THUAI6/CAPI/cpp/grpc/include/absl/hash/hash.h

// 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
//
//      https://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()`,
//     `combine_contiguous()`, and `combine_unordered()`; and 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`.
//
// One should assume that a hash algorithm is chosen randomly at the start of
// each process.  E.g., `absl::Hash<int>{}(9)` in one process and
// `absl::Hash<int>{}(9)` in another process are likely to differ.
//
// `absl::Hash` may also produce different values from different dynamically
// loaded libraries. For this reason, `absl::Hash` values must never cross
// boundries in dynamically loaded libraries (including when used in types like
// hash containers.)
//
// `absl::Hash` is intended to strongly mix input bits with a target of passing
// an [Avalanche Test](https://en.wikipedia.org/wiki/Avalanche_effect).
//
// 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 a free
//   // (non-member) function `AbslHashValue()`, and return the combined hash
//   // state of the existing hash state and the class state. You can add such a
//   // free function using a friend declaration within the body of the class:
//   class Circle {
//    public:
//     ...
//     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 <tuple>
#include <utility>

#include "absl/functional/function_ref.h"
#include "absl/hash/internal/hash.h"

namespace absl
{
    ABSL_NAMESPACE_BEGIN

    // -----------------------------------------------------------------------------
    // `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 `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:
    //     * absl::Cord
    //     * 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 associative containers (provided the elements are
    //    hashable)
    //  * absl types such as the following:
    //    * absl::string_view
    //    * absl::uint128
    //    * absl::Time, absl::Duration, and absl::TimeZone
    //  * absl containers (provided the elements are hashable) such as the
    //    following:
    //    * absl::flat_hash_set, absl::node_hash_set, absl::btree_set
    //    * absl::flat_hash_map, absl::node_hash_map, absl::btree_map
    //    * absl::btree_multiset, absl::btree_multimap
    //    * absl::InlinedVector
    //    * absl::FixedArray
    //
    // When absl::Hash is used to hash an unordered container with a custom hash
    // functor, the elements are hashed using default absl::Hash semantics, not
    // the custom hash functor.  This is consistent with the behavior of
    // operator==() on unordered containers, which compares elements pairwise with
    // operator==() rather than the custom equality functor.  It is usually a
    // mistake to use either operator==() or absl::Hash on unordered collections
    // that use functors incompatible with operator==() equality.
    //
    // 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 `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 three 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.
    //
    // * `H::combine_unordered(state, begin, end)`
    //
    //    Combines a set of elements denoted by an iterator pair into a hash
    //    state, returning the updated state.  Note that the existing hash
    //    state is move-only and must be passed by value.
    //
    //    Unlike the other two methods, the hashing is order-independent.
    //    This can be used to hash unordered collections.
    //
    // -----------------------------------------------------------------------------
    // 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>;

    // HashOf
    //
    // absl::HashOf() is a helper that generates a hash from the values of its
    // arguments.  It dispatches to absl::Hash directly, as follows:
    //  * HashOf(t) == absl::Hash<T>{}(t)
    //  * HashOf(a, b, c) == HashOf(std::make_tuple(a, b, c))
    //
    // HashOf(a1, a2, ...) == HashOf(b1, b2, ...) is guaranteed when
    //  * The argument lists have pairwise identical C++ types
    //  * a1 == b1 && a2 == b2 && ...
    //
    // The requirement that the arguments match in both type and value is critical.
    // It means that `a == b` does not necessarily imply `HashOf(a) == HashOf(b)` if
    // `a` and `b` have different types. For example, `HashOf(2) != HashOf(2.0)`.
    template<int&... ExplicitArgumentBarrier, typename... Types>
    size_t HashOf(const Types&... values)
    {
        auto tuple = std::tie(values...);
        return absl::Hash<decltype(tuple)>{}(tuple);
    }

    // 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)
    //     RunCombineUnordered(H, CombinerF)
    //
    // 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_;
    //     std::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;

        friend class HashState::HashStateBase;

        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>;
            run_combine_unordered_ = &RunCombineUnorderedImpl<T>;
        }

        template<typename HS>
        struct CombineUnorderedInvoker
        {
            template<typename T, typename ConsumerT>
            void operator()(T inner_state, ConsumerT inner_cb)
            {
                f(HashState::Create(&inner_state),
                  [&](HashState& inner_erased)
                  { inner_cb(inner_erased.Real<T>()); });
            }

            absl::FunctionRef<void(HS, absl::FunctionRef<void(HS&)>)> f;
        };

        template<typename T>
        static HashState RunCombineUnorderedImpl(
            HashState state,
            absl::FunctionRef<void(HashState, absl::FunctionRef<void(HashState&)>)>
                f
        )
        {
            // Note that this implementation assumes that inner_state and outer_state
            // are the same type.  This isn't true in the SpyHash case, but SpyHash
            // types are move-convertible to each other, so this still works.
            T& real_state = state.Real<T>();
            real_state = T::RunCombineUnordered(
                std::move(real_state), CombineUnorderedInvoker<HashState>{f}
            );
            return state;
        }

        template<typename CombinerT>
        static HashState RunCombineUnordered(HashState state, CombinerT combiner)
        {
            auto* run = state.run_combine_unordered_;
            return run(std::move(state), std::ref(combiner));
        }

        // Do not erase an already erased state.
        void Init(HashState* state)
        {
            state_ = state->state_;
            combine_contiguous_ = state->combine_contiguous_;
            run_combine_unordered_ = state->run_combine_unordered_;
        }

        template<typename T>
        T& Real()
        {
            return *static_cast<T*>(state_);
        }

        void* state_;
        void (*combine_contiguous_)(void*, const unsigned char*, size_t);
        HashState (*run_combine_unordered_)(
            HashState state,
            absl::FunctionRef<void(HashState, absl::FunctionRef<void(HashState&)>)>
        );
    };

    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_HASH_HASH_H_