76 lines
1.9 KiB
C
76 lines
1.9 KiB
C
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#pragma once
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// We should template this to use __popcnt64 if available
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// but that is premature optimization
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inline uint64_t bitcount(uint64_t c) {
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c = c - ((c >> 1) & 0x5555555555555555);
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c = ((c >> 2) & 0x3333333333333333) +
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(c & 0x3333333333333333);
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c = ((c >> 4) + c) & 0x0F0F0F0F0F0F0F0F;
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c = ((c >> 8) + c) & 0x00FF00FF00FF00FF;
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c = ((c >> 16) + c) & 0x0000FFFF0000FFFF;
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c = ((c >> 32) + c) & 0x00000000FFFFFFFF;
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return c;
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}
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// http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog "Bit Hacks"
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// Find ⌊log2⌋
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// We should template this to use lzcnt64, __builtin_clz or _BitScanReverse if available,
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// but that is premature optimization.
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inline auto rounded_log2(uint32_t v) {
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// This algorithm extends to 64 bits, by adding a step, shrinks to sixteen bits by removing a step.
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decltype(v) r{ 0 }, s;
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// This redundant initialization and redundant |= of r can be eliminated,
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// but eliminating it obfuscates the simplicity of the algorithm.
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s = (v > 0xFFFF) << 4; v >>= s; r |= s;
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s = (v > 0x00FF) << 3; v >>= s; r |= s;
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s = (v > 0x000F) << 2; v >>= s; r |= s;
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s = (v > 0x0003) << 1; v >>= s; r |= s;
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r |= (v >> 1);
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// result of ⌊log2(v)⌋ is in r
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return r;
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}
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// For trailing bits, consider int __builtin_ctz (unsigned int x)
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// http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightLinear
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// Count the consecutive trailing zero bits
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inline auto trailing_zero_bits(uint64_t v) {
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unsigned int c;
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if (v & 0x3F) {
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v = (v ^ (v - 1)) >> 1; // Set v's trailing 0s to 1s and zero rest
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for (c = 0; v; c++) {
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v >>= 1;
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}
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}
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else {
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c = 1;
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if ((v & 0xffffffff) == 0) {
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v >>= 32;
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c += 32;
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}
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if ((v & 0xffff) == 0) {
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v >>= 16;
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c += 16;
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}
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if ((v & 0xff) == 0){
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v >>= 8;
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c += 8;
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}
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if ((v & 0xf) == 0){
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v >>= 4;
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c += 4;
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}
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if ((v & 0x3) == 0) {
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v >>= 2;
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c += 2;
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}
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if ((v & 0x1) == 0) {
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v >>= 1;
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c += 1;
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}
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c -= v & 0x01;
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}
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return c;
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}
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