chromium/third_party/abseil-cpp/absl/hash/internal/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
// -----------------------------------------------------------------------------
//
#ifndef ABSL_HASH_INTERNAL_HASH_H_
#define ABSL_HASH_INTERNAL_HASH_H_

#ifdef __APPLE__
#include <Availability.h>
#include <TargetConditionals.h>
#endif

#include "absl/base/config.h"

// For feature testing and determining which headers can be included.
#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
#include <version>
#else
#include <ciso646>
#endif

#include <algorithm>
#include <array>
#include <bitset>
#include <cmath>
#include <cstddef>
#include <cstring>
#include <deque>
#include <forward_list>
#include <functional>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <set>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#include "absl/base/internal/unaligned_access.h"
#include "absl/base/port.h"
#include "absl/container/fixed_array.h"
#include "absl/hash/internal/city.h"
#include "absl/hash/internal/low_level_hash.h"
#include "absl/meta/type_traits.h"
#include "absl/numeric/bits.h"
#include "absl/numeric/int128.h"
#include "absl/strings/string_view.h"
#include "absl/types/optional.h"
#include "absl/types/variant.h"
#include "absl/utility/utility.h"

#if defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L && \
    !defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
#include <filesystem>  // NOLINT
#endif

#ifdef ABSL_HAVE_STD_STRING_VIEW
#include <string_view>
#endif

namespace absl {
ABSL_NAMESPACE_BEGIN

class HashState;

namespace hash_internal {

// Internal detail: Large buffers are hashed in smaller chunks.  This function
// returns the size of these chunks.
constexpr size_t PiecewiseChunkSize() { … }

// PiecewiseCombiner
//
// PiecewiseCombiner is an internal-only helper class for hashing a piecewise
// buffer of `char` or `unsigned char` as though it were contiguous.  This class
// provides two methods:
//
//   H add_buffer(state, data, size)
//   H finalize(state)
//
// `add_buffer` can be called zero or more times, followed by a single call to
// `finalize`.  This will produce the same hash expansion as concatenating each
// buffer piece into a single contiguous buffer, and passing this to
// `H::combine_contiguous`.
//
//  Example usage:
//    PiecewiseCombiner combiner;
//    for (const auto& piece : pieces) {
//      state = combiner.add_buffer(std::move(state), piece.data, piece.size);
//    }
//    return combiner.finalize(std::move(state));
class PiecewiseCombiner { … };

// is_hashable()
//
// Trait class which returns true if T is hashable by the absl::Hash framework.
// Used for the AbslHashValue implementations for composite types below.
template <typename T>
struct is_hashable;

// HashStateBase
//
// An internal implementation detail that contains common implementation details
// for all of the "hash state objects" objects generated by Abseil.  This is not
// a public API; users should not create classes that inherit from this.
//
// A hash state object is the template argument `H` passed to `AbslHashValue`.
// It represents an intermediate state in the computation of an unspecified hash
// algorithm. `HashStateBase` provides a CRTP style base class for hash state
// implementations. Developers adding type support for `absl::Hash` should not
// rely on any parts of the state object other than the following member
// functions:
//
//   * HashStateBase::combine()
//   * HashStateBase::combine_contiguous()
//   * HashStateBase::combine_unordered()
//
// A derived hash state class of type `H` must provide a public member function
// with a signature similar to the following:
//
//    `static H combine_contiguous(H state, const unsigned char*, size_t)`.
//
// It must also provide a private template method named RunCombineUnordered.
//
// A "consumer" is a 1-arg functor returning void.  Its argument is a reference
// to an inner hash state object, and it may be called multiple times.  When
// called, the functor consumes the entropy from the provided state object,
// and resets that object to its empty state.
//
// A "combiner" is a stateless 2-arg functor returning void.  Its arguments are
// an inner hash state object and an ElementStateConsumer functor.  A combiner
// uses the provided inner hash state object to hash each element of the
// container, passing the inner hash state object to the consumer after hashing
// each element.
//
// Given these definitions, a derived hash state class of type H
// must provide a private template method with a signature similar to the
// following:
//
//    `template <typename CombinerT>`
//    `static H RunCombineUnordered(H outer_state, CombinerT combiner)`
//
// This function is responsible for constructing the inner state object and
// providing a consumer to the combiner.  It uses side effects of the consumer
// and combiner to mix the state of each element in an order-independent manner,
// and uses this to return an updated value of `outer_state`.
//
// This inside-out approach generates efficient object code in the normal case,
// but allows us to use stack storage to implement the absl::HashState type
// erasure mechanism (avoiding heap allocations while hashing).
//
// `HashStateBase` will provide a complete implementation for a hash state
// object in terms of these two methods.
//
// Example:
//
//   // Use CRTP to define your derived class.
//   struct MyHashState : HashStateBase<MyHashState> {
//       static H combine_contiguous(H state, const unsigned char*, size_t);
//       using MyHashState::HashStateBase::combine;
//       using MyHashState::HashStateBase::combine_contiguous;
//       using MyHashState::HashStateBase::combine_unordered;
//     private:
//       template <typename CombinerT>
//       static H RunCombineUnordered(H state, CombinerT combiner);
//   };
template <typename H>
class HashStateBase { … };

// is_uniquely_represented
//
// `is_uniquely_represented<T>` is a trait class that indicates whether `T`
// is uniquely represented.
//
// A type is "uniquely represented" if two equal values of that type are
// guaranteed to have the same bytes in their underlying storage. In other
// words, if `a == b`, then `memcmp(&a, &b, sizeof(T))` is guaranteed to be
// zero. This property cannot be detected automatically, so this trait is false
// by default, but can be specialized by types that wish to assert that they are
// uniquely represented. This makes them eligible for certain optimizations.
//
// If you have any doubt whatsoever, do not specialize this template.
// The default is completely safe, and merely disables some optimizations
// that will not matter for most types. Specializing this template,
// on the other hand, can be very hazardous.
//
// To be uniquely represented, a type must not have multiple ways of
// representing the same value; for example, float and double are not
// uniquely represented, because they have distinct representations for
// +0 and -0. Furthermore, the type's byte representation must consist
// solely of user-controlled data, with no padding bits and no compiler-
// controlled data such as vptrs or sanitizer metadata. This is usually
// very difficult to guarantee, because in most cases the compiler can
// insert data and padding bits at its own discretion.
//
// If you specialize this template for a type `T`, you must do so in the file
// that defines that type (or in this file). If you define that specialization
// anywhere else, `is_uniquely_represented<T>` could have different meanings
// in different places.
//
// The Enable parameter is meaningless; it is provided as a convenience,
// to support certain SFINAE techniques when defining specializations.
template <typename T, typename Enable = void>
struct is_uniquely_represented : std::false_type { … };

// is_uniquely_represented<unsigned char>
//
// unsigned char is a synonym for "byte", so it is guaranteed to be
// uniquely represented.
template <>
struct is_uniquely_represented<unsigned char> : std::true_type { … };

// is_uniquely_represented for non-standard integral types
//
// Integral types other than bool should be uniquely represented on any
// platform that this will plausibly be ported to.
is_uniquely_represented<Integral, typename std::enable_if<std::is_integral<Integral>::value>::type>;

// is_uniquely_represented<bool>
//
//
template <>
struct is_uniquely_represented<bool> : std::false_type { … };

// hash_bytes()
//
// Convenience function that combines `hash_state` with the byte representation
// of `value`.
template <typename H, typename T>
H hash_bytes(H hash_state, const T& value) { … }

// -----------------------------------------------------------------------------
// AbslHashValue for Basic Types
// -----------------------------------------------------------------------------

// Note: Default `AbslHashValue` implementations live in `hash_internal`. This
// allows us to block lexical scope lookup when doing an unqualified call to
// `AbslHashValue` below. User-defined implementations of `AbslHashValue` can
// only be found via ADL.

// AbslHashValue() for hashing bool values
//
// We use SFINAE to ensure that this overload only accepts bool, not types that
// are convertible to bool.
template <typename H, typename B>
typename std::enable_if<std::is_same<B, bool>::value, H>::type AbslHashValue(
    H hash_state, B value) { … }

// AbslHashValue() for hashing enum values
template <typename H, typename Enum>
typename std::enable_if<std::is_enum<Enum>::value, H>::type AbslHashValue(
    H hash_state, Enum e) { … }
// AbslHashValue() for hashing floating-point values
template <typename H, typename Float>
typename std::enable_if<std::is_same<Float, float>::value ||
                            std::is_same<Float, double>::value,
                        H>::type
AbslHashValue(H hash_state, Float value) { … }

// Long double has the property that it might have extra unused bytes in it.
// For example, in x86 sizeof(long double)==16 but it only really uses 80-bits
// of it. This means we can't use hash_bytes on a long double and have to
// convert it to something else first.
template <typename H, typename LongDouble>
typename std::enable_if<std::is_same<LongDouble, long double>::value, H>::type
AbslHashValue(H hash_state, LongDouble value) { … }

// Without this overload, an array decays to a pointer and we hash that, which
// is not likely to be what the caller intended.
template <typename H, typename T, size_t N>
H AbslHashValue(H hash_state, T (&)[N]) { … }

// AbslHashValue() for hashing pointers
template <typename H, typename T>
std::enable_if_t<std::is_pointer<T>::value, H> AbslHashValue(H hash_state,
                                                             T ptr) { … }

// AbslHashValue() for hashing nullptr_t
template <typename H>
H AbslHashValue(H hash_state, std::nullptr_t) { … }

// AbslHashValue() for hashing pointers-to-member
template <typename H, typename T, typename C>
H AbslHashValue(H hash_state, T C::*ptr) { … }

// -----------------------------------------------------------------------------
// AbslHashValue for Composite Types
// -----------------------------------------------------------------------------

// AbslHashValue() for hashing pairs
template <typename H, typename T1, typename T2>
typename std::enable_if<is_hashable<T1>::value && is_hashable<T2>::value,
                        H>::type
AbslHashValue(H hash_state, const std::pair<T1, T2>& p) { … }

// hash_tuple()
//
// Helper function for hashing a tuple. The third argument should
// be an index_sequence running from 0 to tuple_size<Tuple> - 1.
template <typename H, typename Tuple, size_t... Is>
H hash_tuple(H hash_state, const Tuple& t, absl::index_sequence<Is...>) { … }

// AbslHashValue for hashing tuples
template <typename H, typename... Ts>
#if defined(_MSC_VER)
// This SFINAE gets MSVC confused under some conditions. Let's just disable it
// for now.
H
#else   // _MSC_VER
typename std::enable_if<absl::conjunction<is_hashable<Ts>...>::value, H>::type
#endif  // _MSC_VER
AbslHashValue(H hash_state, const std::tuple<Ts...>& t) { … }

// -----------------------------------------------------------------------------
// AbslHashValue for Pointers
// -----------------------------------------------------------------------------

// AbslHashValue for hashing unique_ptr
template <typename H, typename T, typename D>
H AbslHashValue(H hash_state, const std::unique_ptr<T, D>& ptr) { … }

// AbslHashValue for hashing shared_ptr
template <typename H, typename T>
H AbslHashValue(H hash_state, const std::shared_ptr<T>& ptr) { … }

// -----------------------------------------------------------------------------
// AbslHashValue for String-Like Types
// -----------------------------------------------------------------------------

// AbslHashValue for hashing strings
//
// All the string-like types supported here provide the same hash expansion for
// the same character sequence. These types are:
//
//  - `absl::Cord`
//  - `std::string` (and std::basic_string<T, std::char_traits<T>, A> for
//      any allocator A and any T in {char, wchar_t, char16_t, char32_t})
//  - `absl::string_view`, `std::string_view`, `std::wstring_view`,
//    `std::u16string_view`, and `std::u32_string_view`.
//
// For simplicity, we currently support only strings built on `char`, `wchar_t`,
// `char16_t`, or `char32_t`. This support may be broadened, if necessary, but
// with some caution - this overload would misbehave in cases where the traits'
// `eq()` member isn't equivalent to `==` on the underlying character type.
template <typename H>
H AbslHashValue(H hash_state, absl::string_view str) { … }

// Support std::wstring, std::u16string and std::u32string.
template <typename Char, typename Alloc, typename H,
          typename = absl::enable_if_t<std::is_same<Char, wchar_t>::value ||
                                       std::is_same<Char, char16_t>::value ||
                                       std::is_same<Char, char32_t>::value>>
H AbslHashValue(
    H hash_state,
    const std::basic_string<Char, std::char_traits<Char>, Alloc>& str) { … }

#ifdef ABSL_HAVE_STD_STRING_VIEW

// Support std::wstring_view, std::u16string_view and std::u32string_view.
template <typename Char, typename H,
          typename = absl::enable_if_t<std::is_same<Char, wchar_t>::value ||
                                       std::is_same<Char, char16_t>::value ||
                                       std::is_same<Char, char32_t>::value>>
H AbslHashValue(H hash_state, std::basic_string_view<Char> str) { … }

#endif  // ABSL_HAVE_STD_STRING_VIEW

#if defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L && \
    !defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY) && \
    (!defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) ||        \
     __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000) &&       \
    (!defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) ||         \
     __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500)

#define ABSL_INTERNAL_STD_FILESYSTEM_PATH_HASH_AVAILABLE …

// Support std::filesystem::path. The SFINAE is required because some string
// types are implicitly convertible to std::filesystem::path.
template <typename Path, typename H,
          typename = absl::enable_if_t<
              std::is_same_v<Path, std::filesystem::path>>>
H AbslHashValue(H hash_state, const Path& path) { … }

#endif  // ABSL_INTERNAL_STD_FILESYSTEM_PATH_HASH_AVAILABLE

// -----------------------------------------------------------------------------
// AbslHashValue for Sequence Containers
// -----------------------------------------------------------------------------

// AbslHashValue for hashing std::array
template <typename H, typename T, size_t N>
typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
    H hash_state, const std::array<T, N>& array) { … }

// AbslHashValue for hashing std::deque
template <typename H, typename T, typename Allocator>
typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
    H hash_state, const std::deque<T, Allocator>& deque) { … }

// AbslHashValue for hashing std::forward_list
template <typename H, typename T, typename Allocator>
typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
    H hash_state, const std::forward_list<T, Allocator>& list) { … }

// AbslHashValue for hashing std::list
template <typename H, typename T, typename Allocator>
typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
    H hash_state, const std::list<T, Allocator>& list) { … }

// AbslHashValue for hashing std::vector
//
// Do not use this for vector<bool> on platforms that have a working
// implementation of std::hash. It does not have a .data(), and a fallback for
// std::hash<> is most likely faster.
template <typename H, typename T, typename Allocator>
typename std::enable_if<is_hashable<T>::value && !std::is_same<T, bool>::value,
                        H>::type
AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) { … }

// AbslHashValue special cases for hashing std::vector<bool>

#if defined(ABSL_IS_BIG_ENDIAN) && \
    (defined(__GLIBCXX__) || defined(__GLIBCPP__))

// std::hash in libstdc++ does not work correctly with vector<bool> on Big
// Endian platforms therefore we need to implement a custom AbslHashValue for
// it. More details on the bug:
// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102531
template <typename H, typename T, typename Allocator>
typename std::enable_if<is_hashable<T>::value && std::is_same<T, bool>::value,
                        H>::type
AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) {
  typename H::AbslInternalPiecewiseCombiner combiner;
  for (const auto& i : vector) {
    unsigned char c = static_cast<unsigned char>(i);
    hash_state = combiner.add_buffer(std::move(hash_state), &c, sizeof(c));
  }
  return H::combine(combiner.finalize(std::move(hash_state)), vector.size());
}
#else
// When not working around the libstdc++ bug above, we still have to contend
// with the fact that std::hash<vector<bool>> is often poor quality, hashing
// directly on the internal words and on no other state.  On these platforms,
// vector<bool>{1, 1} and vector<bool>{1, 1, 0} hash to the same value.
//
// Mixing in the size (as we do in our other vector<> implementations) on top
// of the library-provided hash implementation avoids this QOI issue.
template <typename H, typename T, typename Allocator>
typename std::enable_if<is_hashable<T>::value && std::is_same<T, bool>::value,
                        H>::type
AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) { … }
#endif

// -----------------------------------------------------------------------------
// AbslHashValue for Ordered Associative Containers
// -----------------------------------------------------------------------------

// AbslHashValue for hashing std::map
template <typename H, typename Key, typename T, typename Compare,
          typename Allocator>
typename std::enable_if<is_hashable<Key>::value && is_hashable<T>::value,
                        H>::type
AbslHashValue(H hash_state, const std::map<Key, T, Compare, Allocator>& map) { … }

// AbslHashValue for hashing std::multimap
template <typename H, typename Key, typename T, typename Compare,
          typename Allocator>
typename std::enable_if<is_hashable<Key>::value && is_hashable<T>::value,
                        H>::type
AbslHashValue(H hash_state,
              const std::multimap<Key, T, Compare, Allocator>& map) { … }

// AbslHashValue for hashing std::set
template <typename H, typename Key, typename Compare, typename Allocator>
typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
    H hash_state, const std::set<Key, Compare, Allocator>& set) { … }

// AbslHashValue for hashing std::multiset
template <typename H, typename Key, typename Compare, typename Allocator>
typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
    H hash_state, const std::multiset<Key, Compare, Allocator>& set) { … }

// -----------------------------------------------------------------------------
// AbslHashValue for Unordered Associative Containers
// -----------------------------------------------------------------------------

// AbslHashValue for hashing std::unordered_set
template <typename H, typename Key, typename Hash, typename KeyEqual,
          typename Alloc>
typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
    H hash_state, const std::unordered_set<Key, Hash, KeyEqual, Alloc>& s) { … }

// AbslHashValue for hashing std::unordered_multiset
template <typename H, typename Key, typename Hash, typename KeyEqual,
          typename Alloc>
typename std::enable_if<is_hashable<Key>::value, H>::type AbslHashValue(
    H hash_state,
    const std::unordered_multiset<Key, Hash, KeyEqual, Alloc>& s) { … }

// AbslHashValue for hashing std::unordered_set
template <typename H, typename Key, typename T, typename Hash,
          typename KeyEqual, typename Alloc>
typename std::enable_if<is_hashable<Key>::value && is_hashable<T>::value,
                        H>::type
AbslHashValue(H hash_state,
              const std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& s) { … }

// AbslHashValue for hashing std::unordered_multiset
template <typename H, typename Key, typename T, typename Hash,
          typename KeyEqual, typename Alloc>
typename std::enable_if<is_hashable<Key>::value && is_hashable<T>::value,
                        H>::type
AbslHashValue(H hash_state,
              const std::unordered_multimap<Key, T, Hash, KeyEqual, Alloc>& s) { … }

// -----------------------------------------------------------------------------
// AbslHashValue for Wrapper Types
// -----------------------------------------------------------------------------

// AbslHashValue for hashing std::reference_wrapper
template <typename H, typename T>
typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
    H hash_state, std::reference_wrapper<T> opt) { … }

// AbslHashValue for hashing absl::optional
template <typename H, typename T>
typename std::enable_if<is_hashable<T>::value, H>::type AbslHashValue(
    H hash_state, const absl::optional<T>& opt) { … }

// VariantVisitor
template <typename H>
struct VariantVisitor { … };

// AbslHashValue for hashing absl::variant
template <typename H, typename... T>
typename std::enable_if<conjunction<is_hashable<T>...>::value, H>::type
AbslHashValue(H hash_state, const absl::variant<T...>& v) { … }

// -----------------------------------------------------------------------------
// AbslHashValue for Other Types
// -----------------------------------------------------------------------------

// AbslHashValue for hashing std::bitset is not defined on Little Endian
// platforms, for the same reason as for vector<bool> (see std::vector above):
// It does not expose the raw bytes, and a fallback to std::hash<> is most
// likely faster.

#if defined(ABSL_IS_BIG_ENDIAN) && \
    (defined(__GLIBCXX__) || defined(__GLIBCPP__))
// AbslHashValue for hashing std::bitset
//
// std::hash in libstdc++ does not work correctly with std::bitset on Big Endian
// platforms therefore we need to implement a custom AbslHashValue for it. More
// details on the bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102531
template <typename H, size_t N>
H AbslHashValue(H hash_state, const std::bitset<N>& set) {
  typename H::AbslInternalPiecewiseCombiner combiner;
  for (size_t i = 0; i < N; i++) {
    unsigned char c = static_cast<unsigned char>(set[i]);
    hash_state = combiner.add_buffer(std::move(hash_state), &c, sizeof(c));
  }
  return H::combine(combiner.finalize(std::move(hash_state)), N);
}
#endif

// -----------------------------------------------------------------------------

// hash_range_or_bytes()
//
// Mixes all values in the range [data, data+size) into the hash state.
// This overload accepts only uniquely-represented types, and hashes them by
// hashing the entire range of bytes.
template <typename H, typename T>
typename std::enable_if<is_uniquely_represented<T>::value, H>::type
hash_range_or_bytes(H hash_state, const T* data, size_t size) { … }

// hash_range_or_bytes()
template <typename H, typename T>
typename std::enable_if<!is_uniquely_represented<T>::value, H>::type
hash_range_or_bytes(H hash_state, const T* data, size_t size) { … }

#if defined(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE) && \
    ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
#define ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_ …
#else
#define ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_ …
#endif

// HashSelect
//
// Type trait to select the appropriate hash implementation to use.
// HashSelect::type<T> will give the proper hash implementation, to be invoked
// as:
//   HashSelect::type<T>::Invoke(state, value)
// Also, HashSelect::type<T>::value is a boolean equal to `true` if there is a
// valid `Invoke` function. Types that are not hashable will have a ::value of
// `false`.
struct HashSelect { … };

template <typename T>
struct is_hashable
    : std::integral_constant<bool, HashSelect::template Apply<T>::value> { … };

// MixingHashState
class ABSL_DLL MixingHashState : public HashStateBase<MixingHashState> { … };

// MixingHashState::CombineContiguousImpl()
inline uint64_t MixingHashState::CombineContiguousImpl(
    uint64_t state, const unsigned char* first, size_t len,
    std::integral_constant<int, 4> /* sizeof_size_t */) { … }

// Overload of MixingHashState::CombineContiguousImpl()
inline uint64_t MixingHashState::CombineContiguousImpl(
    uint64_t state, const unsigned char* first, size_t len,
    std::integral_constant<int, 8> /* sizeof_size_t */) { … }

struct AggregateBarrier { … };

// HashImpl

// Add a private base class to make sure this type is not an aggregate.
// Aggregates can be aggregate initialized even if the default constructor is
// deleted.
struct PoisonedHash : private AggregateBarrier { … };

template <typename T>
struct HashImpl { … };

template <typename T>
struct Hash
    : absl::conditional_t<is_hashable<T>::value, HashImpl<T>, PoisonedHash> { … };

template <typename H>
template <typename T, typename... Ts>
H HashStateBase<H>::combine(H state, const T& value, const Ts&... values) { … }

// HashStateBase::combine_contiguous()
template <typename H>
template <typename T>
H HashStateBase<H>::combine_contiguous(H state, const T* data, size_t size) { … }

// HashStateBase::combine_unordered()
template <typename H>
template <typename I>
H HashStateBase<H>::combine_unordered(H state, I begin, I end) { … }

// HashStateBase::PiecewiseCombiner::add_buffer()
template <typename H>
H PiecewiseCombiner::add_buffer(H state, const unsigned char* data,
                                size_t size) { … }

// HashStateBase::PiecewiseCombiner::finalize()
template <typename H>
H PiecewiseCombiner::finalize(H state) { … }

}  // namespace hash_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_HASH_INTERNAL_HASH_H_