// Copyright 2019 The Dawn 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 DAWNWIRE_CLIENT_OBJECTALLOCATOR_H_
#define DAWNWIRE_CLIENT_OBJECTALLOCATOR_H_

#include "common/Assert.h"
#include "common/Compiler.h"

#include <limits>
#include <memory>
#include <vector>

namespace dawn_wire { namespace client {

    class Client;
    class Device;

    template <typename T>
    class ObjectAllocator {
        using ObjectOwner =
            typename std::conditional<std::is_same<T, Device>::value, Client, Device>::type;

      public:
        struct ObjectAndSerial {
            ObjectAndSerial(std::unique_ptr<T> object, uint32_t generation)
                : object(std::move(object)), generation(generation) {
            }
            std::unique_ptr<T> object;
            uint32_t generation;
        };

        ObjectAllocator() {
            // ID 0 is nullptr
            mObjects.emplace_back(nullptr, 0);
        }

        ObjectAndSerial* New(ObjectOwner* owner) {
            uint32_t id = GetNewId();
            auto object = std::make_unique<T>(owner, 1, id);

            if (id >= mObjects.size()) {
                ASSERT(id == mObjects.size());
                mObjects.emplace_back(std::move(object), 0);
            } else {
                ASSERT(mObjects[id].object == nullptr);

                mObjects[id].generation++;
                // The generation should never overflow. We don't recycle ObjectIds that would
                // overflow their next generation.
                ASSERT(mObjects[id].generation != 0);

                mObjects[id].object = std::move(object);
            }

            return &mObjects[id];
        }
        void Free(T* obj) {
            if (DAWN_LIKELY(mObjects[obj->id].generation != std::numeric_limits<uint32_t>::max())) {
                // Only recycle this ObjectId if the generation won't overflow on the next
                // allocation.
                FreeId(obj->id);
            }
            mObjects[obj->id].object = nullptr;
        }

        T* GetObject(uint32_t id) {
            if (id >= mObjects.size()) {
                return nullptr;
            }
            return mObjects[id].object.get();
        }

        uint32_t GetGeneration(uint32_t id) {
            if (id >= mObjects.size()) {
                return 0;
            }
            return mObjects[id].generation;
        }

      private:
        uint32_t GetNewId() {
            if (mFreeIds.empty()) {
                return mCurrentId++;
            }
            uint32_t id = mFreeIds.back();
            mFreeIds.pop_back();
            return id;
        }
        void FreeId(uint32_t id) {
            mFreeIds.push_back(id);
        }

        // 0 is an ID reserved to represent nullptr
        uint32_t mCurrentId = 1;
        std::vector<uint32_t> mFreeIds;
        std::vector<ObjectAndSerial> mObjects;
        Device* mDevice;
    };
}}  // namespace dawn_wire::client

#endif  // DAWNWIRE_CLIENT_OBJECTALLOCATOR_H_