FeatherMC/src/util/Math.h

#pragma once

#include "../Common.h"
#include <cstdint>

namespace Feather::Math
{
    // de Bruijn Sequence: B(2, 5) = 0000 0111 0111 1100 1011 0101 0011 0001. Contains all 32 distinct 5-bit strings.
    constexpr uint32_t DE_BRUIJN_SEQ = 0x077CB531U;

    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};

    // Is x a power of two?
    inline constexpr bool IsPowerOf2(int x)
    {
        return x != 0 && (x & (x - 1)) == 0;
    }

    // Smallest number 2^z that is greater than or equal to x
    // Precondition: x > 0
    inline constexpr int RoundToPowerOf2(int x)
    {
        x--;

        x |= x >> 1;
        x |= x >> 2;
        x |= x >> 4;
        x |= x >> 8;
        x |= x >> 16;
        return x + 1;
    }

    inline constexpr int CeilLog2(int x)
    {
        // Ensure x is power of 2    
        x = IsPowerOf2(x) ? x : RoundToPowerOf2(x);
        
        // The de Bruijn sequence contains all 32 distinct 5-bit strings
        // 27 is 32 - 5 and 0x1F (31) is the lower 5 bits 0001 1111
        return MULTIPLY_DE_BRUIJN_BIT_POSITION[(int)((int64_t)x * DE_BRUIJN_SEQ >> 27) & 0x1F];
    }

    // log base 2
    inline constexpr int Log2(int x)
    {
        return CeilLog2(x) - (IsPowerOf2(x) ? 0 : 1);
    }

    // Divide and round up
    template<typename T>
    inline constexpr T CeilDiv(const T a, const T b)
    {
        return (a / b) + (a % b != 0);
    }
    
}