Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats: C# 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 C# generated by Kaitai Struct depends on the C# runtime library. You have to install it before you can parse data.

The C# runtime library is available in the NuGet Gallery. Installation instructions can also be found there.

Code

Parse a local file and get structure in memory:

var data = DcmpVariableLengthInteger.FromFile("path/to/local/file.dcmp_variable_length_integer");

Or parse structure from a byte array:

byte[] someArray = new byte[] { ... };
var data = new DcmpVariableLengthInteger(new KaitaiStream(someArray));

After that, one can get various attributes from the structure by accessing 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.

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

DcmpVariableLengthInteger.cs

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



namespace Kaitai
{

    /// <summary>
    /// 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 &lt;= x &lt;= 0x7f`
    ///   (7-bit unsigned integer)
    /// * In 2 bytes,
    ///   if `-0x4000 &lt;= x &lt;= 0x3eff`
    ///   (15-bit signed integer with the highest `0x100` values unavailable)
    /// * In 5 bytes, if `-0x80000000 &lt;= x &lt;= 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.
    /// </summary>
    /// <remarks>
    /// Reference: <a href="https://github.com/dgelessus/python-rsrcfork/tree/master/rsrcfork/compress/common.py">Source</a>
    /// </remarks>
    public partial class DcmpVariableLengthInteger : KaitaiStruct
    {
        public static DcmpVariableLengthInteger FromFile(string fileName)
        {
            return new DcmpVariableLengthInteger(new KaitaiStream(fileName));
        }

        public DcmpVariableLengthInteger(KaitaiStream p__io, KaitaiStruct p__parent = null, DcmpVariableLengthInteger p__root = null) : base(p__io)
        {
            m_parent = p__parent;
            m_root = p__root ?? this;
            f_value = false;
            _read();
        }
        private void _read()
        {
            _first = m_io.ReadU1();
            if (First >= 128) {
                switch (First) {
                case 255: {
                    _more = m_io.ReadS4be();
                    break;
                }
                default: {
                    _more = m_io.ReadU1();
                    break;
                }
                }
            }
        }
        private bool f_value;
        private int _value;

        /// <summary>
        /// The decoded value of the variable-length integer.
        /// </summary>
        public int Value
        {
            get
            {
                if (f_value)
                    return _value;
                _value = (int) ((First == 255 ? More : (First >= 128 ? (((First << 8) | More) - 49152) : First)));
                f_value = true;
                return _value;
            }
        }
        private byte _first;
        private int _more;
        private DcmpVariableLengthInteger m_root;
        private KaitaiStruct m_parent;

        /// <summary>
        /// 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`.
        /// </summary>
        public byte First { get { return _first; } }

        /// <summary>
        /// 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.
        /// </summary>
        public int More { get { return _more; } }
        public DcmpVariableLengthInteger M_Root { get { return m_root; } }
        public KaitaiStruct M_Parent { get { return m_parent; } }
    }
}