Archive build speedup

include/pixie/detail/byte_histogram.h

@@ -0,0 +1,51 @@
+#pragma once
+
+/**
+ * @file byte_histogram.h
+ * @brief Dependency-reduced histogram construction for byte streams.
+ */
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <span>
+
+namespace pixie::detail {
+
+/** @brief Incremental 256-bin byte histogram with four update lanes. */
+class ByteHistogram {
+ public:
+  /** @brief Add one contiguous chunk to the histogram. */
+  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])];
+    }
+  }
+
+  /** @brief Return the combined symbol counts. */
+  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