mtrxgen.h

g729 coding ipaddressing

/**************************************************************************
*
* ROUTINE
*				matrixgen
*
* FUNCTION
*				 
*				This routine is used to generate the H matrix and 
*				syndrome table necessary for Hamming encode and decode.  
*				This routine should be called once before calling
*				encodeham and decodeham.
*
* SYNOPSIS
*				subroutine matrixgen(codelength1,codelength2,
*										hmatrix,syndrometable)
*
*	formal 
*
*						data	I/O
*		name			type	type	function
*		-------------------------------------------------------------------
*		codelength1 	int 	i		number of data bits (63)
*		codelength2 	int 	i		number of information bits (57)
*		hmatrix 		int 	o		vector to encode and decode by
*		syndrometable	int 	o		table containing error masks
*
***************************************************************************
*
* DESCRIPTION
*
*		This subroutine is part of a set of subroutines which perform
*		a Generalized Hamming Code.  As you know, Hamming codes are perfect
*		codes and can only detect and correct one error.  We added an overall
*		parity checkbit, which allows us to detect 2 errors.  When 2 errors 
*		are detected, (in subroutine decodeham) no correction attempt is
*		made.  This would most likely result in more errors.  Instead, a flag
*		is sent to the calling program notifying it of multiple errors so
*		that smoothing may be attempted.  The Hamming codes presently 
*		supported by the routines are (63,57), (31,26), (15,11), and
*		shortened variations thereof.  It could be made even more general 
*		by making minor modifications to the decimal to binary output vector 
*		code in the encodeham procedure.  This routine at present will 
*		calculate a maximum of 6 bits.
*
*		Hamming routines consist of the following files:
*
*				matrixgen - generates the hmatrix and sydrometable.
*				encodeham - generates the codeword and overall paritybit.
*				decodeham - recovers infobits, checks for errors, corrects 1
*							error, and sends out flag for smoothing.
*
*		This routine initializes all of the tables necessary to perform
*		the Hamming code (G Matrix, Syndrome Table) .  
*
***************************************************************************
*
* CALLED BY
*
*		celp
*
* CALLS
*
*
***************************************************************************
*		
* REFERENCES
*
*		Lin and Costello:  Error Control Coding
*		Berlekamp:	Algebraic Coding Theory
*
**************************************************************************/

static void matrixgen(int codelength1, int codelength2,
	int a_hmatrix[], int a_syndrometable[])
{
  int i, temp1;

  /*	This is the data necessary to construct the G Matrix and the 
		Syndrome Table.  If a larger code is desired, this table can 
		be easily added to.  All other routines, except the syndrome 
		table construction,  are general enough to calculate any size 
		Hamming Code.													*/

  static const int itemplate[] = { 1,  2,  4,  8, 16, 32};
  static const int ptemplate[] = 
	  { 3,	5,	6,	7,	9, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20,
	   21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 33, 34, 35, 36,
	   37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51,
	   52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63};

  /*	Construct the parity portion of the hmatrix 					*/
  
  for (i = 0; i < codelength2; i++)
	a_hmatrix[i] = ptemplate[i];

  /*	Construct the identity portion of the hmatrix.					*/

  for (i = 0; i < codelength1 - codelength2; i++)
	a_hmatrix[codelength2 + i] = itemplate[i];
	
  /*	Construct the syndrometable.  This routine is rather simple because
		I chose to arrange my G matrix sequentially (Berlekamp method).  
		I placed the parity bits in front in ascending order then added the 
		bits left over in ascending order.	Since our code is linear I can
		get away with this.  If a larger Hamming code is needed, then a new 
		exception must be generated for each parity bit.				*/

  temp1 = 1;
  for (i = 1; i <= codelength1; i++)
  {
	switch (i)
	{
	  case 1:
		a_syndrometable[i - 1] = codelength2 + 1;
		break;
	  case 2:
		a_syndrometable[i - 1] = codelength2 + 2;
		break;
	  case 4:
		a_syndrometable[i - 1] = codelength2 + 3;
		break;
	  case 8:
		a_syndrometable[i - 1] = codelength2 + 4;
		break;
	  case 16:
		a_syndrometable[i - 1] = codelength2 + 5;
		break;
	  case 32:
		a_syndrometable[i - 1] = codelength2 + 6;
		break;
	  default:
		a_syndrometable[i - 1] = temp1++;
	};
  }
}