r1_ham84.c

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/******************************************************************
*
*   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.
*/

#include "sr_lpc10.h"

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 (INT input, INT * output, INT * errcnt)
{
  INT i, j, parity;
  INT oerr;


/* $$$ */ oerr = *errcnt;

/*  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;
    }
    }

/* $$$ */ *errcnt = oerr;

}

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