bit_utils.h

#pragma once
#include "gaia/config/config.h"
 
#include <cstdint>
 
#include "gaia/core/span.h"
 
namespace gaia {
  namespace core {
    template <uint32_t BlockBits>
    struct bit_view {
      static constexpr uint32_t MaxValue = (1 << BlockBits) - 1;
 
      std::span<uint8_t> m_data;
 
      void set(uint32_t bitPosition, uint8_t value) noexcept {
        GAIA_ASSERT(bitPosition < (m_data.size() * 8));
        GAIA_ASSERT(value <= MaxValue);
 
        const uint32_t idxByte = bitPosition / 8;
        const uint32_t idxBit = bitPosition % 8;
 
        const uint32_t mask = ~(MaxValue << idxBit);
        m_data[idxByte] = (uint8_t)(((uint32_t)m_data[idxByte] & mask) | ((uint32_t)value << idxBit));
 
        const bool overlaps = idxBit + BlockBits > 8;
        if (overlaps) {
          // Value spans over two bytes
          const uint32_t shift2 = 8U - idxBit;
          const uint32_t mask2 = ~(MaxValue >> shift2);
          m_data[idxByte + 1] = (uint8_t)(((uint32_t)m_data[idxByte + 1] & mask2) | ((uint32_t)value >> shift2));
        }
      }
 
      uint8_t get(uint32_t bitPosition) const noexcept {
        GAIA_ASSERT(bitPosition < (m_data.size() * 8));
 
        const uint32_t idxByte = bitPosition / 8;
        const uint32_t idxBit = bitPosition % 8;
 
        const uint8_t byte1 = (m_data[idxByte] >> idxBit) & MaxValue;
 
        const bool overlaps = idxBit + BlockBits > 8;
        if (overlaps) {
          // Value spans over two bytes
          const uint32_t shift2 = uint8_t(8U - idxBit);
          const uint32_t mask2 = MaxValue >> shift2;
          const uint8_t byte2 = uint8_t(((uint32_t)m_data[idxByte + 1] & mask2) << shift2);
          return byte1 | byte2;
        }
 
        return byte1;
      }
    };
 
    template <typename T>
    inline auto swap_bits(T& mask, uint32_t left, uint32_t right) {
      // Swap the bits in the read-write mask
      const uint32_t b0 = (mask >> left) & 1U;
      const uint32_t b1 = (mask >> right) & 1U;
      // XOR the two bits
      const uint32_t bxor = b0 ^ b1;
      // Put the XOR bits back to their original positions
      const uint32_t m = (bxor << left) | (bxor << right);
      // XOR mask with the original one effectively swapping the bits
      mask = mask ^ (uint8_t)m;
    }
  } // namespace core
} // namespace gaia