in reply to Re^4: Detecting whether UV fits into an NV
in thread Detecting whether UV fits into an NV


Nice! I'm glad you got a usful result.

Regarding thinking of that trick: I didn't think of it either, but years ago, I read a document that had a lot of bit-fiddling tricks in it, and remembered that there was a trick for that, and my Google-fu let me dredge it up.


When your only tool is a hammer, all problems look like your thumb.

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Re^6: Detecting whether UV fits into an NV
by syphilis (Bishop) on Mar 06, 2020 at 05:23 UTC
    I didn't think of it either

    It's also worth pointing out that once you've isolated that critical bit, it's still not exactly braindead straightforward as to how best to make use of that info.
    Think of an integer, find its least significant set bit, left-shift its value 53 places, subtract 1, flip all of the bits, and then & that result with the number you first thought of.

    I made a small modification to the sub so that it handled negative and positive IV/UV values.
    I also compounded the guts of the code into 2 lines. (It could be put into 1 line, but I didn't go that far.)
    Here's what I ended up with ... minus explanatory comments.
    int iv_fits_double(SV * t, ...) { dXSARGS; int i, count = 0; for(i = 0; i < items; i++) { IV arg = SvIV(ST(i)); int sign = ( arg > 0 || SvUOK(ST(i)) ) ? 1 : -1; UV valid_bits = ((arg & -arg) << 53) - 1; if(!((arg * sign) & (~valid_bits))) count++; } return count; }