matthew-a-thomas / js-ldpc

A JavaScript implementation of a Low-Density Parity-Check code

js-ldpc

A JavaScript implementation of a Low-Density Parity-Check code.

The goal of this project is to create an implementation of an LDPC (https://en.wikipedia.org/wiki/Low-density_parity-check_code) because an existing one doesn't seem readily available.

Robert Gallager's original paper on the topic can be found here: http://www.rle.mit.edu/rgallager/documents/ldpc.pdf

##Browser support Currently this has only been tested in Chrome >= 46

##Examples

• example.html - shows decoding in the face of symbols being lost
• example2.html - shows how many symbols are necessary from an encoded message to successfully decode

##Documentation

###Constructor

var ldpc = new LDPC(options);

options is an object that can have these properties:

• n the number of symbols in an encoded message
• k the number of non-redundant symbols (number of symbols in unencoded message)
• modulo e.g. 2 for binary digits, 256 for bytes, etc
• randomSeed value to which to initialize an internal random number generator, which is used to build the parity-check and generator matrices. Defaults to Date.now()

###Encoding

var encoded = ldpc.encode(message);
console.log(encoded.join()); // Prints the encoded message

message is an array options.k in length of numbers that are less than options.modulo

###Decoding

var result = ldpc.decode(encoded);
if (result.decoded) {
  // result should have all-positive and non-null numbers
} else {
  // You need more symbols. result may have more decoded symbols than you started with, but it's unlikely
  // Also see result.all, which has all the encoded symbols including any additional which were deduced. Note that the first k symbols are the same as result
}

encoded is an array options.n in length of numbers that are less than options.modulo. Use negative numbers or null to indicate a missing symbol (we're targeting an erasure channel)