ham84.c

this the source code of audio compression standard LPC. It is coded by C.

/******************************************************************
*
*	HAM84 Version 45G
*
******************************************************************
*
*  Hamming 8,4 Decoder - can correct 1 out of seven bits
*   and can detect up to two errors.
*
* INPUT:
*  INPUT  - Seven bit data word ,4 bits parameter and
*           4 bits parity information
* OUTPUTS:
*  OUTPUT - 4 corrected parameter bits
*  ERRCNT - Sums errors detected by Hamming code
*
* This subroutine is entered with an eight bit word in INPUT.
*  The 8th bit is parity and is stripped off.  The remaining 7 bits 
*  address the hamming 8,4 table and the output	OUTPUT from the table
*  gives the 4 bits of corrected data.  If bit 4 is set, no error was
*  detected.  ERRCNT is the number of errors counted.
*/

int dactab[128] = {
	16,0,0,3,0,5,14,7,0,9,14,11,
	14,13,30,14,0,9,2,7,4,7,7,23,9,
	25,10,9,12,9,14,7,0,5,2,11,5,21,
	6,5,8,11,11,27,12,5,14,11,2,1,18,
	2,12,5,2,7,12,9,2,11,28,12,12,15,
	0,3,3,19,4,13,6,3,8,13,10,3,13,29,
	14,13,4,1,10,3,20,4,4,7,10,9,26,
	10,4,13,10,15,8,1,6,3,6,5,22,6,24,
	8,8,11,8,13,6,15,1,17,2,1,4,1,6,
	15,8,1,10,15,12,15,15,31
};
void ham84( input, output, errcnt )
int input, *output, *errcnt;
{
int i, j, parity;



/*  Determine parity of input word 	*/
parity = input & 255;
parity = parity ^ parity/16;
parity = parity ^ parity/4;
parity = parity ^ parity/2;
parity = parity & 1;

i = dactab[ input&127];
*output = i&15;
j = i&16;

if(j!=0) {
/*          No errors detected in seven bits	*/
	if(parity!=0) *errcnt++;
}
else	{
/*          One or two errors detected	*/
	*errcnt++;
	if(parity==0) {
/*             Two errors detected	*/
		*errcnt++;
		*output = -1;
	}
}


}