//===- llvm/Support/HashBuilder.h - Convenient hashing interface-*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements an interface allowing to conveniently build hashes of
// various data types, without relying on the underlying hasher type to know
// about hashed data types.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_SUPPORT_HASHBUILDER_H
#define LLVM_SUPPORT_HASHBUILDER_H
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/type_traits.h"
#include <iterator>
#include <optional>
#include <utility>
namespace llvm {
namespace hashbuilder_detail {
/// Trait to indicate whether a type's bits can be hashed directly (after
/// endianness correction).
template <typename U>
struct IsHashableData
: std::integral_constant<bool, is_integral_or_enum<U>::value> {};
} // namespace hashbuilder_detail
/// Declares the hasher member, and functions forwarding directly to the hasher.
template <typename HasherT> class HashBuilderBase {
public:
template <typename HasherT_ = HasherT>
using HashResultTy = decltype(std::declval<HasherT_ &>().final());
HasherT &getHasher() { return Hasher; }
/// Forward to `HasherT::update(ArrayRef<uint8_t>)`.
///
/// This may not take the size of `Data` into account.
/// Users of this function should pay attention to respect endianness
/// contraints.
void update(ArrayRef<uint8_t> Data) { this->getHasher().update(Data); }
/// Forward to `HasherT::update(ArrayRef<uint8_t>)`.
///
/// This may not take the size of `Data` into account.
/// Users of this function should pay attention to respect endianness
/// contraints.
void update(StringRef Data) {
update(
ArrayRef(reinterpret_cast<const uint8_t *>(Data.data()), Data.size()));
}
/// Forward to `HasherT::final()` if available.
template <typename HasherT_ = HasherT> HashResultTy<HasherT_> final() {
return this->getHasher().final();
}
/// Forward to `HasherT::result()` if available.
template <typename HasherT_ = HasherT> HashResultTy<HasherT_> result() {
return this->getHasher().result();
}
protected:
explicit HashBuilderBase(HasherT &Hasher) : Hasher(Hasher) {}
template <typename... ArgTypes>
explicit HashBuilderBase(ArgTypes &&...Args)
: OptionalHasher(std::in_place, std::forward<ArgTypes>(Args)...),
Hasher(*OptionalHasher) {}
private:
std::optional<HasherT> OptionalHasher;
HasherT &Hasher;
};
/// Implementation of the `HashBuilder` interface.
///
/// `support::endianness::native` is not supported. `HashBuilder` is
/// expected to canonicalize `support::endianness::native` to one of
/// `support::endianness::big` or `support::endianness::little`.
template <typename HasherT, support::endianness Endianness>
class HashBuilderImpl : public HashBuilderBase<HasherT> {
static_assert(Endianness != support::endianness::native,
"HashBuilder should canonicalize endianness");
public:
explicit HashBuilderImpl(HasherT &Hasher)
: HashBuilderBase<HasherT>(Hasher) {}
template <typename... ArgTypes>
explicit HashBuilderImpl(ArgTypes &&...Args)
: HashBuilderBase<HasherT>(Args...) {}
/// Implement hashing for hashable data types, e.g. integral or enum values.
template <typename T>
std::enable_if_t<hashbuilder_detail::IsHashableData<T>::value,
HashBuilderImpl &>
add(T Value) {
return adjustForEndiannessAndAdd(Value);
}
/// Support hashing `ArrayRef`.
///
/// `Value.size()` is taken into account to ensure cases like
/// ```
/// builder.add({1});
/// builder.add({2, 3});
/// ```
/// and
/// ```
/// builder.add({1, 2});
/// builder.add({3});
/// ```
/// do not collide.
template <typename T> HashBuilderImpl &add(ArrayRef<T> Value) {
// As of implementation time, simply calling `addRange(Value)` would also go
// through the `update` fast path. But that would rely on the implementation
// details of `ArrayRef::begin()` and `ArrayRef::end()`. Explicitly call
// `update` to guarantee the fast path.
add(Value.size());
if (hashbuilder_detail::IsHashableData<T>::value &&
Endianness == support::endian::system_endianness()) {
this->update(ArrayRef(reinterpret_cast<const uint8_t *>(Value.begin()),
Value.size() * sizeof(T)));
} else {
for (auto &V : Value)
add(V);
}
return *this;
}
/// Support hashing `StringRef`.
///
/// `Value.size()` is taken into account to ensure cases like
/// ```
/// builder.add("a");
/// builder.add("bc");
/// ```
/// and
/// ```
/// builder.add("ab");
/// builder.add("c");
/// ```
/// do not collide.
HashBuilderImpl &add(StringRef Value) {
// As of implementation time, simply calling `addRange(Value)` would also go
// through `update`. But that would rely on the implementation of
// `StringRef::begin()` and `StringRef::end()`. Explicitly call `update` to
// guarantee the fast path.
add(Value.size());
this->update(ArrayRef(reinterpret_cast<const uint8_t *>(Value.begin()),
Value.size()));
return *this;
}
template <typename T>
using HasAddHashT =
decltype(addHash(std::declval<HashBuilderImpl &>(), std::declval<T &>()));
/// Implement hashing for user-defined `struct`s.
///
/// Any user-define `struct` can participate in hashing via `HashBuilder` by
/// providing a `addHash` templated function.
///
/// ```
/// template <typename HasherT, support::endianness Endianness>
/// void addHash(HashBuilder<HasherT, Endianness> &HBuilder,
/// const UserDefinedStruct &Value);
/// ```
///
/// For example:
/// ```
/// struct SimpleStruct {
/// char c;
/// int i;
/// };
///
/// template <typename HasherT, support::endianness Endianness>
/// void addHash(HashBuilderImpl<HasherT, Endianness> &HBuilder,
/// const SimpleStruct &Value) {
/// HBuilder.add(Value.c);
/// HBuilder.add(Value.i);
/// }
/// ```
///
/// To avoid endianness issues, specializations of `addHash` should
/// generally rely on exising `add`, `addRange`, and `addRangeElements`
/// functions. If directly using `update`, an implementation must correctly
/// handle endianness.
///
/// ```
/// struct __attribute__ ((packed)) StructWithFastHash {
/// int I;
/// char C;
///
/// // If possible, we want to hash both `I` and `C` in a single
/// // `update` call for performance concerns.
/// template <typename HasherT, support::endianness Endianness>
/// friend void addHash(HashBuilderImpl<HasherT, Endianness> &HBuilder,
/// const StructWithFastHash &Value) {
/// if (Endianness == support::endian::system_endianness()) {
/// HBuilder.update(ArrayRef(
/// reinterpret_cast<const uint8_t *>(&Value), sizeof(Value)));
/// } else {
/// // Rely on existing `add` methods to handle endianness.
/// HBuilder.add(Value.I);
/// HBuilder.add(Value.C);
/// }
/// }
/// };
/// ```
///
/// To avoid collisions, specialization of `addHash` for variable-size
/// types must take the size into account.
///
/// For example:
/// ```
/// struct CustomContainer {
/// private:
/// size_t Size;
/// int Elements[100];
///
/// public:
/// CustomContainer(size_t Size) : Size(Size) {
/// for (size_t I = 0; I != Size; ++I)
/// Elements[I] = I;
/// }
/// template <typename HasherT, support::endianness Endianness>
/// friend void addHash(HashBuilderImpl<HasherT, Endianness> &HBuilder,
/// const CustomContainer &Value) {
/// if (Endianness == support::endian::system_endianness()) {
/// HBuilder.update(ArrayRef(
/// reinterpret_cast<const uint8_t *>(&Value.Size),
/// sizeof(Value.Size) + Value.Size * sizeof(Value.Elements[0])));
/// } else {
/// // `addRange` will take care of encoding the size.
/// HBuilder.addRange(&Value.Elements[0], &Value.Elements[0] +
/// Value.Size);
/// }
/// }
/// };
/// ```
template <typename T>
std::enable_if_t<is_detected<HasAddHashT, T>::value &&
!hashbuilder_detail::IsHashableData<T>::value,
HashBuilderImpl &>
add(const T &Value) {
addHash(*this, Value);
return *this;
}
template <typename T1, typename T2>
HashBuilderImpl &add(const std::pair<T1, T2> &Value) {
return add(Value.first, Value.second);
}
template <typename... Ts> HashBuilderImpl &add(const std::tuple<Ts...> &Arg) {
std::apply([this](const auto &...Args) { this->add(Args...); }, Arg);
return *this;
}
/// A convenenience variadic helper.
/// It simply iterates over its arguments, in order.
/// ```
/// add(Arg1, Arg2);
/// ```
/// is equivalent to
/// ```
/// add(Arg1)
/// add(Arg2)
/// ```
template <typename... Ts>
std::enable_if_t<(sizeof...(Ts) > 1), HashBuilderImpl &>
add(const Ts &...Args) {
return (add(Args), ...);
}
template <typename ForwardIteratorT>
HashBuilderImpl &addRange(ForwardIteratorT First, ForwardIteratorT Last) {
add(std::distance(First, Last));
return addRangeElements(First, Last);
}
template <typename RangeT> HashBuilderImpl &addRange(const RangeT &Range) {
return addRange(adl_begin(Range), adl_end(Range));
}
template <typename ForwardIteratorT>
HashBuilderImpl &addRangeElements(ForwardIteratorT First,
ForwardIteratorT Last) {
return addRangeElementsImpl(
First, Last,
typename std::iterator_traits<ForwardIteratorT>::iterator_category());
}
template <typename RangeT>
HashBuilderImpl &addRangeElements(const RangeT &Range) {
return addRangeElements(adl_begin(Range), adl_end(Range));
}
template <typename T>
using HasByteSwapT = decltype(support::endian::byte_swap(
std::declval<T &>(), support::endianness::little));
/// Adjust `Value` for the target endianness and add it to the hash.
template <typename T>
std::enable_if_t<is_detected<HasByteSwapT, T>::value, HashBuilderImpl &>
adjustForEndiannessAndAdd(const T &Value) {
T SwappedValue = support::endian::byte_swap(Value, Endianness);
this->update(ArrayRef(reinterpret_cast<const uint8_t *>(&SwappedValue),
sizeof(SwappedValue)));
return *this;
}
private:
// FIXME: Once available, specialize this function for `contiguous_iterator`s,
// and use it for `ArrayRef` and `StringRef`.
template <typename ForwardIteratorT>
HashBuilderImpl &addRangeElementsImpl(ForwardIteratorT First,
ForwardIteratorT Last,
std::forward_iterator_tag) {
for (auto It = First; It != Last; ++It)
add(*It);
return *this;
}
template <typename T>
std::enable_if_t<hashbuilder_detail::IsHashableData<T>::value &&
Endianness == support::endian::system_endianness(),
HashBuilderImpl &>
addRangeElementsImpl(T *First, T *Last, std::forward_iterator_tag) {
this->update(ArrayRef(reinterpret_cast<const uint8_t *>(First),
(Last - First) * sizeof(T)));
return *this;
}
};
/// Interface to help hash various types through a hasher type.
///
/// Via provided specializations of `add`, `addRange`, and `addRangeElements`
/// functions, various types (e.g. `ArrayRef`, `StringRef`, etc.) can be hashed
/// without requiring any knowledge of hashed types from the hasher type.
///
/// The only method expected from the templated hasher type `HasherT` is:
/// * void update(ArrayRef<uint8_t> Data)
///
/// Additionally, the following methods will be forwarded to the hasher type:
/// * decltype(std::declval<HasherT &>().final()) final()
/// * decltype(std::declval<HasherT &>().result()) result()
///
/// From a user point of view, the interface provides the following:
/// * `template<typename T> add(const T &Value)`
/// The `add` function implements hashing of various types.
/// * `template <typename ItT> void addRange(ItT First, ItT Last)`
/// The `addRange` function is designed to aid hashing a range of values.
/// It explicitly adds the size of the range in the hash.
/// * `template <typename ItT> void addRangeElements(ItT First, ItT Last)`
/// The `addRangeElements` function is also designed to aid hashing a range of
/// values. In contrast to `addRange`, it **ignores** the size of the range,
/// behaving as if elements were added one at a time with `add`.
///
/// User-defined `struct` types can participate in this interface by providing
/// an `addHash` templated function. See the associated template specialization
/// for details.
///
/// This interface does not impose requirements on the hasher
/// `update(ArrayRef<uint8_t> Data)` method. We want to avoid collisions for
/// variable-size types; for example for
/// ```
/// builder.add({1});
/// builder.add({2, 3});
/// ```
/// and
/// ```
/// builder.add({1, 2});
/// builder.add({3});
/// ```
/// . Thus, specializations of `add` and `addHash` for variable-size types must
/// not assume that the hasher type considers the size as part of the hash; they
/// must explicitly add the size to the hash. See for example specializations
/// for `ArrayRef` and `StringRef`.
///
/// Additionally, since types are eventually forwarded to the hasher's
/// `void update(ArrayRef<uint8_t>)` method, endianness plays a role in the hash
/// computation (for example when computing `add((int)123)`).
/// Specifiying a non-`native` `Endianness` template parameter allows to compute
/// stable hash across platforms with different endianness.
template <class HasherT, support::endianness Endianness>
using HashBuilder =
HashBuilderImpl<HasherT, (Endianness == support::endianness::native
? support::endian::system_endianness()
: Endianness)>;
namespace hashbuilder_detail {
class HashCodeHasher {
public:
HashCodeHasher() : Code(0) {}
void update(ArrayRef<uint8_t> Data) {
hash_code DataCode = hash_value(Data);
Code = hash_combine(Code, DataCode);
}
hash_code Code;
};
using HashCodeHashBuilder = HashBuilder<hashbuilder_detail::HashCodeHasher,
support::endianness::native>;
} // namespace hashbuilder_detail
/// Provide a default implementation of `hash_value` when `addHash(const T &)`
/// is supported.
template <typename T>
std::enable_if_t<
is_detected<hashbuilder_detail::HashCodeHashBuilder::HasAddHashT, T>::value,
hash_code>
hash_value(const T &Value) {
hashbuilder_detail::HashCodeHashBuilder HBuilder;
HBuilder.add(Value);
return HBuilder.getHasher().Code;
}
} // end namespace llvm
#endif // LLVM_SUPPORT_HASHBUILDER_H