基于HashHeap的LFU实现

普通heap支持的操作和queue, stack一样，就是push, pop，只是pop出的是最小值，具体点就是add, delMin

hashheap支持一般HashMap的功能，同时维护最小值，和LinkedHashMap是对等的，后者是HashMap加上维护先后关系，hashHeap其实叫HeapedHashMap更科学。

LinkedHashMap -> hashmap ＋ 维护先后关系 －> LRU

HeapedHashMap ->  hashmap + 维护最小值  －> LFU

为什么没有叫HeapedHashMap的？因为有了heap，复杂度就被改变了，从O（1）到O（lgn），hash的意义就不在了。而LinkedHashMap 各种操作仍然都是O（1）的，不影响。

进一步分析：HeapedHashMap可以被stl::set取代，因为要处理堆，HeapedHashMap的所有操作都是 O（logN）的，这和stl::set一样，而后者也支持快速得到最小值（*set.begin())

class LFU<K, V> {
	private static class Counter<V> implements Comparable<Counter<V>> {
		V data;
		Integer freq;
		Counter(V data, Integer freq) {
			this.data = data;
			this.freq = freq;
		}
		@Override
		public int compareTo(Counter<V> e) {
			return this.freq - e.freq;
		}
	}
	int maxSize;
	HashHeap<K, Counter<V>> hh = new HashHeap<K, Counter<V>>();
	public LFU(int maxSize) {
		this.maxSize = maxSize;
	}
	public V get(K key) {
		Counter<V> e = hh.get(key);
		if (e == null) return null;
		e.freq++;
		hh.put(key, e);
		return e.data;
	}
	public void put(K key, V value) {
		Counter<V> e = hh.get(key);
		if (e == null) {
			if (hh.size() == maxSize) {
				System.out.println("expire " + hh.delMin());
			}
		}
		hh.put(key, new Counter<V>(value, 1));
	}
}

public class HashHeap <K, V extends Comparable<V>>  {
	private static class Entry<K, V extends Comparable<V>> {
		K key;
		V value;
		Entry(K k, V v) {
			key = k;
			value = v;
		}
	}
	private Map<K, Integer> addr = new HashMap<K, Integer>();
	private List<Entry<K, V>> heap = new ArrayList<Entry<K, V>>();
	
	public void put(K key, V value) {
		Integer pos = addr.get(key);
		if (pos == null) {
			heap.add(new Entry<K, V>(key, value));
			addr.put(key, heap.size() - 1);
			siftUp(heap.size() - 1);
		}
		else {
			Entry<K, V> e = heap.get(pos);
			V oldValue = e.value;
			e.value = value;
			if (value.compareTo(oldValue) > 0) siftDown(pos);
			else siftUp(pos);
		}
	}
	public V get(K key) {
		Integer pos = addr.get(key);
		if (pos == null) return null;
		return heap.get(pos).value;
	}
	public void remove(K key) {
		Integer pos = addr.get(key);
		if (pos == null) return;
		heap.set(pos, heap.get(heap.size() - 1));
		addr.put(heap.get(pos).key, pos);
		heap.remove(heap.size() - 1);
		siftDown(pos);
		addr.remove(key);
	}
	public K delMin() {
		if (heap.size() == 0) return null;
		Entry<K, V> e = heap.get(0);
		remove(e.key);
		return e.key;
	}
	public int size() {
		return addr.size();
	}
	private void swap(int i, int j) {
		Entry<K, V> temp = heap.get(i);
		heap.set(i, heap.get(j));
		heap.set(j, temp);
		addr.put(heap.get(i).key, i);
		addr.put(heap.get(j).key, j);
	}
	private void siftUp(int i) {
		while (i > 0 && heap.get(i).value.compareTo(heap.get((i - 1) / 2).value) < 0 ) {
			swap(i, (i - 1) / 2);
			i = (i - 1) / 2;
		}
	}
	private void siftDown(int i) {
		while (2 * i + 1 < heap.size()) {
			int j = 2 * i + 1;
			if (j + 1 < heap.size() && heap.get(j + 1).value.compareTo(heap.get(j).value) < 0) ++j;
			if (heap.get(i).value.compareTo(heap.get(j).value) > 0) {
				swap(i, j);
				i = j;
			}
			else break;
		}
	}
}