Wow... That is clean...

---
James.
 
 

> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Andrew Warren
> Sent: 2005 Apr 06, Wed 19:15
> To: microcontroller discussion list - Public.
> Subject: [PIC] Hamming ECC again... In C, this time
> 
> 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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