648db22b |
1 | /* |
2 | * Copyright (c) Meta Platforms, Inc. and affiliates. |
3 | * All rights reserved. |
4 | * |
5 | * This source code is licensed under both the BSD-style license (found in the |
6 | * LICENSE file in the root directory of this source tree) and the GPLv2 (found |
7 | * in the COPYING file in the root directory of this source tree). |
8 | */ |
9 | #pragma once |
10 | |
11 | #include "utils/Range.h" |
12 | |
13 | #include <array> |
14 | #include <cstddef> |
15 | #include <cstdint> |
16 | #include <cstdio> |
17 | |
18 | namespace pzstd { |
19 | /** |
20 | * We put a skippable frame before each frame. |
21 | * It contains a skippable frame magic number, the size of the skippable frame, |
22 | * and the size of the next frame. |
23 | * Each skippable frame is exactly 12 bytes in little endian format. |
24 | * The first 8 bytes are for compatibility with the ZSTD format. |
25 | * If we have N threads, the output will look like |
26 | * |
27 | * [0x184D2A50|4|size1] [frame1 of size size1] |
28 | * [0x184D2A50|4|size2] [frame2 of size size2] |
29 | * ... |
30 | * [0x184D2A50|4|sizeN] [frameN of size sizeN] |
31 | * |
32 | * Each sizeX is 4 bytes. |
33 | * |
34 | * These skippable frames should allow us to skip through the compressed file |
35 | * and only load at most N pages. |
36 | */ |
37 | class SkippableFrame { |
38 | public: |
39 | static constexpr std::size_t kSize = 12; |
40 | |
41 | private: |
42 | std::uint32_t frameSize_; |
43 | std::array<std::uint8_t, kSize> data_; |
44 | static constexpr std::uint32_t kSkippableFrameMagicNumber = 0x184D2A50; |
45 | // Could be improved if the size fits in less bytes |
46 | static constexpr std::uint32_t kFrameContentsSize = kSize - 8; |
47 | |
48 | public: |
49 | // Write the skippable frame to data_ in LE format. |
50 | explicit SkippableFrame(std::uint32_t size); |
51 | |
52 | // Read the skippable frame from bytes in LE format. |
53 | static std::size_t tryRead(ByteRange bytes); |
54 | |
55 | ByteRange data() const { |
56 | return {data_.data(), data_.size()}; |
57 | } |
58 | |
59 | // Size of the next frame. |
60 | std::size_t frameSize() const { |
61 | return frameSize_; |
62 | } |
63 | }; |
64 | } |