Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats: JavaScript parsing library

A variable-length integer, in the format used by the 0xfe chunks in the 'dcmp' (0) and 'dcmp' (1) resource compression formats. See the dcmp_0 and dcmp_1 specs for more information about these compression formats.

This variable-length integer format can store an integer x in any of the following ways:

  • In a single byte, if 0 <= x <= 0x7f (7-bit unsigned integer)
  • In 2 bytes, if -0x4000 <= x <= 0x3eff (15-bit signed integer with the highest 0x100 values unavailable)
  • In 5 bytes, if -0x80000000 <= x <= 0x7fffffff (32-bit signed integer)

In practice, values are always stored in the smallest possible format, but technically any of the larger formats could be used as well.

Application

Mac OS

KS implementation details

License: MIT
Minimal Kaitai Struct required: 0.8

This page hosts a formal specification of Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Runtime library

All parsing code for JavaScript generated by Kaitai Struct depends on the JavaScript runtime library. You have to install it before you can parse data.

The JavaScript runtime library is available at npm:

npm install kaitai-struct

Code

See the usage examples in the JavaScript notes.

Parse structure from an ArrayBuffer:

var arrayBuffer = ...;
var data = new DcmpVariableLengthInteger(new KaitaiStream(arrayBuffer));

After that, one can get various attributes from the structure by accessing fields or properties like:

data.first // => The first byte of the variable-length integer.
This determines which storage format is used.

* For the 1-byte format,
  this encodes the entire value of the value.
* For the 2-byte format,
  this encodes the high 7 bits of the value,
  minus `0xc0`.
  The highest bit of the value,
  i. e. the second-highest bit of this field,
  is the sign bit.
* For the 5-byte format,
  this is always `0xff`.

data.value // => The decoded value of the variable-length integer.

JavaScript source code to parse Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats

DcmpVariableLengthInteger.js

// This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

(function (root, factory) {
  if (typeof define === 'function' && define.amd) {
    define(['kaitai-struct/KaitaiStream'], factory);
  } else if (typeof module === 'object' && module.exports) {
    module.exports = factory(require('kaitai-struct/KaitaiStream'));
  } else {
    root.DcmpVariableLengthInteger = factory(root.KaitaiStream);
  }
}(this, function (KaitaiStream) {
/**
 * A variable-length integer,
 * in the format used by the 0xfe chunks in the `'dcmp' (0)` and `'dcmp' (1)` resource compression formats.
 * See the dcmp_0 and dcmp_1 specs for more information about these compression formats.
 * 
 * This variable-length integer format can store an integer `x` in any of the following ways:
 * 
 * * In a single byte,
 *   if `0 <= x <= 0x7f`
 *   (7-bit unsigned integer)
 * * In 2 bytes,
 *   if `-0x4000 <= x <= 0x3eff`
 *   (15-bit signed integer with the highest `0x100` values unavailable)
 * * In 5 bytes, if `-0x80000000 <= x <= 0x7fffffff`
 *   (32-bit signed integer)
 * 
 * In practice,
 * values are always stored in the smallest possible format,
 * but technically any of the larger formats could be used as well.
 * @see {@link https://github.com/dgelessus/python-rsrcfork/tree/master/rsrcfork/compress/common.py|Source}
 */

var DcmpVariableLengthInteger = (function() {
  function DcmpVariableLengthInteger(_io, _parent, _root) {
    this._io = _io;
    this._parent = _parent;
    this._root = _root || this;

    this._read();
  }
  DcmpVariableLengthInteger.prototype._read = function() {
    this.first = this._io.readU1();
    if (this.first >= 128) {
      switch (this.first) {
      case 255:
        this.more = this._io.readS4be();
        break;
      default:
        this.more = this._io.readU1();
        break;
      }
    }
  }

  /**
   * The decoded value of the variable-length integer.
   */
  Object.defineProperty(DcmpVariableLengthInteger.prototype, 'value', {
    get: function() {
      if (this._m_value !== undefined)
        return this._m_value;
      this._m_value = (this.first == 255 ? this.more : (this.first >= 128 ? (((this.first << 8) | this.more) - 49152) : this.first));
      return this._m_value;
    }
  });

  /**
   * The first byte of the variable-length integer.
   * This determines which storage format is used.
   * 
   * * For the 1-byte format,
   *   this encodes the entire value of the value.
   * * For the 2-byte format,
   *   this encodes the high 7 bits of the value,
   *   minus `0xc0`.
   *   The highest bit of the value,
   *   i. e. the second-highest bit of this field,
   *   is the sign bit.
   * * For the 5-byte format,
   *   this is always `0xff`.
   */

  /**
   * The remaining bytes of the variable-length integer.
   * 
   * * For the 1-byte format,
   *   this is not present.
   * * For the 2-byte format,
   *   this encodes the low 8 bits of the value.
   * * For the 5-byte format,
   *   this encodes the entire value.
   */

  return DcmpVariableLengthInteger;
})();
return DcmpVariableLengthInteger;
}));