> does anyone know if the function /sys/src/cmd/snap/take.c:/^sumr > can return 0 for a buffer that is not all zeros? > > i think it can not, since for sum&1, the next sum at least > has 0x8000 set and otherwise if (sum&1) == 0, then either > sum == 0 or sum>>1 != 0.
>> if(sum & 1) >> sum = 0xffff & ((sum>>1)+*s+0x8000); >> else >> sum = 0xffff & ((sum>>1)+*s); your logic does not account for the addition of *s, which can cause an overflow that clears the 0x8000 bit. ff 80 80 80 80 80 80 80 80 01. > fletcher-16 seems similar except for the extra sum. does anyone > know of any writeups on this algorithm? or at least an example > that shows that sum can return 0 for a non-zeroed buffer? sumr computes the same function as plan 9's sum -r, which is also the same function as seventh edition's sum. i have always assumed the -r stood for "research" as in research unix, although the plan 9 manual describes it as "system v's sum -r" and other manuals describe it as the "historical bsd algorithm." it does seem to have appeared first in v7. russ
