want fast bit set/extract

D. J. Bernstein djb at cr.yp.to
Sat Apr 17 04:45:45 CEST 2004

I often want to compute floor(n/2^s) mod 2^b, given n,b,s. This should
be extremely fast when b is small (particularly when s is a multiple of
32), even if s is somewhere in the middle of the bits of n.

Similarly, I often want to replace n with n + 2^s m. This should be
extremely fast (assuming few carries) when m is small and n already has
enough space allocated.

Please support these two operations! These are _much_ more useful than
isolated right-shift/left-shift operations.

I'm sending this note to the mailing lists for bignum implementors, for
the leading C bignum software, and for the leading C++ bignum software.
already explains one of the ways that these operations are used.

---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago

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