47 lines
1.3 KiB
C
47 lines
1.3 KiB
C
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#pragma once
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#include <cstdint>
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namespace Feather::Math
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{
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// de Bruijn Sequence: B(2, 5) = 0000 0111 0111 1100 1011 0101 0011 0001. Contains all 32 distinct 5-bit strings.
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constexpr uint32_t DE_BRUIJN_SEQ = 0x077CB531U;
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constexpr int MULTIPLY_DE_BRUIJN_BIT_POSITION[32] = {0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9};
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// Is x a power of two?
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inline constexpr bool IsPowerOf2(int x)
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{
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return x != 0 && (x & (x - 1)) == 0;
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}
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// Smallest number 2^z that is greater than or equal to x
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// Precondition: x > 0
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inline constexpr int RoundToPowerOf2(int x)
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{
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x--;
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x |= x >> 1;
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x |= x >> 2;
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x |= x >> 4;
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x |= x >> 8;
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x |= x >> 16;
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return x + 1;
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}
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inline constexpr int CeilLog2(int x)
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{
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// Ensure x is power of 2
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x = IsPowerOf2(x) ? x : RoundToPowerOf2(x);
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// The de Bruijn sequence contains all 32 distinct 5-bit strings
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// 27 is 32 - 5 and 0x1F (31) is the lower 5 bits 0001 1111
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return MULTIPLY_DE_BRUIJN_BIT_POSITION[(int)((int64_t)x * DE_BRUIJN_SEQ >> 27) & 0x1F];
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}
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// log base 2
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inline constexpr int Log2(int x)
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{
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return CeilLog2(x) - (IsPowerOf2(x) ? 0 : 1);
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}
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}
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