NAME¶

fec_new, fec_encode, fec_encode, fec_free — An erasure code in GF(2^m)

SYNOPSIS¶

#include <fec.h>

void *
fec_new(int k, int n);

void
fec_encode(void *code, void *data[], void *dst, int i, int sz);

int
fec_decode(void *code, void *data[], int i[], int sz);

void *
fec_free(void *code);

DESCRIPTION¶

This library implements a simple (n,k) erasure code based on Vandermonde matrices. The encoder takes k packets of size sz each, and is able to produce up to n different encoded packets, numbered from 0 to n-1, such that any subset of k of them permits reconstruction of the original data.

The data structures necessary for the encoding/decoding must first be created using calling fec_new() with the desired parameters. The code descriptor returned by the function must be passed to other functions, and destroyed calling fec_free()

Allowed values for k and n depend on a compile-time value of GF_BITS and must be k <= n <= 2^GF_BITS. Best performance is achieved with GF_BITS=8, although the code supports also GF_BITS=16.

Encoding is done by calling fec_encode() and passing it pointers to the code descriptor, the source and destination data packets, the index of the packet to be produced, and the size of the packet.

fec_decode() with pointers to the code, received packets, indexes of received packets, and packet size. Decoding is done in place, possibly shuffling the arrays passed as parameters. Decoding is deterministic as long as the received packets are different. The decoding procedure does some limited testing on this and returns if parameters are invalid.

EXAMPLE¶

#include <fec.h>

/*
 * example of sender code
 */
void *code ;
int n, k ;

void *src[] ;
void *pkt ;

code = new_code (k, n );

for (i = 0 ; i < k ; i++ )
    src[i] = .. pointer to i-th source packet ..
for (each packet to transmit) {
   i = ... index of the packet ;
   fec_encode(code, src, pkt, i, size) ;
   .. use packet in pkt
}
fec_free(code) ;

/*
 * example of receiver code
 */
void *code ;
int n, k ;

void *data[] ;
int *ix[] ;

code = new_code (k, n );

for (i = 0 ; i < k ; i++ ) {
    ... receive a new packet ...
    data[i] = .. pointer to i-th source packet ..
    ix[i] = .. index of i-th source packet ..
}
fec_decode(code, data, ix, size) ;
/*
 * now data[] has pointers to the source packets
 */
Please direct bug reports to luigi@iet.unipi.it .