Variable length quantity, unsigned integer, base128, big-endian: JavaScript parsing library

A variable-length unsigned integer using base128 encoding. 1-byte groups consist of 1-bit flag of continuation and 7-bit value chunk, and are ordered "most significant group first", i.e. in "big-endian" manner.

This particular encoding is specified and used in:

  • Standard MIDI file format
  • ASN.1 BER encoding
  • RAR 5.0 file format

More information on this encoding is available at https://en.wikipedia.org/wiki/Variable-length_quantity

This particular implementation supports serialized values to up 8 bytes long.

KS implementation details

License: CC0-1.0
Minimal Kaitai Struct required: 0.7

This page hosts a formal specification of Variable length quantity, unsigned integer, base128, big-endian 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 VlqBase128Be(new KaitaiStream(arrayBuffer));

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

data.value // => Resulting value as normal integer

JavaScript source code to parse Variable length quantity, unsigned integer, base128, big-endian

VlqBase128Be.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.VlqBase128Be = factory(root.KaitaiStream);
  }
}(this, function (KaitaiStream) {
/**
 * A variable-length unsigned integer using base128 encoding. 1-byte groups
 * consist of 1-bit flag of continuation and 7-bit value chunk, and are ordered
 * "most significant group first", i.e. in "big-endian" manner.
 * 
 * This particular encoding is specified and used in:
 * 
 * * Standard MIDI file format
 * * ASN.1 BER encoding
 * * RAR 5.0 file format
 * 
 * More information on this encoding is available at
 * https://en.wikipedia.org/wiki/Variable-length_quantity
 * 
 * This particular implementation supports serialized values to up 8 bytes long.
 */

var VlqBase128Be = (function() {
  function VlqBase128Be(_io, _parent, _root) {
    this._io = _io;
    this._parent = _parent;
    this._root = _root || this;

    this._read();
  }
  VlqBase128Be.prototype._read = function() {
    this.groups = []
    var i = 0;
    do {
      var _ = new Group(this._io, this, this._root);
      this.groups.push(_);
      i++;
    } while (!(!(_.hasNext)));
  }

  /**
   * One byte group, clearly divided into 7-bit "value" chunk and 1-bit "continuation" flag.
   */

  var Group = VlqBase128Be.Group = (function() {
    function Group(_io, _parent, _root) {
      this._io = _io;
      this._parent = _parent;
      this._root = _root || this;

      this._read();
    }
    Group.prototype._read = function() {
      this.b = this._io.readU1();
    }

    /**
     * If true, then we have more bytes to read
     */
    Object.defineProperty(Group.prototype, 'hasNext', {
      get: function() {
        if (this._m_hasNext !== undefined)
          return this._m_hasNext;
        this._m_hasNext = (this.b & 128) != 0;
        return this._m_hasNext;
      }
    });

    /**
     * The 7-bit (base128) numeric value chunk of this group
     */
    Object.defineProperty(Group.prototype, 'value', {
      get: function() {
        if (this._m_value !== undefined)
          return this._m_value;
        this._m_value = (this.b & 127);
        return this._m_value;
      }
    });

    return Group;
  })();
  Object.defineProperty(VlqBase128Be.prototype, 'last', {
    get: function() {
      if (this._m_last !== undefined)
        return this._m_last;
      this._m_last = (this.groups.length - 1);
      return this._m_last;
    }
  });

  /**
   * Resulting value as normal integer
   */
  Object.defineProperty(VlqBase128Be.prototype, 'value', {
    get: function() {
      if (this._m_value !== undefined)
        return this._m_value;
      this._m_value = (((((((this.groups[this.last].value + (this.last >= 1 ? (this.groups[(this.last - 1)].value << 7) : 0)) + (this.last >= 2 ? (this.groups[(this.last - 2)].value << 14) : 0)) + (this.last >= 3 ? (this.groups[(this.last - 3)].value << 21) : 0)) + (this.last >= 4 ? (this.groups[(this.last - 4)].value << 28) : 0)) + (this.last >= 5 ? (this.groups[(this.last - 5)].value << 35) : 0)) + (this.last >= 6 ? (this.groups[(this.last - 6)].value << 42) : 0)) + (this.last >= 7 ? (this.groups[(this.last - 7)].value << 49) : 0));
      return this._m_value;
    }
  });

  return VlqBase128Be;
})();
return VlqBase128Be;
}));