package cn.study.concurrency.ch21;

/**
 * 锁分段
 * @author xiaof
 *
 */
public class StripedMap {
  //同步策略：就是对数组进行分段上锁，n个节点用n%LOCKS锁保护
  private static final int N_LOCKS = 16;
  private final Node[] buckets;
  private final Object[] locks;
  
  private static class Node
  {
    private String name;
    private Node next;
    private String key;
    private String value;
    public String getValue() {
      return value;
    }
    public void setValue(String value) {
      this.value = value;
    }
    public String getName() {
      return name;
    }
    public void setName(String name) {
      this.name = name;
    }
    public Node getNext() {
      return next;
    }
    public void setNext(Node next) {
      this.next = next;
    }
    public String getKey() {
      return key;
    }
    public void setKey(String key) {
      this.key = key;
    }
    
  }
  
  public StripedMap(int numBuckets)
  {
    buckets = new Node[numBuckets];
    //创建对应hash的锁
    locks = new Object[N_LOCKS];
    for(int i = 0; i < N_LOCKS; ++ i)
    {
      locks[i] = new Object();
    }
  }
  
  private final int hash(Object key)
  {
    //取绝对值
    return Math.abs(key.hashCode() % buckets.length);
  }
  
  //get和clear
  public Object get(Object key)
  {
    int hash = hash(key);
    synchronized(locks[hash % N_LOCKS])
    {
      //分段上锁
      for(Node m = buckets[hash]; m != null; m = m.next)
      {
        if(m.key.equals(key))
          return m.value;
      }
    }
    
    return null;
  }
  
  /**
   * 清除所有的数据，但是没有要求说要同时获取全部的锁的话，可以进行这样的释放操作
   */
  public void clear()
  {
    for(int i = 0; i < buckets.length; ++i)
    {
      synchronized(locks[i % N_LOCKS])
      {
        buckets[i] = null;
      }
    }
  }
}