For a moment, I thought you were referring to me when you said "Fletcher" since it's my name also. :-)
FletchSOD "Mike Ford" <[EMAIL PROTECTED]> wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > -----Original Message----- > > From: Dave Gervais [mailto:[EMAIL PROTECTED]] > > Sent: 09 January 2003 21:36 > > > > Here is the C code. There is a decode function, but I don't > > need it in > > PHP because I have a C program listening to serial port on > > the other end > > that will validate the checksum. > > > > > > /* > > * operator fletcher_encode > > */ > > fletcher_encode( buffer, count ) > > unsigned char* buffer; > > long count; > > { > > int i; > > unsigned char c0 = 0; > > unsigned char c1 = 0; > > * ( buffer + count - 1 ) = 0; > > * ( buffer + count - 2 ) = 0; > > for( i = 0; i < count; i++) > > { > > c0 = c0 + * ( buffer + i ); > > c1 =c1 +c0; > > } > > * ( buffer + count - 2 ) = c0 - c1; > > * ( buffer + count - 1 ) = c1 - 2*c0; > > } > > > > > > My problem with PHP was with the unsigned char. > > Well, as I guess that would effectively be an integer in the range 0-255, I'd just treat it as an integer and reduce it modulo 256 in places where it might overflow that value. > > Exactly how this translates into your PHP code depends on how you're translating the rest of the routine, and especially what you turn buffer into, but the loop might go something like: > > for ($i=0; $i<$count; $i++): > $c0 = ($c0+$buffer[i])%256; > $c1 = ($c1+$c0)%256; > endfor; > > You could also do the modulo 256 reduction by doing a bitwise and with 0xff (or 0377, or 255), of course -- this is likely to be more efficient, and may, depending on your point of view, be more obvious as to what's going on. Then your loop might look like this: > > for ($i=0; $i<$count; $i++): > $c0 = ($c0+$buffer[i])&0xff; > $c1 = ($c1+$c0)&0xff; > endfor; > > Hope this is helpful and sets you off on the right track. > > Cheers! > > Mike > > --------------------------------------------------------------------- > Mike Ford, Electronic Information Services Adviser, > Learning Support Services, Learning & Information Services, > JG125, James Graham Building, Leeds Metropolitan University, > Beckett Park, LEEDS, LS6 3QS, United Kingdom > Email: [EMAIL PROTECTED] > Tel: +44 113 283 2600 extn 4730 Fax: +44 113 283 3211 -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
