From 48cd2c3f351ff188bc85684b84a91b6e6d17d896 Mon Sep 17 00:00:00 2001 From: Abseil Team Date: Thu, 27 Sep 2018 12:24:54 -0700 Subject: Export of internal Abseil changes. -- 4eacae3ff1b14b1d309e8092185bc10e8a6203cf by Derek Mauro : Release SwissTable - a fast, efficient, cache-friendly hash table. https://www.youtube.com/watch?v=ncHmEUmJZf4 PiperOrigin-RevId: 214816527 -- df8c3dfab3cfb2f4365909a84d0683b193cfbb11 by Derek Mauro : Internal change PiperOrigin-RevId: 214785288 -- 1eabd5266bbcebc33eecc91e5309b751856a75c8 by Abseil Team : Internal change PiperOrigin-RevId: 214722931 -- 2ebbfac950f83146b46253038e7dd7dcde9f2951 by Derek Mauro : Internal change PiperOrigin-RevId: 214701684 GitOrigin-RevId: 4eacae3ff1b14b1d309e8092185bc10e8a6203cf Change-Id: I9ba64e395b22ad7863213d157b8019b082adc19d --- absl/container/internal/hash_function_defaults.h | 148 +++++++++++++++++++++++ 1 file changed, 148 insertions(+) create mode 100644 absl/container/internal/hash_function_defaults.h (limited to 'absl/container/internal/hash_function_defaults.h') diff --git a/absl/container/internal/hash_function_defaults.h b/absl/container/internal/hash_function_defaults.h new file mode 100644 index 00000000..dd6cd8f5 --- /dev/null +++ b/absl/container/internal/hash_function_defaults.h @@ -0,0 +1,148 @@ +// Copyright 2018 The Abseil Authors. +// +// 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. +// +// Define the default Hash and Eq functions for SwissTable containers. +// +// std::hash and std::equal_to are not appropriate hash and equal +// functions for SwissTable containers. There are two reasons for this. +// +// SwissTable containers are power of 2 sized containers: +// +// This means they use the lower bits of the hash value to find the slot for +// each entry. The typical hash function for integral types is the identity. +// This is a very weak hash function for SwissTable and any power of 2 sized +// hashtable implementation which will lead to excessive collisions. For +// SwissTable we use murmur3 style mixing to reduce collisions to a minimum. +// +// SwissTable containers support heterogeneous lookup: +// +// In order to make heterogeneous lookup work, hash and equal functions must be +// polymorphic. At the same time they have to satisfy the same requirements the +// C++ standard imposes on hash functions and equality operators. That is: +// +// if hash_default_eq(a, b) returns true for any a and b of type T, then +// hash_default_hash(a) must equal hash_default_hash(b) +// +// For SwissTable containers this requirement is relaxed to allow a and b of +// any and possibly different types. Note that like the standard the hash and +// equal functions are still bound to T. This is important because some type U +// can be hashed by/tested for equality differently depending on T. A notable +// example is `const char*`. `const char*` is treated as a c-style string when +// the hash function is hash but as a pointer when the hash function is +// hash. +// +#ifndef ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_ +#define ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_ + +#include +#include +#include +#include +#include + +#include "absl/base/config.h" +#include "absl/hash/hash.h" +#include "absl/strings/string_view.h" + +namespace absl { +namespace container_internal { + +// The hash of an object of type T is computed by using absl::Hash. +template +struct HashEq { + using Hash = absl::Hash; + using Eq = std::equal_to; +}; + +struct StringHash { + using is_transparent = void; + + size_t operator()(absl::string_view v) const { + return absl::Hash{}(v); + } +}; + +// Supports heterogeneous lookup for string-like elements. +struct StringHashEq { + using Hash = StringHash; + struct Eq { + using is_transparent = void; + bool operator()(absl::string_view lhs, absl::string_view rhs) const { + return lhs == rhs; + } + }; +}; + +#if defined(HAS_GLOBAL_STRING) +template <> +struct HashEq : StringHashEq {}; +#endif +template <> +struct HashEq : StringHashEq {}; +template <> +struct HashEq : StringHashEq {}; + +// Supports heterogeneous lookup for pointers and smart pointers. +template +struct HashEq { + struct Hash { + using is_transparent = void; + template + size_t operator()(const U& ptr) const { + return absl::Hash{}(HashEq::ToPtr(ptr)); + } + }; + struct Eq { + using is_transparent = void; + template + bool operator()(const A& a, const B& b) const { + return HashEq::ToPtr(a) == HashEq::ToPtr(b); + } + }; + + private: + static const T* ToPtr(const T* ptr) { return ptr; } + template + static const T* ToPtr(const std::unique_ptr& ptr) { + return ptr.get(); + } + template + static const T* ToPtr(const std::shared_ptr& ptr) { + return ptr.get(); + } +}; + +template +struct HashEq> : HashEq {}; +template +struct HashEq> : HashEq {}; + +// This header's visibility is restricted. If you need to access the default +// hasher please use the container's ::hasher alias instead. +// +// Example: typename Hash = typename absl::flat_hash_map::hasher +template +using hash_default_hash = typename container_internal::HashEq::Hash; + +// This header's visibility is restricted. If you need to access the default +// key equal please use the container's ::key_equal alias instead. +// +// Example: typename Eq = typename absl::flat_hash_map::key_equal +template +using hash_default_eq = typename container_internal::HashEq::Eq; + +} // namespace container_internal +} // namespace absl + +#endif // ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_ -- cgit v1.2.3