src/tint/utils/hash.h

// Copyright 2021 The Tint 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.

#ifndef SRC_TINT_UTILS_HASH_H_
#define SRC_TINT_UTILS_HASH_H_

#include <stdint.h>
#include <cstdio>
#include <functional>
#include <tuple>
#include <utility>
#include <variant>
#include <vector>

#include "src/tint/utils/vector.h"

namespace tint::utils {
namespace detail {

/// Helper for obtaining a seed bias value for HashCombine with a bit-width
/// dependent on the size of size_t.
template <int SIZE_OF_SIZE_T>
struct HashCombineOffset {};

/// Specialization of HashCombineOffset for size_t == 4.
template <>
struct HashCombineOffset<4> {
    /// @returns the seed bias value for HashCombine()
    static constexpr inline uint32_t value() { return 0x7f4a7c16; }
};

/// Specialization of HashCombineOffset for size_t == 8.
template <>
struct HashCombineOffset<8> {
    /// @returns the seed bias value for HashCombine()
    static constexpr inline uint64_t value() { return 0x9e3779b97f4a7c16; }
};

}  // namespace detail

/// Forward declarations (see below)
template <typename... ARGS>
size_t Hash(const ARGS&... values);

template <typename... ARGS>
size_t HashCombine(size_t hash, const ARGS&... values);

/// A STL-compatible hasher that does a more thorough job than most implementations of std::hash.
/// Hasher has been optimized for a better quality hash at the expense of increased computation
/// costs.
template <typename T>
struct Hasher {
    /// @param value the value to hash
    /// @returns a hash of the value
    size_t operator()(const T& value) const { return std::hash<T>()(value); }
};

/// Hasher specialization for pointers
/// std::hash<T*> typically uses a reinterpret of the pointer to a size_t.
/// As most pointers a 4 or 16 byte aligned, this usually results in the LSBs of the hash being 0,
/// resulting in bad hashes for hashtables. This implementation mixes up those LSBs.
template <typename T>
struct Hasher<T*> {
    /// @param ptr the pointer to hash
    /// @returns a hash of the pointer
    size_t operator()(T* ptr) const {
        auto hash = std::hash<T*>()(ptr);
        return hash ^ (hash >> 4);
    }
};

/// Hasher specialization for std::vector
template <typename T>
struct Hasher<std::vector<T>> {
    /// @param vector the vector to hash
    /// @returns a hash of the vector
    size_t operator()(const std::vector<T>& vector) const {
        auto hash = Hash(vector.size());
        for (auto& el : vector) {
            hash = HashCombine(hash, el);
        }
        return hash;
    }
};

/// Hasher specialization for utils::vector
template <typename T, size_t N>
struct Hasher<utils::Vector<T, N>> {
    /// @param vector the vector to hash
    /// @returns a hash of the vector
    size_t operator()(const utils::Vector<T, N>& vector) const {
        auto hash = Hash(vector.Length());
        for (auto& el : vector) {
            hash = HashCombine(hash, el);
        }
        return hash;
    }
};

/// Hasher specialization for std::tuple
template <typename... TYPES>
struct Hasher<std::tuple<TYPES...>> {
    /// @param tuple the tuple to hash
    /// @returns a hash of the tuple
    size_t operator()(const std::tuple<TYPES...>& tuple) const {
        return std::apply(Hash<TYPES...>, tuple);
    }
};

/// Hasher specialization for std::tuple
template <typename... TYPES>
struct Hasher<std::variant<TYPES...>> {
    /// @param variant the variant to hash
    /// @returns a hash of the tuple
    size_t operator()(const std::variant<TYPES...>& variant) const {
        return std::visit([](auto&& val) { return Hash(val); }, variant);
    }
};

/// @returns a hash of the variadic list of arguments.
///          The returned hash is dependent on the order of the arguments.
template <typename... ARGS>
size_t Hash(const ARGS&... args) {
    if constexpr (sizeof...(ARGS) == 0) {
        return 0;
    } else if constexpr (sizeof...(ARGS) == 1) {
        using T = std::tuple_element_t<0, std::tuple<ARGS...>>;
        return Hasher<T>()(args...);
    } else {
        size_t hash = 102931;  // seed with an arbitrary prime
        return HashCombine(hash, args...);
    }
}

/// @returns a hash of the variadic list of arguments.
///          The returned hash is dependent on the order of the arguments.
template <typename... ARGS>
size_t HashCombine(size_t hash, const ARGS&... values) {
    constexpr size_t offset = detail::HashCombineOffset<sizeof(size_t)>::value();
    ((hash ^= Hash(values) + offset + (hash << 6) + (hash >> 2)), ...);
    return hash;
}

/// Wrapper for a hashable type enabling the wrapped value to be used as a key
/// for an unordered_map or unordered_set.
template <typename T>
struct UnorderedKeyWrapper {
    /// The wrapped value
    T value;
    /// The hash of value
    size_t hash;

    /// Constructor
    /// @param v the value to wrap
    explicit UnorderedKeyWrapper(const T& v) : value(v), hash(Hash(v)) {}

    /// Move constructor
    /// @param v the value to wrap
    explicit UnorderedKeyWrapper(T&& v) : value(std::move(v)), hash(Hash(value)) {}

    /// @returns true if this wrapper comes before other
    /// @param other the RHS of the operator
    bool operator<(const UnorderedKeyWrapper& other) const { return hash < other.hash; }

    /// @returns true if this wrapped value is equal to the other wrapped value
    /// @param other the RHS of the operator
    bool operator==(const UnorderedKeyWrapper& other) const { return value == other.value; }
};

}  // namespace tint::utils

namespace std {

/// Custom std::hash specialization for tint::utils::UnorderedKeyWrapper
template <typename T>
class hash<tint::utils::UnorderedKeyWrapper<T>> {
  public:
    /// @param w the UnorderedKeyWrapper
    /// @return the hash value
    inline std::size_t operator()(const tint::utils::UnorderedKeyWrapper<T>& w) const {
        return w.hash;
    }
};

}  // namespace std

#endif  // SRC_TINT_UTILS_HASH_H_