hamm7484.c

Hamming（7/4

/*
Here are the functions for Hamming code 7.4 and Extended Hamming code 8.4
encoding and decoding.
For 7.4 code, one error per 7-bit codeword can be corrected.
For 8.4 code, one error per 8-bit codeword can be corrected 
and not less than 2 errors can be detected.

The generator polinomial used: X3+X1+1
Also, X3+X2+1 may be used.

(c)VLV
[e-mail]
*/


#define INVALID_DATA        0xFF

unsigned char const encode_table[16] =
{
0x00,0x0b,0x16,0x1d,0x27,0x2c,0x31,0x3a,
0x45,0x4e,0x53,0x58,0x62,0x69,0x74,0x7f
};

unsigned char const correction_table[8] =
{
0x00,0x01,0x02,0x08,0x04,0x40,0x10,0x20
};

/*
Hamming 7.4 code encoding
input is 4-bit data
output is 7-bit Hamming codeword
*/

unsigned char Encode_74(unsigned char x)
{
return encode_table[x];
}

/*
 Hamming 7.4 code correction and decoding
 input is 7-bit Hamming codeword
 output is 4-bit recovered data
 Corrects 1 error per codeword
*/

unsigned char Decode_74(unsigned char x)
{
return (x^correction_table[x^encode_table[x>>3]])>>3;
}

/*
Parity Extended Hamming 8.4 code encoding
input is 4-bit data
output is 8-bit codeword
*/

unsigned char Encode_84(unsigned char x)
{
unsigned char c,d,p;

c=d=encode_table[x];
p=0;

// Calculate parity

while(d)
 {
 p^=d;
 d>>=1;
 }

return (c<<1)|(p&1);
}

/*
Parity Extended Hamming 8.4 code correction and decoding
input is 8-bit codeword
output is 4-bit decoded data
Corrects 1 error per codeword, detects not less than 2 errors
*/

unsigned char Decode_84(unsigned char x)
{
unsigned char p,s;

// Calculate parity

s=x;
p=0;
while(s)
 {
 p^=s;
 s>>=1;
 }
p&=1;

// Calculate syndrome

s=(x>>1)^encode_table[x>>4];

if(s)
 {
 if(!p) return INVALID_DATA;  // more than 1 error - can't correct

 return (x^(correction_table[s]<<1))>>4;
 }

return x>>4;
}