File: hamm.c

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#include "vt.h"
#include "hamm.h"

// table to decode hamm8/4 encoded bytes.
// the low 4 bits are the (corrected) data bits
// bit 8 is set if there was a single bit error
// bit 12 is set if there was an uncorrectable error

// the idea: you may add up to 15 words and get the
// number of single bit errors in b8-b11 and the number
// of double errors in b12-b15

static unsigned short hammtab[256] =
{
    0x0101, 0x100f, 0x0001, 0x0101, 0x100f, 0x0100, 0x0101, 0x100f,
    0x100f, 0x0102, 0x0101, 0x100f, 0x010a, 0x100f, 0x100f, 0x0107,
    0x100f, 0x0100, 0x0101, 0x100f, 0x0100, 0x0000, 0x100f, 0x0100,
    0x0106, 0x100f, 0x100f, 0x010b, 0x100f, 0x0100, 0x0103, 0x100f,
    0x100f, 0x010c, 0x0101, 0x100f, 0x0104, 0x100f, 0x100f, 0x0107,
    0x0106, 0x100f, 0x100f, 0x0107, 0x100f, 0x0107, 0x0107, 0x0007,
    0x0106, 0x100f, 0x100f, 0x0105, 0x100f, 0x0100, 0x010d, 0x100f,
    0x0006, 0x0106, 0x0106, 0x100f, 0x0106, 0x100f, 0x100f, 0x0107,
    0x100f, 0x0102, 0x0101, 0x100f, 0x0104, 0x100f, 0x100f, 0x0109,
    0x0102, 0x0002, 0x100f, 0x0102, 0x100f, 0x0102, 0x0103, 0x100f,
    0x0108, 0x100f, 0x100f, 0x0105, 0x100f, 0x0100, 0x0103, 0x100f,
    0x100f, 0x0102, 0x0103, 0x100f, 0x0103, 0x100f, 0x0003, 0x0103,
    0x0104, 0x100f, 0x100f, 0x0105, 0x0004, 0x0104, 0x0104, 0x100f,
    0x100f, 0x0102, 0x010f, 0x100f, 0x0104, 0x100f, 0x100f, 0x0107,
    0x100f, 0x0105, 0x0105, 0x0005, 0x0104, 0x100f, 0x100f, 0x0105,
    0x0106, 0x100f, 0x100f, 0x0105, 0x100f, 0x010e, 0x0103, 0x100f,
    0x100f, 0x010c, 0x0101, 0x100f, 0x010a, 0x100f, 0x100f, 0x0109,
    0x010a, 0x100f, 0x100f, 0x010b, 0x000a, 0x010a, 0x010a, 0x100f,
    0x0108, 0x100f, 0x100f, 0x010b, 0x100f, 0x0100, 0x010d, 0x100f,
    0x100f, 0x010b, 0x010b, 0x000b, 0x010a, 0x100f, 0x100f, 0x010b,
    0x010c, 0x000c, 0x100f, 0x010c, 0x100f, 0x010c, 0x010d, 0x100f,
    0x100f, 0x010c, 0x010f, 0x100f, 0x010a, 0x100f, 0x100f, 0x0107,
    0x100f, 0x010c, 0x010d, 0x100f, 0x010d, 0x100f, 0x000d, 0x010d,
    0x0106, 0x100f, 0x100f, 0x010b, 0x100f, 0x010e, 0x010d, 0x100f,
    0x0108, 0x100f, 0x100f, 0x0109, 0x100f, 0x0109, 0x0109, 0x0009,
    0x100f, 0x0102, 0x010f, 0x100f, 0x010a, 0x100f, 0x100f, 0x0109,
    0x0008, 0x0108, 0x0108, 0x100f, 0x0108, 0x100f, 0x100f, 0x0109,
    0x0108, 0x100f, 0x100f, 0x010b, 0x100f, 0x010e, 0x0103, 0x100f,
    0x100f, 0x010c, 0x010f, 0x100f, 0x0104, 0x100f, 0x100f, 0x0109,
    0x010f, 0x100f, 0x000f, 0x010f, 0x100f, 0x010e, 0x010f, 0x100f,
    0x0108, 0x100f, 0x100f, 0x0105, 0x100f, 0x010e, 0x010d, 0x100f,
    0x100f, 0x010e, 0x010f, 0x100f, 0x010e, 0x000e, 0x100f, 0x010e,
};



#if 0	// this information is contained in hamm24par bit 5
// simple parity table (sum of 1 bits modulo 2)

static char odd_parity[256] =
{
    0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
    1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
    1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
    0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
    1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
    0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
    0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
    1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
    1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
    0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
    0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
    1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
    0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
    1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
    1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
    0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0
};
#endif



// this table generates the parity checks for hamm24/18 decoding.
// bit 0 is for test A, 1 for B, ...
// thanks to R. Gancarz for this fine table *g*

static char hamm24par[3][256] =
{
    { // parities of first byte
	 0, 33, 34,  3, 35,  2,  1, 32, 36,  5,  6, 39,  7, 38, 37,  4,
	37,  4,  7, 38,  6, 39, 36,  5,  1, 32, 35,  2, 34,  3,  0, 33,
	38,  7,  4, 37,  5, 36, 39,  6,  2, 35, 32,  1, 33,  0,  3, 34,
	 3, 34, 33,  0, 32,  1,  2, 35, 39,  6,  5, 36,  4, 37, 38,  7,
	39,  6,  5, 36,  4, 37, 38,  7,  3, 34, 33,  0, 32,  1,  2, 35,
	 2, 35, 32,  1, 33,  0,  3, 34, 38,  7,  4, 37,  5, 36, 39,  6,
	 1, 32, 35,  2, 34,  3,  0, 33, 37,  4,  7, 38,  6, 39, 36,  5,
	36,  5,  6, 39,  7, 38, 37,  4,  0, 33, 34,  3, 35,  2,  1, 32,
	40,  9, 10, 43, 11, 42, 41,  8, 12, 45, 46, 15, 47, 14, 13, 44,
	13, 44, 47, 14, 46, 15, 12, 45, 41,  8, 11, 42, 10, 43, 40,  9,
	14, 47, 44, 13, 45, 12, 15, 46, 42, 11,  8, 41,  9, 40, 43, 10,
	43, 10,  9, 40,  8, 41, 42, 11, 15, 46, 45, 12, 44, 13, 14, 47,
	15, 46, 45, 12, 44, 13, 14, 47, 43, 10,  9, 40,  8, 41, 42, 11,
	42, 11,  8, 41,  9, 40, 43, 10, 14, 47, 44, 13, 45, 12, 15, 46,
	41,  8, 11, 42, 10, 43, 40,  9, 13, 44, 47, 14, 46, 15, 12, 45,
	12, 45, 46, 15, 47, 14, 13, 44, 40,  9, 10, 43, 11, 42, 41,  8
    }, { // parities of second byte
	 0, 41, 42,  3, 43,  2,  1, 40, 44,  5,  6, 47,  7, 46, 45,  4,
	45,  4,  7, 46,  6, 47, 44,  5,  1, 40, 43,  2, 42,  3,  0, 41,
	46,  7,  4, 45,  5, 44, 47,  6,  2, 43, 40,  1, 41,  0,  3, 42,
	 3, 42, 41,  0, 40,  1,  2, 43, 47,  6,  5, 44,  4, 45, 46,  7,
	47,  6,  5, 44,  4, 45, 46,  7,  3, 42, 41,  0, 40,  1,  2, 43,
	 2, 43, 40,  1, 41,  0,  3, 42, 46,  7,  4, 45,  5, 44, 47,  6,
	 1, 40, 43,  2, 42,  3,  0, 41, 45,  4,  7, 46,  6, 47, 44,  5,
	44,  5,  6, 47,  7, 46, 45,  4,  0, 41, 42,  3, 43,  2,  1, 40,
	48, 25, 26, 51, 27, 50, 49, 24, 28, 53, 54, 31, 55, 30, 29, 52,
	29, 52, 55, 30, 54, 31, 28, 53, 49, 24, 27, 50, 26, 51, 48, 25,
	30, 55, 52, 29, 53, 28, 31, 54, 50, 27, 24, 49, 25, 48, 51, 26,
	51, 26, 25, 48, 24, 49, 50, 27, 31, 54, 53, 28, 52, 29, 30, 55,
	31, 54, 53, 28, 52, 29, 30, 55, 51, 26, 25, 48, 24, 49, 50, 27,
	50, 27, 24, 49, 25, 48, 51, 26, 30, 55, 52, 29, 53, 28, 31, 54,
	49, 24, 27, 50, 26, 51, 48, 25, 29, 52, 55, 30, 54, 31, 28, 53,
	28, 53, 54, 31, 55, 30, 29, 52, 48, 25, 26, 51, 27, 50, 49, 24
    }, { // parities of third byte
	63, 14, 13, 60, 12, 61, 62, 15, 11, 58, 57,  8, 56,  9, 10, 59,
	10, 59, 56,  9, 57,  8, 11, 58, 62, 15, 12, 61, 13, 60, 63, 14,
	 9, 56, 59, 10, 58, 11,  8, 57, 61, 12, 15, 62, 14, 63, 60, 13,
	60, 13, 14, 63, 15, 62, 61, 12,  8, 57, 58, 11, 59, 10,  9, 56,
	 8, 57, 58, 11, 59, 10,  9, 56, 60, 13, 14, 63, 15, 62, 61, 12,
	61, 12, 15, 62, 14, 63, 60, 13,  9, 56, 59, 10, 58, 11,  8, 57,
	62, 15, 12, 61, 13, 60, 63, 14, 10, 59, 56,  9, 57,  8, 11, 58,
	11, 58, 57,  8, 56,  9, 10, 59, 63, 14, 13, 60, 12, 61, 62, 15,
	31, 46, 45, 28, 44, 29, 30, 47, 43, 26, 25, 40, 24, 41, 42, 27,
	42, 27, 24, 41, 25, 40, 43, 26, 30, 47, 44, 29, 45, 28, 31, 46,
	41, 24, 27, 42, 26, 43, 40, 25, 29, 44, 47, 30, 46, 31, 28, 45,
	28, 45, 46, 31, 47, 30, 29, 44, 40, 25, 26, 43, 27, 42, 41, 24,
	40, 25, 26, 43, 27, 42, 41, 24, 28, 45, 46, 31, 47, 30, 29, 44,
	29, 44, 47, 30, 46, 31, 28, 45, 41, 24, 27, 42, 26, 43, 40, 25,
	30, 47, 44, 29, 45, 28, 31, 46, 42, 27, 24, 41, 25, 40, 43, 26,
	43, 26, 25, 40, 24, 41, 42, 27, 31, 46, 45, 28, 44, 29, 30, 47
    }
};



// table to extract the lower 4 bit from hamm24/18 encoded bytes

static char hamm24val[256] =
{
      0,  0,  0,  0,  1,  1,  1,  1,  0,  0,  0,  0,  1,  1,  1,  1,
      2,  2,  2,  2,  3,  3,  3,  3,  2,  2,  2,  2,  3,  3,  3,  3,
      4,  4,  4,  4,  5,  5,  5,  5,  4,  4,  4,  4,  5,  5,  5,  5,
      6,  6,  6,  6,  7,  7,  7,  7,  6,  6,  6,  6,  7,  7,  7,  7,
      8,  8,  8,  8,  9,  9,  9,  9,  8,  8,  8,  8,  9,  9,  9,  9,
     10, 10, 10, 10, 11, 11, 11, 11, 10, 10, 10, 10, 11, 11, 11, 11,
     12, 12, 12, 12, 13, 13, 13, 13, 12, 12, 12, 12, 13, 13, 13, 13,
     14, 14, 14, 14, 15, 15, 15, 15, 14, 14, 14, 14, 15, 15, 15, 15,
      0,  0,  0,  0,  1,  1,  1,  1,  0,  0,  0,  0,  1,  1,  1,  1,
      2,  2,  2,  2,  3,  3,  3,  3,  2,  2,  2,  2,  3,  3,  3,  3,
      4,  4,  4,  4,  5,  5,  5,  5,  4,  4,  4,  4,  5,  5,  5,  5,
      6,  6,  6,  6,  7,  7,  7,  7,  6,  6,  6,  6,  7,  7,  7,  7,
      8,  8,  8,  8,  9,  9,  9,  9,  8,  8,  8,  8,  9,  9,  9,  9,
     10, 10, 10, 10, 11, 11, 11, 11, 10, 10, 10, 10, 11, 11, 11, 11,
     12, 12, 12, 12, 13, 13, 13, 13, 12, 12, 12, 12, 13, 13, 13, 13,
     14, 14, 14, 14, 15, 15, 15, 15, 14, 14, 14, 14, 15, 15, 15, 15
};



// mapping from parity checks made by table hamm24par to error
// results return by hamm24.
// (0 = no error, 0x0100 = single bit error, 0x1000 = double error)

static short hamm24err[64] =
{
    0x0000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000,
    0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000,
    0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000,
    0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000,
    0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100,
    0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100,
    0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100,
    0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000, 0x1000,
};



// mapping from parity checks made by table hamm24par to faulty bit
// in the decoded 18 bit word.

static int hamm24cor[64] =
{
    0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000,
    0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000,
    0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000,
    0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000,
    0x00000, 0x00000, 0x00000, 0x00001, 0x00000, 0x00002, 0x00004, 0x00008,
    0x00000, 0x00010, 0x00020, 0x00040, 0x00080, 0x00100, 0x00200, 0x00400,
    0x00000, 0x00800, 0x01000, 0x02000, 0x04000, 0x08000, 0x10000, 0x20000,
    0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000, 0x00000,
};



int
hamm8(u8 *p, int *err)
{
    int a = hammtab[p[0]];
    *err += a;
    return a & 15;
}

int
hamm16(u8 *p, int *err)
{
    int a = hammtab[p[0]];
    int b = hammtab[p[1]];
    *err += a;
    *err += b;
    return (a & 15) | (b & 15) * 16;
}

int
hamm24(u8 *p, int *err)
{
    int e = hamm24par[0][p[0]] ^ hamm24par[1][p[1]] ^ hamm24par[2][p[2]];
    int x = hamm24val[p[0]] + p[1] % 128 * 16 + p[2] % 128 * 2048;

    *err += hamm24err[e];
    return x ^ hamm24cor[e];
}

int
chk_parity(u8 *p, int n)
{
    int err;

    for (err = 0; n--; p++)
	if (hamm24par[0][*p] & 32)
	    *p &= 0x7f;
	else
	    *p = BAD_CHAR, err++;
    return err;
}