com.physpics.tools.encoding

Class CompressEngine

java.lang.Object

com.physpics.tools.encoding.CompressEngine

public class CompressEngine
extends java.lang.Object

The compress method reads an InputStream and writes a compressed version to an OutputStream. The compression algorithm is that of the Unix program compress. The output is suitable for HTTP transmission with Content-Encoding: compress. This code was transposed by Fred Hansen, January, 2008, from the original C: compress.c - File compression ala IEEE Computer, June 1984. Authors: Spencer W. Thomas (decvax!utah-cs!thomas) Jim McKie (decvax!mcvax!jim) Steve Davies (decvax!vax135!petsd!peora!srd) Ken Turkowski (decvax!decwrl!turtlevax!ken) James A. Woods (decvax!ihnp4!ames!jaw) Joe Orost (decvax!vax135!petsd!joe) Algorithm from "A Technique for High Performance Data Compression", Terry A. Welch, IEEE Computer Vol 17, No 6 (June 1984), pp 8-19.

Constructor Summary

Constructors

Constructor and Description
CompressEngine(InputStream instr, OutputStream outstr) creates a CompressEngine object that will compress the data in an InputStream and write the result to an OutputStream.

Method Summary

Modifier and Type	Method and Description
boolean	compress() compresses instr to outstr.
void	input_size(int fsize) sets the expected size of the input stream.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

CompressEngine

public CompressEngine(InputStream instr,
                      OutputStream outstr)

creates a CompressEngine object that will compress the data in an InputStream and write the result to an OutputStream.

Parameters:

instr - the data to compress.

outstr - the compressed data, preceded by a three byte magic number.

Method Detail

input_size

public void input_size(int fsize)

sets the expected size of the input stream. The output stream incorporates one or more copies of the hash table. So the output can be bigger than the input. With a smaller input size, the hash table can be smaller and then the output is more likely to be smaller than the input. Must be called before calling the compress method.

Parameters:

fsize - The expected length of the file to be compressed.

compress

public boolean compress()
                 throws IOException

compresses instr to outstr. Algorithm: use open addressing double hashing (no chaining) on the prefix code / next character combination. We do a variant of Knuth's algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime secondary probe. Here, the modular division first probe gives way to a faster exclusive-or manipulation. Also do block compression with an adaptive reset, whereby the code table is cleared when the compression ratio decreases, but after the table fills. The variable-length output codes are re-sized at this point, and a special CLEAR code is generated for the decompressor. Late addition: construct the table according to file size for noticeable speed improvement on small files. Please direct questions about this implementation to ames!jaw.

Returns:

true iff the output is fewer bytes than the input. The InputStream and OutputStream are NOT closed.

Throws:

IOException - if reading or writing fails.