On Tue, 2 Feb 2021 14:00:08 +0100, Pascal Bourguignon said:
Note: if we have access to the implementation of bignum (eg. if we patch the implementation to provide ffs "natively"), we can optimize even more by merging the two solutions.
(- x) would process the whole bignum, as would (logand x (- x)), but after this operation, we have only 0s on the left of the bignum.
Instead, we can process the bignum word by word (starting with the least significant word), computing: (+ (lognot (bignum-word-ref bignum i)) carry) -> i+1, new-carry and (logand (bignum-word-ref bignum i) (+ (lognot (bignum-word-ref bignum i)) carry)) while we get 0. Once we get a value ≠ 0, it has a single bit set which we can identify with integer-length or looping etc (we process a single word here), and stop processing now, because all the other words will give 0 as result.
Even better, we can just do logbitp ≠ 0 in word-sized chunks. I.e. scan from the least significant word until (bignum-word-ref bignum i) is non-zero and then, as you say, process that non-zero word in some way to find the first 1.