1 /* 2 * Copyright (C) 2017 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #include "util/hash/hash.h" 18 19 #include "util/base/macros.h" 20 21 namespace libtextclassifier2 { 22 23 namespace { 24 // Lower-level versions of Get... that read directly from a character buffer 25 // without any bounds checking. 26 inline uint32 DecodeFixed32(const char *ptr) { 27 return ((static_cast<uint32>(static_cast<unsigned char>(ptr[0]))) | 28 (static_cast<uint32>(static_cast<unsigned char>(ptr[1])) << 8) | 29 (static_cast<uint32>(static_cast<unsigned char>(ptr[2])) << 16) | 30 (static_cast<uint32>(static_cast<unsigned char>(ptr[3])) << 24)); 31 } 32 33 // 0xff is in case char is signed. 34 static inline uint32 ByteAs32(char c) { return static_cast<uint32>(c) & 0xff; } 35 } // namespace 36 37 uint32 Hash32(const char *data, size_t n, uint32 seed) { 38 // 'm' and 'r' are mixing constants generated offline. 39 // They're not really 'magic', they just happen to work well. 40 const uint32 m = 0x5bd1e995; 41 const int r = 24; 42 43 // Initialize the hash to a 'random' value 44 uint32 h = static_cast<uint32>(seed ^ n); 45 46 // Mix 4 bytes at a time into the hash 47 while (n >= 4) { 48 uint32 k = DecodeFixed32(data); 49 k *= m; 50 k ^= k >> r; 51 k *= m; 52 h *= m; 53 h ^= k; 54 data += 4; 55 n -= 4; 56 } 57 58 // Handle the last few bytes of the input array 59 switch (n) { 60 case 3: 61 h ^= ByteAs32(data[2]) << 16; 62 TC_FALLTHROUGH_INTENDED; 63 case 2: 64 h ^= ByteAs32(data[1]) << 8; 65 TC_FALLTHROUGH_INTENDED; 66 case 1: 67 h ^= ByteAs32(data[0]); 68 h *= m; 69 } 70 71 // Do a few final mixes of the hash to ensure the last few 72 // bytes are well-incorporated. 73 h ^= h >> 13; 74 h *= m; 75 h ^= h >> 15; 76 return h; 77 } 78 79 } // namespace libtextclassifier2 80
