GCD bug fix #222
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GCD bug fix #222
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The standard-library documentation for `trailingZeroBitCount` states: > If the value is zero, then `trailingZeroBitCount` is equal to `bitWidth`. Thus, if `T` is not a fixed-width integer type, and `x == 0`, and `y` has more trailing zeros than the representation of `x`, then the gcd calculation will be incorrect. In particular, the left-shift by `min(xtz, ytz) == xtz` will not “undo” the earlier right-shift by `ytz`, so the final result will be incorrectly right-shifted by `ytz - xtz`. To fix this I have added an early exit when `x == 0`. I am not sure if this is the optimal solution, and I’m open to alternatives. *** This change ensures that `x != 0` at the top of the loop, so I have also converted it from a `while` to a `repeat-while` loop, in order to eliminate a redundant extra comparison on the first iteration. I don’t know if this actually affects performance, and I’m happy to change it back if that’s preferred.
stephentyrone
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Feb 28, 2022
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The standard-library documentation for
trailingZeroBitCountstates:Thus, if
Tis not a fixed-width integer type, andx == 0, andyhas more trailing zeros than the representation ofx, then the gcd calculation will be incorrect.In particular, the left-shift by
min(xtz, ytz) == xtzwill not “undo” the earlier right-shift byytz, so the final result will be incorrectly right-shifted byytz - xtz.To fix this I have added an early exit when
x == 0. I am not sure if this is the optimal solution, and I’m open to alternatives.This change ensures that
x != 0at the top of the loop, so I have also converted it from awhileto arepeat-whileloop, in order to eliminate a redundant extra comparison on the first iteration.I don’t know if this actually affects performance, and I’m happy to change it back if that’s preferred.