/// <summary>
/// Static constructor: Create the encoding Vandermonde matrix
/// </summary>
static RS_Fec()
{
// To increase runtime encoding performance, we create a large static Vandermonde encoding matrix ahead of time
// that can be shared
// maxChecksumPackets will be the max number of Columns of the encoding static matrix
// maxDataPackets will be the max number of Rows of the encoding static matrix
// Creation of the Vandermonde Matrix over GF16 with the following
// (2^column)^row
// As an example, a 5 x 3 Vandermonde Matrix over GF4 (5 data packets, 3 checksum packets)
// would give the following:
//
// 1^0 2^0 4^0
// 1^1 2^1 4^1
// 1^2 2^2 4^2
// 1^3 2^3 4^3
// 1^4 2^4 4^4
//
// Which gives:
//
// 1 1 1
// 1 2 4
// 1 4 16
// 1 8 64
// 1 16 256
enc = FEC_Matrix.CreateVandermondeMatrix(MaxChecksumPackets, MaxDataPackets);
}
/// <summary>
/// Invert the matrix by using Gauss-Jordan using GF16 operations
/// </summary>
/// <remarks>
/// The inversion is done by adding the identity matrix to the right
/// and using Gauss-Jordan algorithm (elementary
/// operations on rows) until we get the identity matrix
/// on the left. Then we strip the identity on the left
/// in order to get the reverted matrix.
/// </remarks>
/// <param name="mtxIn_GF16">The matrix to invert</param>
/// <returns>The inverted matrix</returns>
public static GF16[,] Invert(GF16[,] mtxIn_GF16)
{
// Add the ID to the right to be able to apply Gauss-Jordan to revert the matrix
GF16[,] mtxWithID_GF16 = FEC_Matrix.AddIdentityRight(mtxIn_GF16);
// Apply Gauss Jordan algo to revert the matrix
GF16[,] mtxInvert_GF16 = FEC_Matrix.GaussJordan(mtxWithID_GF16);
// Remove the ID on the left
GF16[,] mtxInvertStripped_GF16 = FEC_Matrix.StripIdentity(mtxInvert_GF16);
// Return the inverted matrix
return(mtxInvertStripped_GF16);
}
// write the data into the provided RtpPacket's BufferChunks in the recovery[]
/// <summary>
/// Decode and insert in place the recovered packet(s) lost in the data BufferChunk array
/// </summary>
/// <param name="data">Array of BufferChunk that represents the data received including
/// checksum (with a null entrie for each packet lost)</param>
/// <param name="checksum">Number of checksum used to encode</param>
/// <param name="recovery">Array of BufferChunk to store the recovered data packets</param>
public void Decode(BufferChunk[] data, int checksum, BufferChunk[] recovery)
{
// TODO: We should first validate that #null entries in data BufferChunk
// array is equal to recovery.Length and is also equal to checksum
// Generate a decoding matrix from the BufferChunk array of packet received
GF16[,] mtxDecode = FEC_BufferChunk.DecodingMatrixGeneration(data, checksum);
// Invert the decoding matrix
// TODO: To optimize we could generate the invert matrix only with the
// row needed for decoding missing packets instead of the full inverted matrix
GF16[,] mtxDecodeInverted = FEC_Matrix.Invert(mtxDecode);
// Decode by multiplying the received data BufferChunk array
// with the inverted decoding matrix. The operations are done over
// GF16 (Galois Field 16 bits)
FEC_BufferChunk.DecodeRS(checksum, data, mtxDecodeInverted, recovery);
}
