// Copyright 2022 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_UNIQUE_ALLOCATOR_H_
#define SRC_TINT_UTILS_UNIQUE_ALLOCATOR_H_

#include <functional>
#include <unordered_set>
#include <utility>

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

namespace tint::utils {

/// UniqueAllocator is used to allocate unique instances of the template type
/// `T`.
template <typename T,
          typename HASH = std::hash<T>,
          typename EQUAL = std::equal_to<T>>
class UniqueAllocator {
 public:
  /// @param args the arguments used to construct the object.
  /// @return a pointer to an instance of `T` with the provided arguments.
  ///         If an existing instance of `T` has been constructed, then the same
  ///         pointer is returned.
  template <typename TYPE = T, typename... ARGS>
  TYPE* Get(ARGS&&... args) {
    // Create a temporary T instance on the stack so that we can hash it, and
    // use it for equality lookup for the std::unordered_set. If the item is not
    // found in the set, then we create the persisted instance with the
    // allocator.
    TYPE key{args...};
    auto hash = HASH{}(key);
    auto it = items.find(Entry{hash, &key});
    if (it != items.end()) {
      return static_cast<TYPE*>(it->ptr);
    }
    auto* ptr = allocator.template Create<TYPE>(std::forward<ARGS>(args)...);
    items.emplace_hint(it, Entry{hash, ptr});
    return ptr;
  }

 protected:
  /// Entry is used as the entry to the unordered_set
  struct Entry {
    /// The pre-calculated hash of the entry
    size_t hash;
    /// Tge pointer to the unique object
    T* ptr;
  };
  /// Comparator is the hashing and equality function used by the unordered_set
  struct Comparator {
    /// Hashing function
    /// @param e the entry
    /// @returns the hash of the entry
    size_t operator()(Entry e) const { return e.hash; }

    /// Equality function
    /// @param a the first entry to compare
    /// @param b the second entry to compare
    /// @returns true if the two entries are equal
    bool operator()(Entry a, Entry b) const { return EQUAL{}(*a.ptr, *b.ptr); }
  };

  /// The block allocator used to allocate the unique objects
  BlockAllocator<T> allocator;
  /// The unordered_set of unique item entries
  std::unordered_set<Entry, Comparator, Comparator> items;
};

}  // namespace tint::utils

#endif  // SRC_TINT_UTILS_UNIQUE_ALLOCATOR_H_