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Diffstat (limited to 'absl/hash/hash.h')
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diff --git a/absl/hash/hash.h b/absl/hash/hash.h new file mode 100644 index 00000000..2c8982b8 --- /dev/null +++ b/absl/hash/hash.h @@ -0,0 +1,314 @@ +// 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. +// +// ----------------------------------------------------------------------------- +// File: hash.h +// ----------------------------------------------------------------------------- +// +// This header file defines the Abseil `hash` library and the Abseil hashing +// framework. This framework consists of the following: +// +// * The `absl::Hash` functor, which is used to invoke the hasher within the +// Abseil hashing framework. `absl::Hash<T>` supports most basic types and +// a number of Abseil types out of the box. +// * `AbslHashValue`, an extension point that allows you to extend types to +// support Abseil hashing without requiring you to define a hashing +// algorithm. +// * `HashState`, a type-erased class which implements the manipulation of the +// hash state (H) itself, contains member functions `combine()` and +// `combine_contiguous()`, which you can use to contribute to an existing +// hash state when hashing your types. +// +// Unlike `std::hash` or other hashing frameworks, the Abseil hashing framework +// provides most of its utility by abstracting away the hash algorithm (and its +// implementation) entirely. Instead, a type invokes the Abseil hashing +// framework by simply combining its state with the state of known, hashable +// types. Hashing of that combined state is separately done by `absl::Hash`. +// +// Example: +// +// // Suppose we have a class `Circle` for which we want to add hashing +// class Circle { +// public: +// ... +// private: +// std::pair<int, int> center_; +// int radius_; +// }; +// +// // To add hashing support to `Circle`, we simply need to add an ordinary +// // function `AbslHashValue()`, and return the combined hash state of the +// // existing hash state and the class state: +// +// template <typename H> +// friend H AbslHashValue(H h, const Circle& c) { +// return H::combine(std::move(h), c.center_, c.radius_); +// } +// +// For more information, see Adding Type Support to `absl::Hash` below. +// +#ifndef ABSL_HASH_HASH_H_ +#define ABSL_HASH_HASH_H_ + +#include "absl/hash/internal/hash.h" + +namespace absl { +inline namespace lts_2018_12_18 { + +// ----------------------------------------------------------------------------- +// `absl::Hash` +// ----------------------------------------------------------------------------- +// +// `absl::Hash<T>` is a convenient general-purpose hash functor for any type `T` +// satisfying any of the following conditions (in order): +// +// * T is an arithmetic or pointer type +// * T defines an overload for `AbslHashValue(H, const T&)` for an arbitrary +// hash state `H`. +// - T defines a specialization of `HASH_NAMESPACE::hash<T>` +// - T defines a specialization of `std::hash<T>` +// +// `absl::Hash` intrinsically supports the following types: +// +// * All integral types (including bool) +// * All enum types +// * All floating-point types (although hashing them is discouraged) +// * All pointer types, including nullptr_t +// * std::pair<T1, T2>, if T1 and T2 are hashable +// * std::tuple<Ts...>, if all the Ts... are hashable +// * std::unique_ptr and std::shared_ptr +// * All string-like types including: +// * std::string +// * std::string_view (as well as any instance of std::basic_string that +// uses char and std::char_traits) +// * All the standard sequence containers (provided the elements are hashable) +// * All the standard ordered associative containers (provided the elements are +// hashable) +// * absl types such as the following: +// * absl::string_view +// * absl::InlinedVector +// * absl::FixedArray +// * absl::uint128 +// * absl::Time, absl::Duration, and absl::TimeZone +// +// Note: the list above is not meant to be exhaustive. Additional type support +// may be added, in which case the above list will be updated. +// +// ----------------------------------------------------------------------------- +// absl::Hash Invocation Evaluation +// ----------------------------------------------------------------------------- +// +// When invoked, `absl::Hash<T>` searches for supplied hash functions in the +// following order: +// +// * Natively supported types out of the box (see above) +// * Types for which an `AbslHashValue()` overload is provided (such as +// user-defined types). See "Adding Type Support to `absl::Hash`" below. +// * Types which define a `HASH_NAMESPACE::hash<T>` specialization (aka +// `__gnu_cxx::hash<T>` for gcc/Clang or `stdext::hash<T>` for MSVC) +// * Types which define a `std::hash<T>` specialization +// +// The fallback to legacy hash functions exists mainly for backwards +// compatibility. If you have a choice, prefer defining an `AbslHashValue` +// overload instead of specializing any legacy hash functors. +// +// ----------------------------------------------------------------------------- +// The Hash State Concept, and using `HashState` for Type Erasure +// ----------------------------------------------------------------------------- +// +// The `absl::Hash` framework relies on the Concept of a "hash state." Such a +// hash state is used in several places: +// +// * Within existing implementations of `absl::Hash<T>` to store the hashed +// state of an object. Note that it is up to the implementation how it stores +// such state. A hash table, for example, may mix the state to produce an +// integer value; a testing framework may simply hold a vector of that state. +// * Within implementations of `AbslHashValue()` used to extend user-defined +// types. (See "Adding Type Support to absl::Hash" below.) +// * Inside a `HashState`, providing type erasure for the concept of a hash +// state, which you can use to extend the `absl::Hash` framework for types +// that are otherwise difficult to extend using `AbslHashValue()`. (See the +// `HashState` class below.) +// +// The "hash state" concept contains two member functions for mixing hash state: +// +// * `H::combine(state, values...)` +// +// Combines an arbitrary number of values into a hash state, returning the +// updated state. Note that the existing hash state is move-only and must be +// passed by value. +// +// Each of the value types T must be hashable by H. +// +// NOTE: +// +// state = H::combine(std::move(state), value1, value2, value3); +// +// must be guaranteed to produce the same hash expansion as +// +// state = H::combine(std::move(state), value1); +// state = H::combine(std::move(state), value2); +// state = H::combine(std::move(state), value3); +// +// * `H::combine_contiguous(state, data, size)` +// +// Combines a contiguous array of `size` elements into a hash state, +// returning the updated state. Note that the existing hash state is +// move-only and must be passed by value. +// +// NOTE: +// +// state = H::combine_contiguous(std::move(state), data, size); +// +// need NOT be guaranteed to produce the same hash expansion as a loop +// (it may perform internal optimizations). If you need this guarantee, use a +// loop instead. +// +// ----------------------------------------------------------------------------- +// Adding Type Support to `absl::Hash` +// ----------------------------------------------------------------------------- +// +// To add support for your user-defined type, add a proper `AbslHashValue()` +// overload as a free (non-member) function. The overload will take an +// existing hash state and should combine that state with state from the type. +// +// Example: +// +// template <typename H> +// H AbslHashValue(H state, const MyType& v) { +// return H::combine(std::move(state), v.field1, ..., v.fieldN); +// } +// +// where `(field1, ..., fieldN)` are the members you would use on your +// `operator==` to define equality. +// +// Notice that `AbslHashValue` is not a class member, but an ordinary function. +// An `AbslHashValue` overload for a type should only be declared in the same +// file and namespace as said type. The proper `AbslHashValue` implementation +// for a given type will be discovered via ADL. +// +// Note: unlike `std::hash', `absl::Hash` should never be specialized. It must +// only be extended by adding `AbslHashValue()` overloads. +// +template <typename T> +using Hash = absl::hash_internal::Hash<T>; + +// HashState +// +// A type erased version of the hash state concept, for use in user-defined +// `AbslHashValue` implementations that can't use templates (such as PImpl +// classes, virtual functions, etc.). The type erasure adds overhead so it +// should be avoided unless necessary. +// +// Note: This wrapper will only erase calls to: +// combine_contiguous(H, const unsigned char*, size_t) +// +// All other calls will be handled internally and will not invoke overloads +// provided by the wrapped class. +// +// Users of this class should still define a template `AbslHashValue` function, +// but can use `absl::HashState::Create(&state)` to erase the type of the hash +// state and dispatch to their private hashing logic. +// +// This state can be used like any other hash state. In particular, you can call +// `HashState::combine()` and `HashState::combine_contiguous()` on it. +// +// Example: +// +// class Interface { +// public: +// template <typename H> +// friend H AbslHashValue(H state, const Interface& value) { +// state = H::combine(std::move(state), std::type_index(typeid(*this))); +// value.HashValue(absl::HashState::Create(&state)); +// return state; +// } +// private: +// virtual void HashValue(absl::HashState state) const = 0; +// }; +// +// class Impl : Interface { +// private: +// void HashValue(absl::HashState state) const override { +// absl::HashState::combine(std::move(state), v1_, v2_); +// } +// int v1_; +// string v2_; +// }; +class HashState : public hash_internal::HashStateBase<HashState> { + public: + // HashState::Create() + // + // Create a new `HashState` instance that wraps `state`. All calls to + // `combine()` and `combine_contiguous()` on the new instance will be + // redirected to the original `state` object. The `state` object must outlive + // the `HashState` instance. + template <typename T> + static HashState Create(T* state) { + HashState s; + s.Init(state); + return s; + } + + HashState(const HashState&) = delete; + HashState& operator=(const HashState&) = delete; + HashState(HashState&&) = default; + HashState& operator=(HashState&&) = default; + + // HashState::combine() + // + // Combines an arbitrary number of values into a hash state, returning the + // updated state. + using HashState::HashStateBase::combine; + + // HashState::combine_contiguous() + // + // Combines a contiguous array of `size` elements into a hash state, returning + // the updated state. + static HashState combine_contiguous(HashState hash_state, + const unsigned char* first, size_t size) { + hash_state.combine_contiguous_(hash_state.state_, first, size); + return hash_state; + } + using HashState::HashStateBase::combine_contiguous; + + private: + HashState() = default; + + template <typename T> + static void CombineContiguousImpl(void* p, const unsigned char* first, + size_t size) { + T& state = *static_cast<T*>(p); + state = T::combine_contiguous(std::move(state), first, size); + } + + template <typename T> + void Init(T* state) { + state_ = state; + combine_contiguous_ = &CombineContiguousImpl<T>; + } + + // Do not erase an already erased state. + void Init(HashState* state) { + state_ = state->state_; + combine_contiguous_ = state->combine_contiguous_; + } + + void* state_; + void (*combine_contiguous_)(void*, const unsigned char*, size_t); +}; + +} // inline namespace lts_2018_12_18 +} // namespace absl +#endif // ABSL_HASH_HASH_H_ |