
    headerBuffer.writeUInt32BE(length, 0);
    pointer += HEADER_LENGTH;

    // ASCII value for the letter WBHD (0x57 0x42 0x48 0x44 in hexadecimal)
    headerBuffer.writeUInt32BE(0x57424844, pointer);
    pointer += HEADER_CHUNK_TYPE;

    // Compression method
    headerBuffer.writeUInt8(0, pointer);
    pointer += HEADER_COMPRESSION_METHOD;

    // Compile level
    headerBuffer.writeUInt8(0, pointer);
    pointer += HEADER_COMPILE_LEVEL;

    // Bytecode version
    headerBuffer.writeUInt8(0, pointer);
    pointer += HEADER_BYTECODE_VERSION;

    // Additional data zone
    for (let i = 0; i < HEADER_ADDITIONAL_DATA; i++) {
      headerBuffer.writeUint8(0, pointer);
      pointer += 1;
    }

    // Checksum
    const adler32Value = this.getAdler32(headerBuffer.slice(0, pointer));
    // Only the CRC32 value of the first 14 bytes is calculated because the last CRC32 field is reserved
    // Write the calculated CRC32 value to the last 4 bytes of the Buffer
    headerBuffer.writeUInt32BE(adler32Value, pointer);
    return headerBuffer;
  }

  calculateHeaderLength() {
    return HEADER_FIELDS.reduce((sum, value) => sum + value, 0);
  }

  generateBody(oriBody) {
    let pointer = 0;
    var bodyChunk = oriBody;
    let length = BODY_LENGTH + BODY_CHUNK_TYPE + bodyChunk.length + BODY_CRC32;
    const bodyBuffer = Buffer.alloc(length);

    //length
    bodyBuffer.writeUInt32BE(length, 0);

    //ASCII value for the letter WBDY (0x57 0x42 0x44 0x59 in hexadecimal)
    pointer += BODY_LENGTH;
    bodyBuffer.writeUInt32BE(0x57424459, pointer);

    //body chunk
    pointer += BODY_CHUNK_TYPE;
    bodyChunk.copy(bodyBuffer, pointer);

    //crc32
    pointer += bodyChunk.length;
    const adler32Value = this.getAdler32(bodyBuffer.slice(0, pointer));

    // Write the calculated CRC32 value to the last 4 bytes of the Buffer
    bodyBuffer.writeUInt32BE(adler32Value, pointer);
    return bodyBuffer;
  }

  generateEnd() {
    let pointer = 0;
    let length = END_LENGTH + END_CHUNK_TYPE;
    const endBuffer = Buffer.alloc(length);

    //length
    endBuffer.writeUInt32BE(length, 0);

    //The ASCII values for the letters 'WEND' (0x57 0x45 0x4e 0x44 in hexadecimal).
    pointer += END_LENGTH;
    endBuffer.writeUInt32BE(0x57454e44, pointer);
    return endBuffer;
  }
}

exports.Wbc = Wbc