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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
// This file depends on internal implementation details in abseil. In a library upgrade you may have
// to re-write this file significantly.
#include <absl/container/internal/hash_function_defaults.h>
/**
* To be safe we let abseil hash the produced hash one more time to protect ourselves against bad
* hash functions. If you know your hash function is good and can be trusted you get mark it as
* trusted by specializing a template like this:
*
* namespace mongo {
* template <>
* struct IsTrustedHasher<YourHash, YourKey> : std::true_type {};
* }
*/
namespace mongo {
template <typename Key>
using DefaultHasher = absl::container_internal::hash_default_hash<Key>;
template <typename Hasher, typename Key>
struct IsTrustedHasher : std::is_same<Hasher, DefaultHasher<Key>> {};
template <typename Hasher, typename Key>
struct HashImprover : private Hasher {
HashImprover(const Hasher& hasher = Hasher()) : Hasher(hasher) {}
std::size_t operator()(const Key& k) const {
return absl::Hash<std::size_t>{}(Hasher::operator()(k));
}
};
template <typename Hasher, typename Key>
using EnsureTrustedHasher =
std::conditional_t<IsTrustedHasher<Hasher, Key>::value, Hasher, HashImprover<Hasher, Key>>;
} // namespace mongo
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