| /* |
| * Copyright (C) 2017 The Android Open Source Project |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); |
| * you may not use this file except in compliance with the License. |
| * You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| #include "util/hash/hash.h" |
| |
| #include "utils/base/macros.h" |
| |
| namespace libtextclassifier2 { |
| |
| namespace { |
| // Lower-level versions of Get... that read directly from a character buffer |
| // without any bounds checking. |
| inline uint32 DecodeFixed32(const char *ptr) { |
| return ((static_cast<uint32>(static_cast<unsigned char>(ptr[0]))) | |
| (static_cast<uint32>(static_cast<unsigned char>(ptr[1])) << 8) | |
| (static_cast<uint32>(static_cast<unsigned char>(ptr[2])) << 16) | |
| (static_cast<uint32>(static_cast<unsigned char>(ptr[3])) << 24)); |
| } |
| |
| // 0xff is in case char is signed. |
| static inline uint32 ByteAs32(char c) { return static_cast<uint32>(c) & 0xff; } |
| } // namespace |
| |
| uint32 Hash32(const char *data, size_t n, uint32 seed) { |
| // 'm' and 'r' are mixing constants generated offline. |
| // They're not really 'magic', they just happen to work well. |
| const uint32 m = 0x5bd1e995; |
| const int r = 24; |
| |
| // Initialize the hash to a 'random' value |
| uint32 h = static_cast<uint32>(seed ^ n); |
| |
| // Mix 4 bytes at a time into the hash |
| while (n >= 4) { |
| uint32 k = DecodeFixed32(data); |
| k *= m; |
| k ^= k >> r; |
| k *= m; |
| h *= m; |
| h ^= k; |
| data += 4; |
| n -= 4; |
| } |
| |
| // Handle the last few bytes of the input array |
| switch (n) { |
| case 3: |
| h ^= ByteAs32(data[2]) << 16; |
| TC3_FALLTHROUGH_INTENDED; |
| case 2: |
| h ^= ByteAs32(data[1]) << 8; |
| TC3_FALLTHROUGH_INTENDED; |
| case 1: |
| h ^= ByteAs32(data[0]); |
| h *= m; |
| } |
| |
| // Do a few final mixes of the hash to ensure the last few |
| // bytes are well-incorporated. |
| h ^= h >> 13; |
| h *= m; |
| h ^= h >> 15; |
| return h; |
| } |
| |
| } // namespace libtextclassifier2 |