dropshell release 2025.0513.2134

build_amd64/_deps/zstd-src/contrib/pzstd/SkippableFrame.h

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+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ */
+#pragma once
+
+#include "utils/Range.h"
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+
+namespace pzstd {
+/**
+ * We put a skippable frame before each frame.
+ * It contains a skippable frame magic number, the size of the skippable frame,
+ * and the size of the next frame.
+ * Each skippable frame is exactly 12 bytes in little endian format.
+ * The first 8 bytes are for compatibility with the ZSTD format.
+ * If we have N threads, the output will look like
+ *
+ * [0x184D2A50|4|size1] [frame1 of size size1]
+ * [0x184D2A50|4|size2] [frame2 of size size2]
+ * ...
+ * [0x184D2A50|4|sizeN] [frameN of size sizeN]
+ *
+ * Each sizeX is 4 bytes.
+ *
+ * These skippable frames should allow us to skip through the compressed file
+ * and only load at most N pages.
+ */
+class SkippableFrame {
+ public:
+  static constexpr std::size_t kSize = 12;
+
+ private:
+  std::uint32_t frameSize_;
+  std::array<std::uint8_t, kSize> data_;
+  static constexpr std::uint32_t kSkippableFrameMagicNumber = 0x184D2A50;
+  // Could be improved if the size fits in less bytes
+  static constexpr std::uint32_t kFrameContentsSize = kSize - 8;
+
+ public:
+   // Write the skippable frame to data_ in LE format.
+  explicit SkippableFrame(std::uint32_t size);
+
+  // Read the skippable frame from bytes in LE format.
+  static std::size_t tryRead(ByteRange bytes);
+
+  ByteRange data() const {
+    return {data_.data(), data_.size()};
+  }
+
+  // Size of the next frame.
+  std::size_t frameSize() const {
+    return frameSize_;
+  }
+};
+}