https://github.com/nclack/arithcode

Arithmetic coding in C
https://github.com/nclack/arithcode

Last synced: about 2 months ago
JSON representation

Arithmetic coding in C

• Host: GitHub
• URL: https://github.com/nclack/arithcode
• Owner: nclack
• License: mit
• Created: 2012-04-24T00:54:35.000Z (over 12 years ago)
• Default Branch: master
• Last Pushed: 2024-10-17T18:01:42.000Z (3 months ago)
• Last Synced: 2024-10-20T03:00:18.838Z (3 months ago)
• Language: C
• Homepage: http://nclack.github.io/arithcode/
• Size: 269 KB
• Stars: 7
• Watchers: 3
• Forks: 4
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

Awesome Lists containing this project

README

        
# Arithmetic Coding

## Introduction

Implementation after Amir Said's Algorithms 22-29([1]).  This was mostly a

learning exercise.  It hasn't been optimized.

[1]:  http://www.hpl.hp.com/techreports/2004/HPL-2004-76.pdf

      Said, A. "Introduction to Arithmetic Coding - Theory and Practice."

      Hewlett Packard Laboratories Report: 2004-2076.

      

## Features

  - Encoding/decoding to/from variable symbol alphabets.

  - Implicit use of a STOP symbol means that you don't need to know the number of symbols in the decoded message in order

    to decode something.

  - Encode messages stored as signed or unsigned chars, shorts, longs or long longs. Keep in mind, however, that the

    number of encodable symbols may be limiting.

  - Encode to streams of variable symbol width; either 1,4,8, or 16 bits. There are two tradeoffs here.

      - Smaller bit-width (e.g. 1) gives better compression than larger bit-width (e.g. 16), but compression is slower (I think?).

      - The implementation limits the smallest probability of an encoded symbol.  Smaller bit-width (e.g. 1) can accommodate

        a larger range of probabilities than large bit-width (e.g. 16).

        

## Example

```C

void encode()

{ unsigned char *input, *output=0;      // input and output buffer

  size_t countof_input, countof_output; // number of symbols in input and output buffer

  float *cdf=0;

  size_t nsym;                          // number of symbols in the input alphabet

  // somehow load the data into the input array

  cdf_build(&cdf,&nsym,input,countof_input);  // get the symbol statistics

  encode_u1_u8(                         // encode unsigned chars to a string of bits (1 bit per output symbol)

    (void**)&out,&countof_output,

           input, countof_input,

             cdf, nsym);

  // do something with the output

  free(out);

  free(cdf);

}

```