| 16 | namespace { |
| 17 | |
| 18 | class FixedPatternScanner { |
| 19 | public: |
| 20 | explicit FixedPatternScanner(std::span<const uint8_t> pattern) |
| 21 | : pattern_(pattern.begin(), pattern.end()) { |
| 22 | const size_t patternSize = pattern_.size(); |
| 23 | skip_.fill(patternSize == 0 ? 1 : patternSize); |
| 24 | if (patternSize <= 1) return; |
| 25 | |
| 26 | for (size_t i = 0; i + 1 < patternSize; ++i) { |
| 27 | skip_[pattern_[i]] = patternSize - 1 - i; |
| 28 | } |
| 29 | } |
| 30 | |
| 31 | std::optional<size_t> Find(std::span<const uint8_t> bytes) const { |
| 32 | const size_t patternSize = pattern_.size(); |
| 33 | if (patternSize == 0 || bytes.size() < patternSize) return std::nullopt; |
| 34 | |
| 35 | size_t offset = 0; |
| 36 | while (offset <= bytes.size() - patternSize) { |
| 37 | const uint8_t tail = bytes[offset + patternSize - 1]; |
| 38 | if (tail == pattern_[patternSize - 1]) { |
| 39 | size_t i = patternSize - 1; |
| 40 | while (i > 0 && bytes[offset + i - 1] == pattern_[i - 1]) { |
| 41 | --i; |
| 42 | } |
| 43 | if (i == 0) return offset; |
| 44 | } |
| 45 | |
| 46 | offset += skip_[tail]; |
| 47 | } |
| 48 | |
| 49 | return std::nullopt; |
| 50 | } |
| 51 | |
| 52 | size_t PatternSize() const { return pattern_.size(); } |
| 53 | |
| 54 | private: |
| 55 | std::vector<uint8_t> pattern_; |
| 56 | std::array<size_t, 256> skip_{}; |
| 57 | }; |
| 58 | |
| 59 | // Open for overlapped (async) sequential reads. FILE_FLAG_SEQUENTIAL_SCAN biases |
| 60 | // the cache manager toward aggressive readahead and evict-behind; FILE_FLAG_OVERLAPPED |