/// <summary>
/// Encode the length of the packets that are in the data BufferChunk array and place the
/// encoded length over GF16 in the 2 first byte of the checksum packets that are in
/// the result BufferChunk array
/// </summary>
/// <param name="data">The BufferChunk array containing the data packets to get the size</param>
/// <param name="checksum">The number of checksum packets to generate.
/// This number is actually also the number of column of the Vandermonde
/// encoding matrix</param>
/// <param name="encode">The Vandermonde encoding matrix (size min should be
/// Vandermonde #column: checksum, Vandermonde #row: data.Length)
/// </param>
/// <param name="result">The BufferChunk checksum packets array of containing the encoded length (over GF16)
/// in the first 2 bytes</param>
private static void EncodeRSLength(BufferChunk[] data, BufferChunk[] result, UInt16[,] encode)
{
// Get the length once to avoid having the overhead of getting it again inside a inner (performance)
// Note that this value is also used outside of this method, so we could have had a parameter too,
// but I prefer to get it here to avoid inconsistencies
int dataPackets = data.Length;
int checksum = result.Length;
// Note: The size of the Vandermonde matrix used to encode the length is
// Vandermonde #column: checksum, Vandermonde #row: dataPackets
// TODO: Validate if Vandermonde # max column >= checksum and
// Vandermonde #row >= dataPackets
// TODO: Validate that there are no null entries in data and result
// TODO: Validate that all the checksum packet have at least 2 bytes
// Length encoding
for (int encodeColumn = 0; encodeColumn < checksum; encodeColumn++)
{
UInt16 encodeValue = 0;
for (int dataPacket = 0; dataPacket < dataPackets; dataPacket++)
{
// TODO: performance - lengths of BC don't change between loops, store once? JVE
UInt16 dataValue = GF16.Multiply((UInt16)data[dataPacket].Length, encode[encodeColumn, dataPacket]);
encodeValue = GF16.Add(encodeValue, dataValue);
}
result[encodeColumn] += encodeValue;
}
}
/// <summary>
/// Create a Vandermonde matrix of size row x column over GF16
/// </summary>
/// <remarks>
/// The Vandermonde matrix is typically used to create the encoding matrix where:
/// - The number of Columns of the matrix correspond to number of checksum
/// packets.
/// - The number of Rows of the matrix correspond to number of data packets.
/// </remarks>
/// <param name="columns">The number of columns of the Vandermonde matrix</param>
/// <param name="rows">The number of rows of the Vandermode matrix</param>
/// <returns></returns>
public static UInt16[,] CreateVandermondeMatrix(int columns, int rows)
{
// TODO: Add input validation
// 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
UInt16[,] vandermondeMtx = new UInt16[columns, rows];
// Creation of the Vandermonde Matrix over GF16 with the following
// (2^column)^row
// As an example, a 5 x 3 Vandermonde Matrix over GF16 (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
for (int col = 0; col < columns; col++)
{
// multFactor is the number to multiply to get the value in the next row
// for a given column of the Vandermonde matrix
UInt16 multFactor = GF16.Power(2, (uint)col);
for (int row = 0; row < rows; row++)
{
if (row == 0)
{
// Special case the first row (power of zero)
vandermondeMtx[col, row] = 1;
}
else
{
// Each element of the Vandermonde matrix is calculated as (2^column)^row over GF16
// This algorithm uses the previous row to compute the next one to improve
// the performances (instead of recalculating (2^column)^row)
vandermondeMtx[col, row] = GF16.Multiply(vandermondeMtx[col, row - 1], multFactor);
}
}
}
return(vandermondeMtx);
}
/// <summary>
/// Decode to retrieve the missing data packet(s) (null entries) in the data BufferChunk array and place the
/// decoded result over GF16 in order in the recovery BufferChunk array
/// </summary>
/// <param name="data">The data BufferChunk array</param>
/// <param name="checksum">The number of checksum packet(s) that were used to encode</param>
/// <param name="decode">The decoding GF16 matrix</param>
/// <param name="recovery">The recovery BufferChunk array to place the recovered result</param>
private static void DecodeRSData(BufferChunk[] data, int checksum, GF16[,] decode, BufferChunk[] recovery)
{
// Important! For now I assume the following in the firstMatrix:
// - The array is always of size data + checksum
// - #checksum not null entries = #packet lost
// - recovery.length is exactly the number of data packet lost
// - checksum param is # checksum packets that were used to encode
// Reconstruct data
// Get the length once to avoid having the overhead of getting it again inside a inner (performance)
// Note that this value is also used outside of this method, so we could have had a parameter too,
// but I prefer to get it here to avoid inconsitencies
int dataLength = data.Length;
// Scan column after column
int recoveryColumn = 0;
for (int dataColumn = 0; dataColumn < dataLength - checksum; dataColumn++)
{
// recover only the missing column
if (data[dataColumn] == null)
{
int recoveryRowLength = recovery[recoveryColumn].Length;
// For each column fill out the output mtx row after row
for (int recoveryRow = 0; recoveryRow < recoveryRowLength; recoveryRow += 2)
{
int imRowIndex = 0;
// nnDataColumn - not null data index
for (int nnDataColumn = 0; nnDataColumn < dataLength; nnDataColumn++)
{
// Perform the core operation mult with add in GF16
// TODO: We could crunch the column so we avoid this test
if (data[nnDataColumn] != null)
{
if (recoveryRow < data[nnDataColumn].Length)
{
UInt16 currentValue = GF16.Multiply(data[nnDataColumn].GetUInt16(recoveryRow),
decode[dataColumn, imRowIndex].Value);
currentValue = GF16.Add(recovery[recoveryColumn].GetUInt16(recoveryRow), currentValue);
recovery[recoveryColumn].SetUInt16(recoveryRow, currentValue);
}
imRowIndex++;
}
}
}
recoveryColumn++;
}
}
}
/// <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;
}
/// <summary>
/// Remove the identity at the front of the matrix
/// </summary>
/// <param name="matrixIn">The input matrix</param>
/// <returns>Stripped Matrix without the identity at the front</returns>
private static GF16[,] StripIdentity(GF16[,] matrixIn)
{
int rowLength = matrixIn.GetLength(1);
int columnLength = matrixIn.GetLength(0);
GF16[,] matrixOut = new GF16[columnLength - rowLength, rowLength];
for (int row = 0; row < rowLength; row++)
{
for (int column = rowLength; column < columnLength; column++)
{
matrixOut[column - rowLength, row] = matrixIn[column, row];
}
}
return(matrixOut);
}
/// <summary>
/// Add identity to the right side of a matrix
/// </summary>
/// <remarks>This is use when doing Jordan-Gauss elimination</remarks>
/// <param name="matrixIn">The matrix</param>
/// <returns>The new matrix with the identity</returns>
private static GF16[,] AddIdentityRight(GF16[,] matrixIn)
{
Int32 rowLength = matrixIn.GetLength(1);
Int32 columnLength = matrixIn.GetLength(0);
GF16[,] matrixOut = new GF16[columnLength * 2, rowLength];
// Add in identity portion to the top
for (Int32 column = 0; column < columnLength; column++)
{
matrixOut[column + columnLength, column] = (UInt16)1;
}
// Copy the previous matrix to the bottom
MatrixCopyTo(matrixIn, matrixOut);
return(matrixOut);
}
/// <summary>
/// Power is used to create teh Vandermonde matrix
/// </summary>
/// <param name="exponent"></param>
/// <returns></returns>
public GF16 Power(int exponent)
{
if (Value == 0)
{
return(this);
}
if (exponent == 0)
{
return(new GF16((UInt16)1));
}
GF16 original = this;
for (int i = 1; i < exponent; i++)
{
this *= original;
}
return(this);
}
/// <summary>
/// Encode the packets that are in the data BufferChunk array and place the
/// encoded result over GF16 in the checksum packets that are in
/// the result BufferChunk array
/// </summary>
/// <param name="data">The BufferChunk array containing the data packets</param>
/// <param name="checksum">The number of checksum packets to generate.
/// This number is actually also the number of column of the Vandermonde
/// encoding matrix</param>
/// <param name="checksum">The BufferChunk checksum packets array of containing the encoded data (over GF16)
/// after the first 2 bytes (the first 2 bytes are used to encode the length)</param>
/// <param name="encode">The Vandermonde encoding matrix (size min should be
/// Vandermonde #column: checksum, Vandermonde #row: data.Length)
/// </param>
/// <param name="checksumRowsInt16">The number of row (in int 16 chunks) of the checksum packets</param>
private static void EncodeRSData(BufferChunk[] data, BufferChunk[] checksum, UInt16[,] encode, int maxDataLength)
{
// Note: The size of the Vandermonde matrix used to encode the length is
// Vandermonde #column: checksumLength, Vandermonde #row: dataPackets
// TODO: Validate if Vandermonde # max column >= checksum and
// Vandermonde #row >= dataPackets
// TODO: Validate that there are no null entries in data and checksum
// Get the length once to avoid having the overhead of getting it again inside a inner (performance)
int dataPackets = data.Length;
int checksumLength = checksum.Length;
// Note that we scan column after column, so we generate the checksum packets
// one after the other
for (int checksumColumn = 0; checksumColumn < checksumLength; checksumColumn++)
{
// For each column fill out the checksum mtx row after row
for (int checksumRow = 0; checksumRow < maxDataLength; checksumRow += 2)
{
for (int encodeRow = 0; encodeRow < dataPackets; encodeRow++)
{
// If we pass the size of the current data packet, so no operation are required
//
if ((checksumRow) < data[encodeRow].Length)
{
UInt16 currentValue = GF16.Multiply(data[encodeRow].GetUInt16(checksumRow),
encode[checksumColumn, encodeRow]);
currentValue = GF16.Add(checksum[checksumColumn].GetUInt16(checksumRow), currentValue);
checksum[checksumColumn].SetUInt16(checksumRow, currentValue);
}
}
}
}
}
/// <summary>
/// Overload the - operator
/// </summary>
/// <param name="a">First operande</param>
/// <param name="b">Second operande</param>
/// <remarks>
/// The subtraction is the same as the addition
/// because with Galois fields, each number is its
/// own negative. So there is no need to have a Sub
/// method
/// </remarks>
/// <returns>Subrtraction of the 2 Galois Fields (XOR)</returns>
public static GF16 operator -(GF16 a, GF16 b)
{
return(GF16.Add(a.Value, b.Value));
}
/// <summary>
/// Decode to retrieve the length of the missing data packet(s) (null entries) in the
/// data BufferChunk array and set the length of the packets in the recovery BufferChunk array.
/// We have to do that because every data packet could have a different length.
/// </summary>
/// <param name="data">The data BufferChunk array</param>
/// <param name="checksum">The number of checksum packet(s) that were used to encode</param>
/// <param name="decode">The decoding GF16 matrix</param>
/// <param name="recovery">The recovery BufferChunk array to set the length of the recovered packets</param>
private static void DecodeRSLength(BufferChunk[] data, int checksum, GF16[,] decode, BufferChunk[] recovery)
{
// Important! For now I assume the following in the firstMatrix:
// - The array is always of size data + checksum
// - #checksum not null entries = #packet lost
// - recovery.length is exactly the number of data packet lost
// - checksum param is # checksum packets that were used to encode
// Get the length once to avoid having the overhead of getting it again inside a inner (performance)
// Note that this value is also used outside of this method, so we could have had a parameter too,
// but I prefer to get it here to avoid inconsitencies
int dataLength = data.Length;
int recoveryColumn = 0;
for (int dataColumn = 0; dataColumn < dataLength - checksum; dataColumn++)
{
// recover only the missing column
if (data[dataColumn] == null)
{
// Inverted matrix row index
int imRowIndex = 0;
// length of the recovered buffer
UInt16 currentLength = 0;
for (int dataIndex = 0; dataIndex < dataLength; dataIndex++)
{
// Perform the core operation mult with add in GF16
// TODO: We could crunch the column so we avoid this test
if (data[dataIndex] != null)
{
UInt16 length = 0;
if (dataIndex < dataLength - checksum) // For the data part, we get the length of the BufferChunk
{
length = (UInt16)data[dataIndex].Length;
}
else // For the checksum part, the encoded length is inside the first 2 bytes
{
length = data[dataIndex].GetUInt16(0);
}
UInt16 currentValue = GF16.Multiply(length, decode[dataColumn, imRowIndex].Value);
currentLength = GF16.Add(currentLength, currentValue);
imRowIndex++;
} // if
} // for column (do elementary operations)
BufferChunk bc = recovery[recoveryColumn];
bc.Reset(bc.Index, currentLength);
// Reset all of the recovery packets so they have no data
// TODO - Temporary workaround for column based approach - JVE 7/6/2004
bc.Clear();
recoveryColumn++;
} // if
} // for dataColumn
}
/// <summary>
/// Add identity to the right side of a matrix
/// </summary>
/// <remarks>This is use when doing Jordan-Gauss elimination</remarks>
/// <param name="matrixIn">The matrix</param>
/// <returns>The new matrix with the identity</returns>
private static GF16[,] AddIdentityRight (GF16[,] matrixIn)
{
Int32 rowLength = matrixIn.GetLength(1);
Int32 columnLength = matrixIn.GetLength(0);
GF16[,] matrixOut = new GF16[columnLength * 2, rowLength];
// Add in identity portion to the top
for (Int32 column = 0; column < columnLength; column++)
{
matrixOut[column + columnLength, column] = (UInt16)1;
}
// Copy the previous matrix to the bottom
MatrixCopyTo(matrixIn, matrixOut);
return matrixOut;
}
/// <summary>
/// Create the decoding matrix
/// </summary>
/// <param name="nbDataPackets">Number of data packet</param>
/// <param name="nbChecksumPackets">Number of checksum packet</param>
/// <param name="rtpPackets">Array of rtpPackets</param>
/// <returns>The decoding matrix</returns>
/// <example>
/// For example, if you have 3 data packets and 2 checksum
/// packets and get the last data packet, you will get
/// the following matrix
/// 0 1 1
/// 0 1 2
/// 1 1 4
/// </example>
// TODO: To be more generic, we should use System.Array instead of array of BufferChunks
public static GF16[,] DecodingMatrixGeneration(BufferChunk[] dataReceived, int nbChecksumPackets)
{
// nbTotalPackets is the total # rtpPackets to create (data packets and checksum packets)
int nbTotalPackets = dataReceived.Length;
int nbDataPackets = nbTotalPackets - nbChecksumPackets;
// Let's suppose that we lost the 2 first packets
// So [E'] that should have been
// 1 0 0 1 1
// 0 1 0 1 2
// 0 0 1 1 4
// is actually
// 0 1 1
// 0 1 2
// 1 1 4
// The reverted decoding matrix will be multiplied by data received to get all the datas
// for instance [D0..D2] = [D2 C0 C1] x 1/[E']
// => So the number of columns should correspond to the number of data recieved + checksum received
// and the number of rows should correspond of the number of data packet K
// count the number of packets received
int nbPacketsReceived = ReceivedPacketsCount(nbTotalPackets, dataReceived);
// nbPacketsReceived columns, K rows
// Note that the # columns is smaller than the number of packet received
// in case the number of packet lost was smaller than the number than
// the number of checksum packets (we short the matrix: It's like if we lost the last
// checksum(s) packet(s)
GF16[,] mtxEprime_GF16 = new GF16[nbPacketsReceived, nbDataPackets];
// Construct the matrix mtxEprime_GF16 from the rtpPackets received
// Note see also Jay's Decode method
// Loop trough the data rtpPackets and put a 1 on the row of the packet
// received
int foundIndex = 0;
for (int index = 0; index < nbDataPackets; index++)
{
if (dataReceived[index] != null) // not a packet lost
{
// Generate a row in the mtxEprime_GF16 (decode matrix)
if (index < nbDataPackets)
{
// TODO: To be clean, we should have initialized all the other items
// on the row explicitely to zero or make sure their are set them to zero
// when creating the new matrix
mtxEprime_GF16[foundIndex, index] = (UInt16)1;
}
foundIndex++;
// if (foundIndex >= nbPacketsReceived-nbAdditionalPacketsReceived)
// return mtxEprime_GF16;
}
}
// Generate a Vandermonde Mtx for the other items
// TODO: Change that to use the Vandermonde static mtx
for (UInt32 m = 0; m < nbChecksumPackets; m++)
{
// Note: If a checksum packet has been lost, the column is skiped
if (dataReceived[m + nbDataPackets] != null) // not a packet lost
{
for (UInt32 k = 0; k < nbDataPackets; k++)
{
// Each element of the Vandermonde matrix is calculated as (m+1)^k over GF16
// mtx[column, row] = (column+1)^row
// With Vandermonde: 1^x, 2^x, 3^x, ... the code was:
// mtxEprime_GF16[foundIndex, k] = GF16.Power((UInt16)(m+1), (UInt32)k);
// I experimented problems (no pivot found during invertion of Eprime)
// when losing 3 packets (worked fine when losing 2 packets)
// So I changed, to use Vandermonde matrix were mtx[column, row] = (2^column)^row
mtxEprime_GF16[foundIndex, k] = GF16.Power((UInt16)GF16.Power(2, (m)), (UInt32)k);
// TODO: Check if Jay's approach is more efficient: mtxE_GF16[m, k] = GF16.gf_exp[GF16.Modnn(k*m)];
}
foundIndex++;
// if (foundIndex >= nbPacketsReceived-nbAdditionalPacketsReceived)
// return mtxEprime_GF16;
}
} // for
return(mtxEprime_GF16);
}
static GF16()
{
GF16.fillLogTables();
}
/// <summary>
/// Gauss - Jordan elimination.
/// This algorithm takes a matrix and use elementary
/// row operations to put the matrix in a row echelon form. This
/// means that the result matrix will have the identity matrix at the
/// begining.
/// This method can be used in 2 specific scenarios:
/// - To solve a linear system numerically
/// - To invert a matrix
/// </summary>
/// <example>
/// For instance the matrix:
/// 0 1 1 1 0 0
/// 0 1 2 0 1 0
/// 1 1 4 0 0 1
///
/// Will become (over int):
/// [ ID ] [ rest ]
/// 1 0 0 2 -3 1
/// 0 1 0 2 -1 0
/// 0 0 1 -1 1 0
/// </example>
/// <remarks>
/// For the Reed Solomon encoding, Gauss - Jordan elimination
/// is used to invert the decoding matrix. This is needed to
/// recover from packet loss.
/// Important note: The input is modified
/// </remarks>
/// <param name="matrixIn">The matrix to transform in a row echelon form</param>
/// <returns>The matrix in a row echelon from</returns>
private static GF16[,] GaussJordan(GF16[,] matrixIn)
{
// Matrix size variables
int rowLength = matrixIn.GetLength(1);
int columnLength = matrixIn.GetLength(0);
// The number of pivots needed is the min between row/column
// For instance the matrix:
// 0 1 1 1 0 0
// 0 1 2 0 1 0
// 1 1 4 0 0 1
// has 3 pivots
int pivots = Math.Min(rowLength, columnLength);
// Loop through all the pivot columns
for (int pivotColumn = 0; pivotColumn < pivots; pivotColumn++)
{
// TODO: Place Step 1 in a separate method
// Step 1: Find the Pivot Row and reduce the pivot to have a factor of 1
int pivotRow = -1;
// Loop through all the rows to find a pivot for the pivotColumn
for (int row = 0; row < rowLength; row++)
{
// A pivot must be non-zero...
if (matrixIn[pivotColumn, row].Value != 0)
{
// ...and don't pivot on a row that has previous non-zero column entries
// Loop through columns of the row Check is the row found respect the non-zero column rule
bool isRowWithPreviousZero = true;
for (int column = 0; column < pivotColumn; column++)
{
// Check for non-zero
if (matrixIn[column, row].Value != 0)
{
// The row is not a pivot because it has previous non-zero entries
isRowWithPreviousZero = false;
break;
}
}
// If the row found has previous zero, it is a pivot row
if (isRowWithPreviousZero)
{
// The current row is the pivot
pivotRow = row;
GF16 factorPivot = matrixIn[pivotColumn, row];
// Reduce the pivot row to have a factor of 1
for (int column = pivotColumn; column < columnLength; column++)
{
matrixIn[column, row] /= factorPivot;
}
// We can exit the loop through rows in Step 1
break;
}
}
}
// Ensure that a pivot has been found
if (pivotRow == -1)
{
throw new ApplicationException(Strings.NoPivotRowFound);
}
// TODO: Place Step 2 in a separate method
// Step 2: Do elementary operation to all rows (except the pivot row and
// the rows that have zero on the pivot column)
// The final goal is to have a zero for all rows on the column corresponding to the
// pivot column
// Loop trough the rows to do elementary operation (subtraction)
for (int row = 0; row < rowLength; row++)
{
// Note: We skip the pivot row and the rows that has zero on the pivot
// column (no operation requiered)
if ((row != pivotRow) && (matrixIn[pivotColumn, row].Value != 0))
{
// Now, determine the factor by which the pivot row must be added to the target
// row to cancel out the pivot value
GF16 factorOperation = matrixIn[pivotColumn, row];
// Perform the operation for each column of the current row
for (int column = 0; column < columnLength; column++)
{
// Perform a subtraction. The final goal is to have a zero for all rows
// on the column corresponding to the pivot column
matrixIn[column, row] -= factorOperation * matrixIn[column, pivotRow];
}
}
}
}
// At this point the work manipulation is done but we might get the rows
// out of order.
// For instance, the input matrix:
// 0 1 1 1 0 0
// 0 1 2 0 1 0
// 1 1 4 0 0 1
//
// will become the following (over int operation):
// 0 1 0 2 -1 0
// 0 0 1 -1 1 0
// 1 0 0 2 -3 1
//
// so now, we need to swap the row to place them in order to obtain the following:
// [ ID ] [ rest ]
// 1 0 0 2 -3 1
// 0 1 0 2 -1 0
// 0 0 1 -1 1 0
// TODO: Place Step 3 in a separate method
// Step 3: Swap raw into order to have an identity matrix at the front
for (int row = 0; row < rowLength; row++) // can use row < rowLength -1, because last row should always be in order?
{
// Skip rows that are in proper order
if (matrixIn[row, row].Value != 1)
{
// Not in proper order, search down through the matrix
bool found = false;
for (int searchRow = 0; searchRow < rowLength; searchRow++)
{
if (matrixIn[row, searchRow].Value == 1) // found the row we're looking for
{
found = true;
// Do the swap operation
// Save the target row to temporary swap space first
GF16[] swapRowTemp = new GF16[columnLength];
for (int swapRowColumn = 0; swapRowColumn < columnLength; swapRowColumn++)
{
swapRowTemp[swapRowColumn] = matrixIn[swapRowColumn, row];
}
// Replace the target row contents with the found row contents
for (int swapRowColumn = 0; swapRowColumn < columnLength; swapRowColumn++)
{
matrixIn[swapRowColumn, row] = matrixIn[swapRowColumn, searchRow];
}
// Replace the found row contents with the temporary swap space (previous target row contents)
for (int swapRowColumn = 0; swapRowColumn < columnLength; swapRowColumn++)
{
matrixIn[swapRowColumn, searchRow] = swapRowTemp[swapRowColumn];
}
}
}
if (!found)
{
throw new ApplicationException(Strings.SwapRowNotFound);
}
}
}
return(matrixIn);
}
/// <summary>
/// Decode to retrieve the length of the missing data packet(s) (null entries) in the
/// data BufferChunk array and set the length of the packets in the recovery BufferChunk array.
/// We have to do that because every data packet could have a different length.
/// </summary>
/// <param name="data">The data BufferChunk array</param>
/// <param name="checksum">The number of checksum packet(s) that were used to encode</param>
/// <param name="decode">The decoding GF16 matrix</param>
/// <param name="recovery">The recovery BufferChunk array to set the length of the recovered packets</param>
private static void DecodeRSLength(BufferChunk[] data, int checksum, GF16[,] decode, BufferChunk[] recovery)
{
// Important! For now I assume the following in the firstMatrix:
// - The array is always of size data + checksum
// - #checksum not null entries = #packet lost
// - recovery.length is exactly the number of data packet lost
// - checksum param is # checksum packets that were used to encode
// Get the length once to avoid having the overhead of getting it again inside a inner (performance)
// Note that this value is also used outside of this method, so we could have had a parameter too,
// but I prefer to get it here to avoid inconsitencies
int dataLength = data.Length;
int recoveryColumn = 0;
for (int dataColumn = 0; dataColumn < dataLength - checksum; dataColumn++)
{
// recover only the missing column
if (data[dataColumn] == null)
{
// Inverted matrix row index
int imRowIndex = 0;
// length of the recovered buffer
UInt16 currentLength = 0;
for (int dataIndex = 0; dataIndex < dataLength; dataIndex++)
{
// Perform the core operation mult with add in GF16
// TODO: We could crunch the column so we avoid this test
if (data[dataIndex] != null)
{
UInt16 length = 0;
if (dataIndex < dataLength - checksum) // For the data part, we get the length of the BufferChunk
{
length = (UInt16) data[dataIndex].Length;
}
else // For the checksum part, the encoded length is inside the first 2 bytes
{
length = data[dataIndex].GetUInt16(0);
}
UInt16 currentValue = GF16.Multiply(length, decode[dataColumn, imRowIndex].Value);
currentLength = GF16.Add(currentLength, currentValue);
imRowIndex++;
} // if
} // for column (do elementary operations)
BufferChunk bc = recovery[recoveryColumn];
bc.Reset(bc.Index, currentLength);
// Reset all of the recovery packets so they have no data
// TODO - Temporary workaround for column based approach - JVE 7/6/2004
bc.Clear();
recoveryColumn++;
} // if
} // for dataColumn
}
/// <summary>
/// Decode to retrieve the missing data packet(s) (null entries) in the data BufferChunk array and place the
/// decoded result over GF16 in order in the recovery BufferChunk array with the right length (every
/// data packet can have a different length)
/// </summary>
/// <param name="checksum">The number of checksum packet(s) that were used to encode</param>
/// <param name="data">The data BufferChunk array</param>
/// <param name="decode">The decoding GF16 matrix</param>
/// <param name="recovery">The recovery BufferChunk array to place the recovered result</param>
public static void DecodeRS(int checksum, BufferChunk[] data, GF16[,] decode, BufferChunk[] recovery)
{
// TODO: Validation discussed with Jason:
// Check in data array if: the #data lost = # checksum and if this number = recovery.length
// Decode the length and set it to the recovery packets
DecodeRSLength(data, checksum, decode, recovery);
// ... and don't forget to adjust the index of the checksum BufferChunk because
// we don't want to take in account the length anymore
int dataLength = data.Length;
for (int i = dataLength - checksum; i < dataLength; i++)
{
BufferChunk bc = data[i];
if(bc != null)
{
bc.Reset(bc.Index + CFec.SIZE_OVERHEAD, bc.Length - CFec.SIZE_OVERHEAD);
}
}
// Adjust all of the data (and recovery) to a 16bit boundary to avoid byte operations
// Important: This assumes the provided BufferChunks have room to grow to 16bit boundary
Queue paddedBufferChunks = new Queue();
AddPadding(data, checksum, paddedBufferChunks);
AddPadding(recovery, 0, paddedBufferChunks);
// Decode the data and set the result into the recovery packets
DecodeRSData(data, checksum, decode, recovery);
// Reset all the packets back to their initial length
RemovePadding(paddedBufferChunks);
// The caller is responsible for resetting all BufferChunks so there is no need
// to reset the checksum packets
}
/// <summary>
/// Remove the identity at the front of the matrix
/// </summary>
/// <param name="matrixIn">The input matrix</param>
/// <returns>Stripped Matrix without the identity at the front</returns>
private static GF16[,] StripIdentity(GF16[,] matrixIn)
{
int rowLength = matrixIn.GetLength(1);
int columnLength = matrixIn.GetLength(0);
GF16[,] matrixOut = new GF16[columnLength - rowLength, rowLength];
for (int row = 0; row < rowLength; row++)
for (int column = rowLength; column < columnLength; column++)
matrixOut[column - rowLength, row] = matrixIn[column, row];
return matrixOut;
}
/// <summary>
/// Copy a matrix to a destination matrix
/// </summary>
/// <param name="matrixIn">The matrix to copy</param>
/// <param name="matrixDestination">The destination matrix</param>
private static void MatrixCopyTo(GF16[,] matrixIn, GF16[,] matrixDestination)
{
int rowLength = matrixIn.GetLength(1);
int columnLength = matrixIn.GetLength(0);
for (int row = 0; row < rowLength; row++)
for (int column = 0; column < columnLength; column++)
matrixDestination[column, row] = matrixIn[column, row];
}
/// <summary>
/// Gauss - Jordan elimination.
/// This algorithm takes a matrix and use elementary
/// row operations to put the matrix in a row echelon form. This
/// means that the result matrix will have the identity matrix at the
/// begining.
/// This method can be used in 2 specific scenarios:
/// - To solve a linear system numerically
/// - To invert a matrix
/// </summary>
/// <example>
/// For instance the matrix:
/// 0 1 1 1 0 0
/// 0 1 2 0 1 0
/// 1 1 4 0 0 1
///
/// Will become (over int):
/// [ ID ] [ rest ]
/// 1 0 0 2 -3 1
/// 0 1 0 2 -1 0
/// 0 0 1 -1 1 0
/// </example>
/// <remarks>
/// For the Reed Solomon encoding, Gauss - Jordan elimination
/// is used to invert the decoding matrix. This is needed to
/// recover from packet loss.
/// Important note: The input is modified
/// </remarks>
/// <param name="matrixIn">The matrix to transform in a row echelon form</param>
/// <returns>The matrix in a row echelon from</returns>
private static GF16[,] GaussJordan(GF16[,] matrixIn)
{
// Matrix size variables
int rowLength = matrixIn.GetLength(1);
int columnLength = matrixIn.GetLength(0);
// The number of pivots needed is the min between row/column
// For instance the matrix:
// 0 1 1 1 0 0
// 0 1 2 0 1 0
// 1 1 4 0 0 1
// has 3 pivots
int pivots = Math.Min(rowLength, columnLength);
// Loop through all the pivot columns
for (int pivotColumn = 0; pivotColumn < pivots; pivotColumn++)
{
// TODO: Place Step 1 in a separate method
// Step 1: Find the Pivot Row and reduce the pivot to have a factor of 1
int pivotRow = -1;
// Loop through all the rows to find a pivot for the pivotColumn
for (int row = 0; row < rowLength; row++)
{
// A pivot must be non-zero...
if (matrixIn[pivotColumn, row].Value != 0)
{
// ...and don't pivot on a row that has previous non-zero column entries
// Loop through columns of the row Check is the row found respect the non-zero column rule
bool isRowWithPreviousZero = true;
for (int column = 0; column < pivotColumn; column++)
{
// Check for non-zero
if (matrixIn[column, row].Value != 0)
{
// The row is not a pivot because it has previous non-zero entries
isRowWithPreviousZero = false;
break;
}
}
// If the row found has previous zero, it is a pivot row
if (isRowWithPreviousZero)
{
// The current row is the pivot
pivotRow = row;
GF16 factorPivot = matrixIn[pivotColumn, row];
// Reduce the pivot row to have a factor of 1
for(int column = pivotColumn; column < columnLength; column++)
{
matrixIn[column, row] /= factorPivot;
}
// We can exit the loop through rows in Step 1
break;
}
}
}
// Ensure that a pivot has been found
if (pivotRow == -1)
{
throw new ApplicationException("No pivot row found!");
}
// TODO: Place Step 2 in a separate method
// Step 2: Do elementary operation to all rows (except the pivot row and
// the rows that have zero on the pivot column)
// The final goal is to have a zero for all rows on the column corresponding to the
// pivot column
// Loop trough the rows to do elementary operation (subtraction)
for (int row = 0; row < rowLength; row++)
{
// Note: We skip the pivot row and the rows that has zero on the pivot
// column (no operation requiered)
if ((row != pivotRow) && (matrixIn[ pivotColumn, row].Value != 0))
{
// Now, determine the factor by which the pivot row must be added to the target
// row to cancel out the pivot value
GF16 factorOperation = matrixIn[ pivotColumn, row ];
// Perform the operation for each column of the current row
for(int column = 0; column < columnLength; column++)
{
// Perform a subtraction. The final goal is to have a zero for all rows
// on the column corresponding to the pivot column
matrixIn[column, row] -= factorOperation * matrixIn[column, pivotRow];
}
}
}
}
// At this point the work manipulation is done but we might get the rows
// out of order.
// For instance, the input matrix:
// 0 1 1 1 0 0
// 0 1 2 0 1 0
// 1 1 4 0 0 1
//
// will become the following (over int operation):
// 0 1 0 2 -1 0
// 0 0 1 -1 1 0
// 1 0 0 2 -3 1
//
// so now, we need to swap the row to place them in order to obtain the following:
// [ ID ] [ rest ]
// 1 0 0 2 -3 1
// 0 1 0 2 -1 0
// 0 0 1 -1 1 0
// TODO: Place Step 3 in a separate method
// Step 3: Swap raw into order to have an identity matrix at the front
for(int row = 0; row < rowLength; row++) // can use row < rowLength -1, because last row should always be in order?
{
// Skip rows that are in proper order
if (matrixIn[row,row].Value != 1)
{
// Not in proper order, search down through the matrix
bool found = false;
for (int searchRow = 0; searchRow < rowLength; searchRow++)
{
if (matrixIn[row, searchRow].Value == 1) // found the row we're looking for
{
found = true;
// Do the swap operation
// Save the target row to temporary swap space first
GF16[] swapRowTemp = new GF16[columnLength];
for(int swapRowColumn = 0; swapRowColumn < columnLength; swapRowColumn++)
{
swapRowTemp[swapRowColumn] = matrixIn[swapRowColumn, row];
}
// Replace the target row contents with the found row contents
for(int swapRowColumn = 0; swapRowColumn < columnLength; swapRowColumn++)
{
matrixIn[swapRowColumn, row] = matrixIn[swapRowColumn,searchRow];
}
// Replace the found row contents with the temporary swap space (previous target row contents)
for(int swapRowColumn = 0; swapRowColumn < columnLength; swapRowColumn++)
{
matrixIn[swapRowColumn,searchRow] = swapRowTemp[swapRowColumn];
}
}
}
if(!found)
throw new ApplicationException("Swap row not found!");
}
}
return matrixIn;
}
/// <summary>
/// Create the decoding matrix
/// </summary>
/// <param name="nbDataPackets">Number of data packet</param>
/// <param name="nbChecksumPackets">Number of checksum packet</param>
/// <param name="rtpPackets">Array of rtpPackets</param>
/// <returns>The decoding matrix</returns>
/// <example>
/// For example, if you have 3 data packets and 2 checksum
/// packets and get the last data packet, you will get
/// the following matrix
/// 0 1 1
/// 0 1 2
/// 1 1 4
/// </example>
// TODO: To be more generic, we should use System.Array instead of array of BufferChunks
public static GF16[,] DecodingMatrixGeneration(BufferChunk[] dataReceived, int nbChecksumPackets)
{
// nbTotalPackets is the total # rtpPackets to create (data packets and checksum packets)
int nbTotalPackets = dataReceived.Length;
int nbDataPackets = nbTotalPackets - nbChecksumPackets;
// Let's suppose that we lost the 2 first packets
// So [E'] that should have been
// 1 0 0 1 1
// 0 1 0 1 2
// 0 0 1 1 4
// is actually
// 0 1 1
// 0 1 2
// 1 1 4
// The reverted decoding matrix will be multiplied by data received to get all the datas
// for instance [D0..D2] = [D2 C0 C1] x 1/[E']
// => So the number of columns should correspond to the number of data recieved + checksum received
// and the number of rows should correspond of the number of data packet K
// count the number of packets received
int nbPacketsReceived = ReceivedPacketsCount(nbTotalPackets, dataReceived);
// nbPacketsReceived columns, K rows
// Note that the # columns is smaller than the number of packet received
// in case the number of packet lost was smaller than the number than
// the number of checksum packets (we short the matrix: It's like if we lost the last
// checksum(s) packet(s)
GF16[,] mtxEprime_GF16 = new GF16[nbPacketsReceived, nbDataPackets];
// Construct the matrix mtxEprime_GF16 from the rtpPackets received
// Note see also Jay's Decode method
// Loop trough the data rtpPackets and put a 1 on the row of the packet
// received
int foundIndex = 0;
for (int index=0; index < nbDataPackets; index++)
{
if (dataReceived[index] != null) // not a packet lost
{
// Generate a row in the mtxEprime_GF16 (decode matrix)
if (index < nbDataPackets)
{
// TODO: To be clean, we should have initialized all the other items
// on the row explicitely to zero or make sure their are set them to zero
// when creating the new matrix
mtxEprime_GF16[foundIndex, index] = (UInt16)1;
}
foundIndex++;
// if (foundIndex >= nbPacketsReceived-nbAdditionalPacketsReceived)
// return mtxEprime_GF16;
}
}
// Generate a Vandermonde Mtx for the other items
// TODO: Change that to use the Vandermonde static mtx
for (UInt32 m = 0; m < nbChecksumPackets; m++)
{
// Note: If a checksum packet has been lost, the column is skiped
if (dataReceived[m+nbDataPackets] != null) // not a packet lost
{
for (UInt32 k = 0; k < nbDataPackets; k++)
{
// Each element of the Vandermonde matrix is calculated as (m+1)^k over GF16
// mtx[column, row] = (column+1)^row
// With Vandermonde: 1^x, 2^x, 3^x, ... the code was:
// mtxEprime_GF16[foundIndex, k] = GF16.Power((UInt16)(m+1), (UInt32)k);
// I experimented problems (no pivot found during invertion of Eprime)
// when losing 3 packets (worked fine when losing 2 packets)
// So I changed, to use Vandermonde matrix were mtx[column, row] = (2^column)^row
mtxEprime_GF16[foundIndex, k] = GF16.Power((UInt16)GF16.Power(2, (m)), (UInt32)k);
// TODO: Check if Jay's approach is more efficient: mtxE_GF16[m, k] = GF16.gf_exp[GF16.Modnn(k*m)];
}
foundIndex++;
// if (foundIndex >= nbPacketsReceived-nbAdditionalPacketsReceived)
// return mtxEprime_GF16;
}
} // for
return mtxEprime_GF16;
}
/// <summary>
/// Overload the * operator
/// </summary>
/// <param name="a">First operande</param>
/// <param name="b">Second operande</param>
/// <returns>Multiplication of the 2 Galois Fields</returns>
public static GF16 operator *(GF16 a, GF16 b)
{
return(GF16.Multiply(a.Value, b.Value));
}
/// <summary>
/// Decode to retrieve the missing data packet(s) (null entries) in the data BufferChunk array and place the
/// decoded result over GF16 in order in the recovery BufferChunk array
/// </summary>
/// <param name="data">The data BufferChunk array</param>
/// <param name="checksum">The number of checksum packet(s) that were used to encode</param>
/// <param name="decode">The decoding GF16 matrix</param>
/// <param name="recovery">The recovery BufferChunk array to place the recovered result</param>
private static void DecodeRSData(BufferChunk[] data, int checksum, GF16[,] decode, BufferChunk[] recovery)
{
// Important! For now I assume the following in the firstMatrix:
// - The array is always of size data + checksum
// - #checksum not null entries = #packet lost
// - recovery.length is exactly the number of data packet lost
// - checksum param is # checksum packets that were used to encode
// Reconstruct data
// Get the length once to avoid having the overhead of getting it again inside a inner (performance)
// Note that this value is also used outside of this method, so we could have had a parameter too,
// but I prefer to get it here to avoid inconsitencies
int dataLength = data.Length;
// Scan column after column
int recoveryColumn = 0;
for (int dataColumn = 0; dataColumn < dataLength - checksum; dataColumn++)
{
// recover only the missing column
if (data[dataColumn] == null)
{
int recoveryRowLength = recovery[recoveryColumn].Length;
// For each column fill out the output mtx row after row
for (int recoveryRow = 0; recoveryRow < recoveryRowLength; recoveryRow += 2)
{
int imRowIndex = 0;
// nnDataColumn - not null data index
for (int nnDataColumn = 0; nnDataColumn < dataLength; nnDataColumn++)
{
// Perform the core operation mult with add in GF16
// TODO: We could crunch the column so we avoid this test
if (data[nnDataColumn] != null)
{
if(recoveryRow < data[nnDataColumn].Length)
{
UInt16 currentValue = GF16.Multiply(data[nnDataColumn].GetUInt16(recoveryRow),
decode[dataColumn, imRowIndex].Value);
currentValue = GF16.Add(recovery[recoveryColumn].GetUInt16(recoveryRow), currentValue);
recovery[recoveryColumn].SetUInt16(recoveryRow, currentValue);
}
imRowIndex++;
}
}
}
recoveryColumn++;
}
}
}
/// <summary>
/// Overload the / operator
/// </summary>
/// <param name="a">First operande</param>
/// <param name="b">Second operande</param>
/// <returns>a div b in Galois Fields</returns>
public static GF16 operator /(GF16 a, GF16 b)
{
return(GF16.Divide(a.Value, b.Value));
}