Philip Pemberton <piclist@mit.edu> wrote:

> Nicely done.. Only one thing though:
> 
> >     const uint8 fixbits[8] = {0x00, 0x90, 0xA0, 0x81,
> >                               0xC0, 0x82, 0x84, 0x88}
> > 
> >     uint8 receive(uint8 x)
> >     {
> >         return x ^ fixbits[(encode[x & 0x0F] ^ x) >> 4];
> >     }
> 
> You're declaring fixbits as an 8-byte array. That means your array
> elements are numbered 0 to 7. Now, if x = 10 (say), because you're
> ANDing by 0x0F (to get the low nibble), this code will try and get the
> low 4 bits (range 0-15), meaning it'll try and get entry 10 from
> fixbits[8]. Which doesn't exist. End result? The program jumps off
> into never-never land.

Phil:  

You've misread my code.  If x = 10, then the code will try to get 
entry 10 from the encode[16] array, not from the fixbits[8] array.  

Since all the entries in the encode[] array have bit 7 cleared to 0, 
and x also has bit 7 cleared to 0, the "^x" will leave bit 7 cleared, 
and the ">> 4" is guaranteed to produce a number in the range 0-7... 
And it's THAT number that gets used as an index into the fixbits[8] 
array.

-Andy

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

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