/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */

package org.apache.iotdb.db.metadata.cache.dualkeycache.impl;

import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;

public class FIFOCacheEntryManager<FK, SK, V>
    implements ICacheEntryManager<FK, SK, V, FIFOCacheEntryManager.FIFOCacheEntry<SK, V>> {

  private static final int SLOT_NUM = 128;

  private final FIFOLinkedList[] fifoLinkedLists = new FIFOLinkedList[SLOT_NUM];

  private final AtomicInteger cachePutRoundRobinIndex = new AtomicInteger(0);

  private final AtomicInteger cacheEvictRoundRobinIndex = new AtomicInteger(0);

  @Override
  public FIFOCacheEntry<SK, V> createCacheEntry(
      SK secondKey, V value, ICacheEntryGroup<FK, SK, V, FIFOCacheEntry<SK, V>> cacheEntryGroup) {
    return new FIFOCacheEntry<>(secondKey, value, cacheEntryGroup);
  }

  @Override
  public void access(FIFOCacheEntry<SK, V> cacheEntry) {
    // do nothing
  }

  @Override
  public void put(FIFOCacheEntry<SK, V> cacheEntry) {
    getNextList(cachePutRoundRobinIndex).add(cacheEntry);
  }

  @Override
  public FIFOCacheEntry<SK, V> evict() {
    int startIndex = getNextIndex(cacheEvictRoundRobinIndex);
    FIFOLinkedList fifoLinkedList;
    FIFOCacheEntry<SK, V> cacheEntry;
    for (int i = 0; i < SLOT_NUM; i++) {
      if (startIndex == SLOT_NUM) {
        startIndex = 0;
      }
      fifoLinkedList = fifoLinkedLists[startIndex];
      if (fifoLinkedList != null) {
        cacheEntry = fifoLinkedList.evict();
        if (cacheEntry != null) {
          return cacheEntry;
        }
      }
      startIndex++;
    }
    return null;
  }

  @Override
  public void cleanUp() {
    synchronized (fifoLinkedLists) {
      for (int i = 0; i < SLOT_NUM; i++) {
        fifoLinkedLists[i] = null;
      }
    }
  }

  private FIFOLinkedList getNextList(AtomicInteger roundRobinIndex) {
    int listIndex = getNextIndex(roundRobinIndex);
    FIFOLinkedList fifoLinkedList = fifoLinkedLists[listIndex];
    if (fifoLinkedList == null) {
      synchronized (fifoLinkedLists) {
        fifoLinkedList = fifoLinkedLists[listIndex];
        if (fifoLinkedList == null) {
          fifoLinkedList = new FIFOLinkedList();
          fifoLinkedLists[listIndex] = fifoLinkedList;
        }
      }
    }
    return fifoLinkedList;
  }

  private int getNextIndex(AtomicInteger roundRobinIndex) {
    return roundRobinIndex.getAndUpdate(
        currentValue -> {
          currentValue = currentValue + 1;
          return currentValue >= SLOT_NUM ? 0 : currentValue;
        });
  }

  static class FIFOCacheEntry<SK, V> implements ICacheEntry<SK, V> {

    private final SK secondKey;
    private final ICacheEntryGroup cacheEntryGroup;

    private V value;

    private FIFOCacheEntry<SK, V> pre;

    private FIFOCacheEntry(SK secondKey, V value, ICacheEntryGroup cacheEntryGroup) {
      this.secondKey = secondKey;
      this.value = value;
      this.cacheEntryGroup = cacheEntryGroup;
    }

    @Override
    public SK getSecondKey() {
      return secondKey;
    }

    @Override
    public V getValue() {
      return value;
    }

    @Override
    public ICacheEntryGroup getBelongedGroup() {
      return cacheEntryGroup;
    }

    @Override
    public void replaceValue(V newValue) {
      this.value = newValue;
    }

    @Override
    public boolean equals(Object o) {
      if (this == o) return true;
      if (o == null || getClass() != o.getClass()) return false;
      FIFOCacheEntry<?, ?> that = (FIFOCacheEntry<?, ?>) o;
      return Objects.equals(secondKey, that.secondKey)
          && Objects.equals(cacheEntryGroup, that.cacheEntryGroup);
    }

    @Override
    public int hashCode() {
      return cacheEntryGroup.hashCode() * 31 + secondKey.hashCode();
    }
  }

  private static class FIFOLinkedList {

    private FIFOCacheEntry head;
    private FIFOCacheEntry tail;

    synchronized void add(FIFOCacheEntry cacheEntry) {
      if (head == null) {
        head = cacheEntry;
        tail = cacheEntry;
        return;
      }

      head.pre = cacheEntry;

      head = cacheEntry;
    }

    synchronized FIFOCacheEntry evict() {
      if (tail == null) {
        return null;
      }

      FIFOCacheEntry cacheEntry = tail;
      tail = tail.pre;

      if (tail == null) {
        head = null;
      }

      cacheEntry.pre = null;

      return cacheEntry;
    }
  }
}
