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[REBOL] Re: subtracting binaries

From: greggirwin:mindspring at: 3-Apr-2004 10:33

Hi Maarten, MK> I tested it some more, and it works like a charm! Thanks for the MK> time-saver. Especially the fact that it works modulo (max value of the MK> supplied binary) is really nice. Great! I wasn't sure if I had overlooked something, or if it was what you wanted. I won't spend time on it now, but eliminating FOR is an obvious speed trap (~30%): add-bitsets: func [a b /local c i n carry] [ carry: 0 c: make bitset! length? a repeat i (length? a) - 1 [ n: i - 1 switch carry + add get-bit a n get-bit b n [ 0 [] 1 [set-bit c n carry: 0] 2 [carry: 1] 3 [set-bit c n carry: 1] ] ] if carry <> 0 [ i: 0 while [bit-set? c i][ clear-bit c i i: i + 1 ] set-bit c i ] c ] MK> BTW: I never really "got" bitsets (shame) so if anybody wants to MK> elaborate on how the various operation on bitsets work... The docs don't MK> mention the bitset! value at all, except for parse (where I used it). I just derived my stuff via experimentation. Occasionally I look at old libraries I've done and port pieces of them. I had an old bit-vector class I pseudo-ported (haven't done bit-matrix yet :) and just played around to see how they worked. Things like UNION, DIFFERENCE, and INTERSECT also work on them, which is nice. Now, if only we had native SHIFT and ROTATE functions... -- Gregg