test/source/blender/blenlib/BLI_hash.hh

/* SPDX-FileCopyrightText: 2023 Blender Authors
 *
 * SPDX-License-Identifier: GPL-2.0-or-later */

#pragma once

/** \file
 * \ingroup bli
 *
 * A specialization of `blender::DefaultHash<T>` provides a hash function for values of type T.
 * This hash function is used by default in hash table implementations in blenlib.
 *
 * The actual hash function is in the `operator()` method of `DefaultHash<T>`. The following code
 * computes the hash of some value using DefaultHash.
 *
 *   T value = ...;
 *   DefaultHash<T> hash_function;
 *   uint32_t hash = hash_function(value);
 *
 * Hash table implementations like blender::Set support heterogeneous key lookups. That means that
 * one can do a lookup with a key of type A in a hash table that stores keys of type B. This is
 * commonly done when B is std::string, because the conversion from e.g. a #StringRef to
 * std::string can be costly and is unnecessary. To make this work, values of type A and B that
 * compare equal have to have the same hash value. This is achieved by defining potentially
 * multiple `operator()` in a specialization of #DefaultHash. All those methods have to compute the
 * same hash for values that compare equal.
 *
 * The computed hash is an unsigned 64 bit integer. Ideally, the hash function would generate
 * uniformly random hash values for a set of keys. However, in many cases trivial hash functions
 * are faster and produce a good enough distribution. In general it is better when more information
 * is in the lower bits of the hash. By choosing a good probing strategy, the effects of a bad hash
 * function are less noticeable though. In this context a good probing strategy is one that takes
 * all bits of the hash into account eventually. One has to check on a case by case basis to see if
 * a better but more expensive or trivial hash function works better.
 *
 * There are three main ways to provide a hash table implementation with a custom hash function.
 *
 * - When you want to provide a default hash function for your own custom type: Add a `hash()`
 *   member function to it. The function should return `uint64_t` and take no arguments. This
 *   method will be called by the default implementation of #DefaultHash. It will automatically be
 *   used by hash table implementations.
 *
 * - When you want to provide a default hash function for a type that you cannot modify: Add a new
 *   specialization to the #DefaultHash struct. This can be done by writing code like below in
 *   either global or `blender` namespace.
 *
 *     template<> struct blender::DefaultHash<TheType> {
 *       uint64_t operator()(const TheType &value) const {
 *         return ...;
 *       }
 *     };
 *
 * - When you want to provide a different hash function for a type that already has a default hash
 *   function: Implement a struct like the one below and pass it as template parameter to the hash
 *   table explicitly.
 *
 *     struct MyCustomHash {
 *       uint64_t operator()(const TheType &value) const {
 *         return ...;
 *       }
 *     };
 */

#include <memory>
#include <string>
#include <utility>

#include "BLI_hash_fwd.hh"
#include "BLI_string_ref.hh"

namespace blender {

/**
 * If there is no other specialization of #DefaultHash for a given type, look for a hash function
 * on the type itself. Implementing a `hash()` method on a type is often significantly easier than
 * specializing #DefaultHash.
 *
 * To support heterogeneous lookup, a type can also implement a static `hash_as(const OtherType &)`
 * function.
 *
 * In the case of an enum type, the default hash is just to cast the enum value to an integer.
 */
template<typename T> struct DefaultHash {
  uint64_t operator()(const T &value) const
  {
    if constexpr (std::is_enum_v<T>) {
      /* For enums use the value as hash directly. */
      return uint64_t(value);
    }
    else {
      /* Try to call the `hash()` function on the value. */
      /* If this results in a compiler error, no hash function for the type has been found. */
      return value.hash();
    }
  }

  template<typename U> uint64_t operator()(const U &value) const
  {
    /* Try calling the static `T::hash_as(value)` function with the given value. The returned hash
     * should be "compatible" with `T::hash()`. Usually that means that if `value` is converted to
     * `T` its hash does not change. */
    /* If this results in a compiler error, no hash function for the heterogeneous lookup has been
     * found. */
    return T::hash_as(value);
  }
};

/**
 * Use the same hash function for const and non const variants of a type.
 */
template<typename T> struct DefaultHash<const T> {
  uint64_t operator()(const T &value) const
  {
    return DefaultHash<T>{}(value);
  }
};

#define TRIVIAL_DEFAULT_INT_HASH(TYPE) \
  template<> struct DefaultHash<TYPE> { \
    uint64_t operator()(TYPE value) const \
    { \
      return uint64_t(value); \
    } \
  }

/**
 * We cannot make any assumptions about the distribution of keys, so use a trivial hash function by
 * default. The default probing strategy is designed to take all bits of the hash into account
 * to avoid worst case behavior when the lower bits are all zero. Special hash functions can be
 * implemented when more knowledge about a specific key distribution is available.
 */
TRIVIAL_DEFAULT_INT_HASH(int8_t);
TRIVIAL_DEFAULT_INT_HASH(uint8_t);
TRIVIAL_DEFAULT_INT_HASH(int16_t);
TRIVIAL_DEFAULT_INT_HASH(uint16_t);
TRIVIAL_DEFAULT_INT_HASH(int32_t);
TRIVIAL_DEFAULT_INT_HASH(uint32_t);
TRIVIAL_DEFAULT_INT_HASH(int64_t);
TRIVIAL_DEFAULT_INT_HASH(uint64_t);

/**
 * One should try to avoid using floats as keys in hash tables, but sometimes it is convenient.
 */
template<> struct DefaultHash<float> {
  uint64_t operator()(float value) const
  {
    /* Explicit `uint64_t` cast to suppress CPPCHECK warning. */
    return uint64_t(*reinterpret_cast<uint32_t *>(&value));
  }
};

template<> struct DefaultHash<double> {
  uint64_t operator()(double value) const
  {
    return *reinterpret_cast<uint64_t *>(&value);
  }
};

template<> struct DefaultHash<bool> {
  uint64_t operator()(bool value) const
  {
    return uint64_t((value != false) * 1298191);
  }
};

inline uint64_t hash_string(StringRef str)
{
  uint64_t hash = 5381;
  for (char c : str) {
    hash = hash * 33 + c;
  }
  return hash;
}

template<> struct DefaultHash<std::string> {
  /**
   * Take a #StringRef as parameter to support heterogeneous lookups in hash table implementations
   * when std::string is used as key.
   */
  uint64_t operator()(StringRef value) const
  {
    return hash_string(value);
  }
};

template<> struct DefaultHash<StringRef> {
  uint64_t operator()(StringRef value) const
  {
    return hash_string(value);
  }
};

template<> struct DefaultHash<StringRefNull> {
  uint64_t operator()(StringRef value) const
  {
    return hash_string(value);
  }
};

template<> struct DefaultHash<std::string_view> {
  uint64_t operator()(StringRef value) const
  {
    return hash_string(value);
  }
};

/**
 * While we cannot guarantee that the lower 4 bits of a pointer are zero, it is often the case.
 */
template<typename T> struct DefaultHash<T *> {
  uint64_t operator()(const T *value) const
  {
    uintptr_t ptr = uintptr_t(value);
    uint64_t hash = uint64_t(ptr >> 4);
    return hash;
  }
};

namespace detail {
static constexpr std::array<uint64_t, 3> default_hash_factors = {19349669, 83492791, 3632623};

template<size_t... I, typename... Args>
inline uint64_t get_default_hash_array(std::index_sequence<I...> /*indices*/, const Args &...args)
{
  static_assert(sizeof...(Args) == sizeof...(I));
  static_assert(sizeof...(Args) <= default_hash_factors.size());
  return (0 ^ ... ^ (default_hash_factors[I] * DefaultHash<std::decay_t<Args>>{}(args)));
}

}  // namespace detail

template<typename T, typename... Args>
inline uint64_t get_default_hash(const T &v, const Args &...args)
{
  return DefaultHash<std::decay_t<T>>{}(v) ^
         detail::get_default_hash_array(std::make_index_sequence<sizeof...(Args)>(), args...);
}

/** Support hashing different kinds of pointer types. */
template<typename T> struct PointerHashes {
  template<typename U> uint64_t operator()(const U &value) const
  {
    return get_default_hash(&*value);
  }
};

template<typename T> struct DefaultHash<std::unique_ptr<T>> : public PointerHashes<T> {};
template<typename T> struct DefaultHash<std::shared_ptr<T>> : public PointerHashes<T> {};

template<typename T> struct DefaultHash<std::reference_wrapper<T>> {
  uint64_t operator()(const std::reference_wrapper<T> &value) const
  {
    return get_default_hash(value.get());
  }
};

template<typename T1, typename T2> struct DefaultHash<std::pair<T1, T2>> {
  uint64_t operator()(const std::pair<T1, T2> &value) const
  {
    return get_default_hash(value.first, value.second);
  }
};

}  // namespace blender