Huffman Zipper  v-1.0
Data Compression and Decompression using Greedy Huffman Algorithm
Huffman Zipper Documentation

Huffman Zipper

Data Compression and Decompression using Greedy Huffman Algorithm

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Table of Contents

Introduction

In computer science and information theory, a Huffman code is a particular type of optimal prefix code that is commonly used for lossless data compression. The process of finding or using such a code proceeds by means of Huffman coding, an algorithm developed by David A. Huffman while he was a Sc.D. student at MIT, and published in the 1952 paper "A Method for the Construction of Minimum-Redundancy Codes".

The output from Huffman's algorithm can be viewed as a variable-length code table for encoding a source symbol (such as a character in a file). The algorithm derives this table from the estimated probability or frequency of occurrence (weight) for each possible value of the source symbol. As in other entropy encoding methods, more common symbols are generally represented using fewer bits than less common symbols. Huffman's method can be efficiently implemented, finding a code in time linear to the number of input weights if these weights are sorted. However, although optimal among methods encoding symbols separately, Huffman coding is not always optimal among all compression methods - it is replaced with arithmetic coding or asymmetric numeral systems if better compression ratio is required.

Features

  1. Compress and decompress files according to Greedy Huffman Algorithm
  2. Robust program which handles any files (incuding binary files) as input
  3. Best compression is obtained for text (.txt) files
  4. Can compress single file or multiple files into a compressed file (.huff file)
  5. Can compress a directory and its content recursively preserving file path hierarchy.
  6. Extracts file, multiple files or directory from compressed file (.huff) into a folder according to compression done previously.

This repo has three projects among which huffman-zipper generates static .lib file, huffmanCLI is for command line interface and huffmanGUI is for graphical user interface.

Installation

  - Clone this repo to your PC
  - Go to its project directory
  - Open huffman-zipper.sln in Visual Studio 2019
  - Set HuffmanCLI or HuffmanGUI as startup project
  - To build HuffamnGUI project, configure wxWidgets for Visual Studio 2019.
  - Build and run the project.

How Huffman Compression Algorithm Works

The process to build a hufffman binary tree for compression is very simple and straightforward.

  1. Read through the whole file and count the number of occurrence of each character
  2. Create a a frequencyMap (Key/ Value pair) with character as key and its frequency as value.
  3. For each symbol in frequencyMap, wrap it in a binary node. Insert them into a MinHeap Priority Queue which pops out binary node in decreasing trend of their frequency.
  4. Pop the two least probable symbols from the priority queue. Combine these two symbols by adding their frequencies into a single binary node. And insert it back to the priority queue.
  5. Set the two popped nodes as left and right child of the newly created node.
  6. Repeat step 4 and 5 until only one tree node left in the queue which is the root node of the tree.
  7. Create a CodeMap (Key/ Value pair) with character as key and its huffman code as value by assigning each symbol with its path from root node to its node, with left being 0 and right being 1.
  8. Use the codeMap to translate the characters in input files to its equivalent Huffman code and write the code to the newly created compressed file.

Compression

As per current implementation of file compression using Huffman Compression Algorithm, writing only the encoded string is insufficient to decode the compressed file during decompression. The same Huffman Tree which was previously used during compression is required to decode the encoded characters to its original form. So, it is necessary to to write the File header in the compressed file which contains Huffman Tree and other meta data required by the decompressor.

1. How to write a file header?

The compressed file header has following structure :

Tree 2 bytes File Size File Name
Pre-order traversal of the tree number of files to be compressed number of chars in the each file File name of each file

2. How to save the tree in the compressed file?

One of approach to write the tree in to the file is to use a pre order traversal. And later during decompression, using same traversal algorithm, we can reconstruct the tree from the file header part.

While writing tree to the compressed file, when we visit a leaf node, write bit 1 followed by the symbol in 8 bits.And when we visit an internal node (or the root node),simply write bit 0 only. So in this way, during decompression, when the program reads a bit 1, it can read the next 8 bits and wrap the char in a binary leaf node. When the program reads a bit 0, it create a new internal node and connect the two nodes as left and right child using preorder traversal algorithm.

3. How to mark EOF of the output file?

There are many methods to mark the EOF. For example, you can choose a character that is not present in the input file as the EOF character. However, binary files may contain character which was used pesudoEOF. So, an alternative approach would be to store file size in the file header. So the decompression program first reads the file size and, then continue to work the decompression till the required file size is reached.

Decompression

The decompression is much more easier than compression. During decompression, the program first reads the header from the compressed file and reconstruct the tree from it. And using the tree, it decodes the rest of the file to original characters and thus completes the decompression.

Conclusion

The compression ratio and performance of the Huffman coding depends on the size of input text and the frequency of distinct characters in the file. The current implementation of huffman coding may produce file with increase size for small input files due to the overhead of storing Huffman Tree in the compressed file, which is required at the time of decompression.

But, as the size of input file increases, the compression ratio becomes nearly 50%.

Documentation

The huffman-zipper documentation is available here.

References

  1. Wikipedia: Huffman coding
  2. Huffman Coding | Greedy Algo-3
  3. Thomas H. Cormen, Charles E. Leiserson, Ronald L.Rivest, Clifford Stein "Introduction to Algorithms 3rd Edition"
  4. Clifford A. Shaffer "Data Structures and Algorithm Analysis Edition 3.2 (C++ Version)"

Team

Ashish Lamsal Aaditya Subedi

License

This project is licensed under the MIT License - see the [LICENSE](LICENSE) file for details