iciql/src/com/iciql/util/WeakIdentityHashMap.java

/*
 * Copyright 2004-2011 H2 Group.
 * Copyright 2011 James Moger.
 *
 * 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.
 */

package com.iciql.util;

import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.Map;
import java.util.Set;

import com.iciql.IciqlException;

/**
 * This hash map uses weak references, so that elements that are no longer
 * referenced elsewhere can be garbage collected. It also uses object identity
 * to compare keys. The garbage collection happens when trying to add new data,
 * or when resizing.
 * 
 * @param <K>
 *            the keys
 * @param <V>
 *            the value
 */

public class WeakIdentityHashMap<K, V> implements Map<K, V> {

	private static final int MAX_LOAD = 90;
	private static final WeakReference<Object> DELETED_KEY = new WeakReference<Object>(null);
	private int mask, len, size, deletedCount, level;
	private int maxSize, minSize, maxDeleted;
	private WeakReference<K>[] keys;
	private V[] values;

	public WeakIdentityHashMap() {
		reset(2);
	}

	public int size() {
		return size;
	}

	private void checkSizePut() {
		if (deletedCount > size) {
			rehash(level);
		}
		if (size + deletedCount >= maxSize) {
			rehash(level + 1);
		}
	}

	private void checkSizeRemove() {
		if (size < minSize && level > 0) {
			rehash(level - 1);
		} else if (deletedCount > maxDeleted) {
			rehash(level);
		}
	}

	private int getIndex(Object key) {
		return System.identityHashCode(key) & mask;
	}

	@SuppressWarnings("unchecked")
	private void reset(int newLevel) {
		minSize = size * 3 / 4;
		size = 0;
		level = newLevel;
		len = 2 << level;
		mask = len - 1;
		maxSize = (int) (len * MAX_LOAD / 100L);
		deletedCount = 0;
		maxDeleted = 20 + len / 2;
		keys = new WeakReference[len];
		values = (V[]) new Object[len];
	}

	public V put(K key, V value) {
		checkSizePut();
		int index = getIndex(key);
		int plus = 1;
		int deleted = -1;
		do {
			WeakReference<K> k = keys[index];
			if (k == null) {
				// found an empty record
				if (deleted >= 0) {
					index = deleted;
					deletedCount--;
				}
				size++;
				keys[index] = new WeakReference<K>(key);
				values[index] = value;
				return null;
			} else if (k == DELETED_KEY) {
				if (deleted < 0) {
					// found the first deleted record
					deleted = index;
				}
			} else {
				Object r = k.get();
				if (r == null) {
					delete(index);
				} else if (r == key) {
					// update existing
					V old = values[index];
					values[index] = value;
					return old;
				}
			}
			index = (index + plus++) & mask;
		} while (plus <= len);
		throw new IciqlException("Hashmap is full");
	}

	public V remove(Object key) {
		checkSizeRemove();
		int index = getIndex(key);
		int plus = 1;
		do {
			WeakReference<K> k = keys[index];
			if (k == null) {
				// found an empty record
				return null;
			} else if (k == DELETED_KEY) {
				// continue
			} else {
				Object r = k.get();
				if (r == null) {
					delete(index);
				} else if (r == key) {
					// found the record
					V old = values[index];
					delete(index);
					return old;
				}
			}
			index = (index + plus++) & mask;
			k = keys[index];
		} while (plus <= len);
		// not found
		return null;
	}

	@SuppressWarnings("unchecked")
	private void delete(int index) {
		keys[index] = (WeakReference<K>) DELETED_KEY;
		values[index] = null;
		deletedCount++;
		size--;
	}

	private void rehash(int newLevel) {
		WeakReference<K>[] oldKeys = keys;
		V[] oldValues = values;
		reset(newLevel);
		for (int i = 0; i < oldKeys.length; i++) {
			WeakReference<K> k = oldKeys[i];
			if (k != null && k != DELETED_KEY) {
				K key = k.get();
				if (key != null) {
					put(key, oldValues[i]);
				}
			}
		}
	}

	public V get(Object key) {
		int index = getIndex(key);
		int plus = 1;
		do {
			WeakReference<K> k = keys[index];
			if (k == null) {
				return null;
			} else if (k == DELETED_KEY) {
				// continue
			} else {
				Object r = k.get();
				if (r == null) {
					delete(index);
				} else if (r == key) {
					return values[index];
				}
			}
			index = (index + plus++) & mask;
		} while (plus <= len);
		return null;
	}

	public void clear() {
		reset(2);
	}

	public boolean containsKey(Object key) {
		return get(key) != null;
	}

	public boolean containsValue(Object value) {
		if (value == null) {
			return false;
		}
		for (V item : values) {
			if (value.equals(item)) {
				return true;
			}
		}
		return false;
	}

	public Set<java.util.Map.Entry<K, V>> entrySet() {
		throw new UnsupportedOperationException();
	}

	public boolean isEmpty() {
		return size == 0;
	}

	public Set<K> keySet() {
		throw new UnsupportedOperationException();
	}

	public void putAll(Map<? extends K, ? extends V> m) {
		throw new UnsupportedOperationException();
	}

	public Collection<V> values() {
		throw new UnsupportedOperationException();
	}

}