Functions for interleaving two or three numbers bitwise.

interleave.cpp

#ifndef UTIL_BITS_HPP
#define UTIL_BITS_HPP

#include <cstddef>
#include <cstdint>
#include <limits>

constexpr uint64_t ileave2(uint32_t x, uint32_t y)
{
    constexpr uint64_t MASKS_2[] = {
        0x5555'5555'5555'5555,
        0x3333'3333'3333'3333,
        0x0F0F'0F0F'0F0F'0F0F,
        0x00FF'00FF'00FF'00FF,
        0x0000'FFFF'0000'FFFF
    };

    uint64_t result = 0;
    uint32_t *nums[] = {&x, &y};

    for (size_t i = 0; i < 2; ++i) {
        uint64_t n = *nums[i];
        for (size_t i = 4; i != std::numeric_limits<size_t>::max(); --i) {
            n |= n << (1 << i);
            n &= MASKS_2[i];
        }
        result |= n << (1 - i);
    }

    return result;
}

static_assert(ileave2(0b1111'1111, 0) == 0b1010'1010'1010'1010);
static_assert(ileave2(0, 0b1'1111'1111) == 0b01'0101'0101'0101'0101);
static_assert(ileave2(0, 0xffff'ffff) == 0x5555'5555'5555'5555);
static_assert(ileave2(0, ileave2(0, 0b11)) == 0b10001);

constexpr uint64_t ileave3(uint32_t x, uint32_t y, uint32_t z)
{
    constexpr size_t numInputs = 3;
    constexpr uint64_t MASKS_2[] = {
      0x9249'2492'4924'9249,
      0x30C3'0C30'C30C'30C3,
      0xF00F'00F0'0F00'F00F,
      0x00FF'0000'FF00'00FF,
      0xFFFF'0000'0000'FFFF
    };

    uint64_t result = 0;
    uint32_t *nums[] = {&x, &y, &z};

    for (size_t i = 0; i < numInputs; ++i) {
       uint64_t n = *nums[i];
       for (size_t i = 4; i != std::numeric_limits<size_t>::max(); --i) {
           const auto shift = (numInputs - 1) * (1 << i);
           n |= n << shift;
           n &= MASKS_2[i];
       }
       result |= n << (numInputs - i - 1);
    }

    return result;
}

static_assert(ileave3(0, 0, 0b1111) == 0b001001001001);
static_assert(ileave3(0b1111, 0, 0) == 0b100100100100);

#endif  // UTIL_BITS_HPP