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// Copyright 2018 The Cobalt Authors. All Rights Reserved.
//
// 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 "starboard/common/murmurhash2.h"
namespace starboard {
// Aligns data if it isn't already.
uint32_t MurmurHash2_32(const void* src, uint32_t size, uint32_t prev_hash) {
if ((reinterpret_cast<uintptr_t>(src) & 0x3) == 0) {
// Already aligned.
return MurmurHash2_32_Aligned(src, size, prev_hash);
} else {
enum { kInlineSize = 1024 };
if (size <= kInlineSize) {
uint32_t aligned_src[kInlineSize / 4];
memcpy(aligned_src, src, size);
return MurmurHash2_32_Aligned(aligned_src, size, prev_hash);
} else {
scoped_array<char> aligned_src(new char[size]);
memcpy(aligned_src.get(), src, size);
return MurmurHash2_32_Aligned(aligned_src.get(), size, prev_hash);
}
}
}
// Warning - do not change the output of this function because certain platforms
// rely on this hash being stable across runs.
uint32_t MurmurHash2_32_Aligned(const void* src,
uint32_t size,
uint32_t prev_hash) {
// 'm' and 'r' are mixing constants generated offline.
// They're not really 'magic', they just happen to work well.
static const uint32_t m = 0x5bd1e995;
static const int r = 24;
const uint8_t* data = reinterpret_cast<const uint8_t*>(src);
// Initialize the hash to the previous hash value.
uint32_t h = prev_hash;
// Mix 4 bytes at a time into the hash
while (size >= 4) {
uint32_t k = *reinterpret_cast<const uint32_t*>(data);
k *= m;
k ^= k >> r;
k *= m;
h *= m;
h ^= k;
size -= 4;
data += 4;
}
// Handle the last few bytes of the input array
switch (size) {
case 3:
h ^= data[2] << 16;
case 2:
h ^= data[1] << 8;
case 1:
h ^= 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 starboard