byte_histogram.h

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#pragma once

 
#include <array>
#include <cstddef>
#include <cstdint>
#include <span>
 
namespace pixie::detail {

class ByteHistogram {
 public:
  void add(std::span<const std::byte> bytes) {
    std::size_t index = 0;
    for (; index + kLaneCount <= bytes.size(); index += kLaneCount) {
      ++partial_[0][std::to_integer<std::uint8_t>(bytes[index])];
      ++partial_[1][std::to_integer<std::uint8_t>(bytes[index + 1])];
      ++partial_[2][std::to_integer<std::uint8_t>(bytes[index + 2])];
      ++partial_[3][std::to_integer<std::uint8_t>(bytes[index + 3])];
    }
    for (; index < bytes.size(); ++index) {
      ++partial_[index & (kLaneCount - 1)]
                [std::to_integer<std::uint8_t>(bytes[index])];
    }
  }

  std::array<std::size_t, 256> counts() const {
    std::array<std::size_t, 256> result{};
    for (std::size_t symbol = 0; symbol < result.size(); ++symbol) {
      for (const auto& lane : partial_) {
        result[symbol] += lane[symbol];
      }
    }
    return result;
  }
 
 private:
  static constexpr std::size_t kLaneCount = 4;
  static_assert((kLaneCount & (kLaneCount - 1)) == 0);
 
  alignas(64) std::array<std::array<std::size_t, 256>, kLaneCount> partial_{};
};
 
}  // namespace pixie::detail