106 lines
3.0 KiB
C
106 lines
3.0 KiB
C
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#include <stdio.h>
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#include <stdlib.h>
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#include <stdint.h>
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static inline unsigned long statistical_prng()
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{
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static unsigned long statistical_prng_state = 2463534242U;
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unsigned long x = statistical_prng_state;
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if (sizeof(long) <= 4) {
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x ^= x << 13;
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x ^= x >> 17;
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x ^= x << 5;
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} else {
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x ^= x << 13;
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x ^= x >> 7;
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x ^= x << 17;
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}
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return statistical_prng_state = x;
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}
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/* returns the position of one bit set in <v>, starting at position <bit>, and
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* searching in other halves if not found. This is intended to be used to
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* report the position of one bit set among several based on a counter or a
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* random generator while preserving a relatively good distribution so that
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* values made of holes in the middle do not see one of the bits around the
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* hole being returned much more often than the other one. It can be seen as a
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* disturbed ffsl() where the initial search starts at bit <bit>. The look up
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* is performed in O(logN) time for N bit words, yielding a bit among 64 in
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* about 16 cycles. Passing value 0 for <v> makes no sense and -1 is returned
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* in this case.
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*/
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int one_among(unsigned long v, int bit)
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{
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/* note, these masks may be produced by ~0UL/((1UL<<scale)+1) but
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* that's more expensive.
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*/
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static const unsigned long halves[] = {
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(unsigned long)0x5555555555555555ULL,
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(unsigned long)0x3333333333333333ULL,
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(unsigned long)0x0F0F0F0F0F0F0F0FULL,
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(unsigned long)0x00FF00FF00FF00FFULL,
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(unsigned long)0x0000FFFF0000FFFFULL,
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(unsigned long)0x00000000FFFFFFFFULL
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};
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unsigned long halfword = ~0UL;
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int scope = 0;
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int mirror;
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int scale;
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if (!v)
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return -1;
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/* we check if the exact bit is set or if it's present in a mirror
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* position based on the current scale we're checking, in which case
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* it's returned with its current (or mirrored) value. Otherwise we'll
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* make sure there's at least one bit in the half we're in, and will
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* scale down to a smaller scope and try again, until we find the
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* closest bit.
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*/
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for (scale = (sizeof(long) > 4) ? 5 : 4; scale >= 0; scale--) {
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halfword >>= (1UL << scale);
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scope |= (1UL << scale);
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mirror = bit ^ (1UL << scale);
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if (v & ((1UL << bit) | (1UL << mirror)))
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return (v & (1UL << bit)) ? bit : mirror;
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if (!((v >> (bit & scope)) & halves[scale] & halfword))
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bit = mirror;
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}
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return bit;
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}
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int main(int argc, char **argv)
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{
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unsigned long mask;
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int bit;
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if (argc < 2) {
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unsigned long long tests = 0;
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int ret;
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while (1) {
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mask = statistical_prng(); // note: cannot be zero
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bit = statistical_prng() % (sizeof(long) * 8);
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ret = one_among(mask, bit);
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if (ret < 0 || !((mask >> ret) & 1))
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printf("###ERR### mask=%#lx bit=%d ret=%d\n", mask, bit, ret);
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if (!(tests & 0xffffff))
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printf("count=%Ld mask=%lx bit=%d ret=%d\n", tests, mask, bit, ret);
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tests++;
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}
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}
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mask = atol(argv[1]);
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if (argc < 3) {
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for (bit = 0; bit < 8*sizeof(long); bit++)
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printf("v %#x bit %d best %d\n", mask, bit, one_among(mask, bit));
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} else {
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bit = atoi(argv[2]);
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printf("v %#x bit %d best %d\n", mask, bit, one_among(mask, bit));
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}
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return 0;
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}
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