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

The Java runtime library is published in the Maven Central Repository. Refer to the artifact page for instructions how to add it into your project with the build tool that you use.

Code

Parse a local file and get structure in memory:

DcmpVariableLengthInteger data = DcmpVariableLengthInteger.fromFile("path/to/local/file.Variable-length integer used in Apple `'dcmp' (0)` and `'dcmp' (1)` compressed resource formats");

Or parse structure from a byte array:

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

After that, one can get various attributes from the structure by invoking getter methods 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.

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

DcmpVariableLengthInteger.java

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

import io.kaitai.struct.ByteBufferKaitaiStream;
import io.kaitai.struct.KaitaiStruct;
import io.kaitai.struct.KaitaiStream;
import java.io.IOException;


/**
 * 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 <a href="https://github.com/dgelessus/python-rsrcfork/tree/master/rsrcfork/compress/common.py">Source</a>
 */
public class DcmpVariableLengthInteger extends KaitaiStruct {
    public static DcmpVariableLengthInteger fromFile(String fileName) throws IOException {
        return new DcmpVariableLengthInteger(new ByteBufferKaitaiStream(fileName));
    }

    public DcmpVariableLengthInteger(KaitaiStream _io) {
        this(_io, null, null);
    }

    public DcmpVariableLengthInteger(KaitaiStream _io, KaitaiStruct _parent) {
        this(_io, _parent, null);
    }

    public DcmpVariableLengthInteger(KaitaiStream _io, KaitaiStruct _parent, DcmpVariableLengthInteger _root) {
        super(_io);
        this._parent = _parent;
        this._root = _root == null ? this : _root;
        _read();
    }
    private void _read() {
        this.first = this._io.readU1();
        if (first() >= 128) {
            switch (first()) {
            case 255: {
                this.more = this._io.readS4be();
                break;
            }
            default: {
                this.more = (int) (this._io.readU1());
                break;
            }
            }
        }
    }
    private Integer value;

    /**
     * The decoded value of the variable-length integer.
     */
    public Integer value() {
        if (this.value != null)
            return this.value;
        int _tmp = (int) ((first() == 255 ? more() : (first() >= 128 ? (((first() << 8) | more()) - 49152) : first())));
        this.value = _tmp;
        return this.value;
    }
    private int first;
    private Integer more;
    private DcmpVariableLengthInteger _root;
    private KaitaiStruct _parent;

    /**
     * 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`.
     */
    public int first() { return first; }

    /**
     * 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.
     */
    public Integer more() { return more; }
    public DcmpVariableLengthInteger _root() { return _root; }
    public KaitaiStruct _parent() { return _parent; }
}