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