Bzip2 Implementation

This project is a C++ implementation of the Bzip2 compression algorithm. The implementation includes both the encoder and decoder for compressing and decompressing files. The encoder compresses a file using Burrows-Wheeler Transform (BWT), Move-to-Front Transform (MTF), Run-Length Encoding (RLE), and Huffman coding. The decoder reverses these transformations to restore the original file.

Bzip2 Algorithm Overview

Bzip2 is a block-sorting file compressor that uses a combination of the Burrows-Wheeler Transform, Move-to-Front Transform, Run-Length Encoding, and Huffman Coding to compress data. Here’s a brief overview of each step:

• Burrows-Wheeler Transform (BWT): Rearranges the input text to group similar characters together, which is more favorable for subsequent compression steps.
• Move-to-Front (MTF) Transform: Converts the sequence of characters into a sequence of indices, which tends to produce many zeros and small numbers when used after BWT.
• Run-Length Encoding (RLE): Encodes runs of the same value into a single value and a count, reducing repetitive sequences to smaller representations.
• Huffman Coding: Compresses the final sequence of symbols into a binary format, where more frequent symbols have shorter codes.

For more detailed information about the Bzip2 algorithm, you can visit the wikipedia page.

Preformance

The following table illustrates the performance of this Bzip2 implementation on sample data generated using Lorem Ipsum:

Original Size	Compressed Size	Compression Ratio	Space Savings (%)
1 KB	628 Bytes	0.613	38.7%
10 KB	3.4 KB	0.340	66.0%
100 KB	30 KB	0.300	70.0%
1 MB	292 KB	0.292	70.8%
10 MB	2.8 MB	0.280	72.0%

The graph shows the compression ratio at different file sizes, with the ratio stabilizing around 72%:

Usage

Build

Run make to compile the project, creating bzip2_compress and bzip2_decompress executables.

Encoding

To compress a file, use the command:

./bzip2_compress <input_file>

Decoding

To decompress a file, use the command:

./bzip2_decompress <compressed_file>

Adds the ._bz2 extension to the compressed file name.

File Structure

The project directory is structured as follows:

.
├── Makefile
├── README.md
├── decoder.cpp
├── encoder.cpp
├── inc
│   ├── burrows_wheeler.h
│   ├── huffman.h
│   ├── move_to_front.h
│   ├── run_length_encoding.h
│   ├── transform.h
│   └── utils.h
└── src
    ├── burrows_wheeler.cpp
    ├── huffman.cpp
    ├── move_to_front.cpp
    ├── run_length_encoding.cpp
    └── utils.cpp