Just wrote this for a friend who needed to do simple error-
correction.  I've previously posted (4,7) Hamming encode/decode 
routines in assembly language, but he needed it in C, so...

It's two small tables and two lines of code.  Maybe someone else will 
find it useful.

    const uint8 encode[16] = {0x00, 0x31, 0x52, 0x63,
                              0x64, 0x55, 0x36, 0x07,
                              0x78, 0x49, 0x2A, 0x1B,
                              0x1C, 0x2D, 0x4E, 0x7F}

    const uint8 fixbits[8] = {0x00, 0x90, 0xA0, 0x81,
                              0xC0, 0x82, 0x84, 0x88}

    uint8 transmit(uint8 x)
    {
        return encode[x];
    }

    uint8 receive(uint8 x)
    {
        return x ^ fixbits[(encode[x & 0x0F] ^ x) >> 4];
    }

The transmit() function takes as input a 4-bit number (0x00-0x0F) and 
returns a 7-bit codeword (original input unchanged in bits 0-3, three 
check bits in bits 4-6, and bit 7 clear).  

The receive() function takes as input a 7-bit codeword (0x00-0x7F) 
with up to two errors.  If the codeword contains no errors, the 
function returns the codeword unchanged (original data in bits 0-3, 
correct check bits in bits 4-6, bit 7 clear).  If the codeword 
contains one error, the function returns a corrected codeword 
(original data in bits 0-3, correct check bits in bits 4-6, but bit 7 
set to indicate that an error was detected).  If the codeword 
contains two errors, the function returns garbage in bits 0-6, but 
with bit 7 set to indicate that an error was detected.       

-Andy

=== Andrew Warren - fastfwd@ix.netcom.com

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