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/*
* Copyright 2015 WebAssembly Community Group participants
*
* 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.
*/
#ifndef wasm_support_hash_h
#define wasm_support_hash_h
#include <functional>
#include <stdint.h>
namespace wasm {
// Computes the digest of `value`.
template<typename T> inline std::size_t hash(const T& value) {
return std::hash<T>{}(value);
}
// Combines two digests into the first digest. Use instead of `rehash` if
// `otherDigest` is another digest and not a `size_t` value. This is also useful
// when you want deterministic behavior across systems, as this method does not
// call std::hash, so it does not depend on the behavior of the local machine's
// C++ standard library implementation.
inline void hash_combine(std::size_t& digest, const std::size_t otherDigest) {
// see: boost/container_hash/hash.hpp
// The constant is the N-bits reciprocal of the golden ratio:
// phi = (1 + sqrt(5)) / 2
#if SIZE_MAX == UINT64_MAX
// trunc(2^64 / phi) = 0x9e3779b97f4a7c15
digest ^= otherDigest + 0x9e3779b97f4a7c15 + (digest << 12) + (digest >> 4);
#else
// trunc(2^32 / phi) = 0x9e3779b9
digest ^= otherDigest + 0x9e3779b9 + (digest << 6) + (digest >> 2);
#endif
}
// Hashes `value` and combines the resulting digest into the existing digest.
// Use instead of `hash_combine` if `value` is not another digest.
template<typename T> inline void rehash(std::size_t& digest, const T& value) {
hash_combine(digest, hash(value));
}
} // namespace wasm
namespace std {
// Hashing pairs is often useful
template<typename T1, typename T2> struct hash<pair<T1, T2>> {
size_t operator()(const pair<T1, T2>& p) const {
auto digest = wasm::hash(p.first);
wasm::rehash(digest, p.second);
return digest;
}
};
// And hashing tuples is useful, too.
template<typename T, typename... Ts> struct hash<tuple<T, Ts...>> {
private:
template<size_t... I>
static void rehash(const tuple<T, Ts...>& tup,
size_t& digest,
std::index_sequence<I...>) {
(wasm::rehash(digest, std::get<1 + I>(tup)), ...);
}
public:
size_t operator()(const tuple<T, Ts...>& tup) const {
auto digest = wasm::hash(std::get<0>(tup));
rehash(tup, digest, std::index_sequence_for<Ts...>{});
return digest;
}
};
} // namespace std
#endif // wasm_support_hash_h