diff options
Diffstat (limited to 'abseil-cpp/absl/hash/hash.h')
| -rw-r--r-- | abseil-cpp/absl/hash/hash.h | 121 |
1 files changed, 110 insertions, 11 deletions
diff --git a/abseil-cpp/absl/hash/hash.h b/abseil-cpp/absl/hash/hash.h index 5de132c..470cca4 100644 --- a/abseil-cpp/absl/hash/hash.h +++ b/abseil-cpp/absl/hash/hash.h @@ -26,9 +26,9 @@ // 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. +// 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 @@ -40,6 +40,11 @@ // 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 +// boundaries 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). // @@ -73,6 +78,10 @@ #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 { @@ -101,18 +110,34 @@ ABSL_NAMESPACE_BEGIN // * 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) +// * std::string (as well as any instance of std::basic_string that +// uses one of {char, wchar_t, char16_t, char32_t} and its associated +// std::char_traits) +// * std::string_view (as well as any instance of std::basic_string_view +// that uses one of {char, wchar_t, char16_t, char32_t} and its associated +// std::char_traits) // * All the standard sequence containers (provided the elements are hashable) -// * All the standard ordered associative containers (provided the elements are +// * All the standard 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 +// * 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. @@ -151,7 +176,8 @@ ABSL_NAMESPACE_BEGIN // 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: +// The "hash state" concept contains three member functions for mixing hash +// state: // // * `H::combine(state, values...)` // @@ -185,6 +211,15 @@ ABSL_NAMESPACE_BEGIN // (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` // ----------------------------------------------------------------------------- @@ -214,6 +249,26 @@ ABSL_NAMESPACE_BEGIN 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 @@ -221,8 +276,9 @@ using Hash = absl::hash_internal::Hash<T>; // classes, virtual functions, etc.). The type erasure adds overhead so it // should be avoided unless necessary. // -// Note: This wrapper will only erase calls to: +// 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. @@ -296,6 +352,8 @@ class HashState : public hash_internal::HashStateBase<HashState> { private: HashState() = default; + friend class HashState::HashStateBase; + template <typename T> static void CombineContiguousImpl(void* p, const unsigned char* first, size_t size) { @@ -307,16 +365,57 @@ class HashState : public hash_internal::HashStateBase<HashState> { 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 |