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package edu.stanford.nlp.util;
import java.util.*;
/**
* Implements a heap as an ArrayList.
* Values are all implicit in the comparator
* passed in on construction. Decrease key is supported, though only
* lg(n). Unlike the previous implementation of this class, this
* heap interprets the addition of an existing element as a "change
* key" which gets ignored unless it actually turns out to be a
* decrease key. Note that in this implementation, changing the key
* of an object should trigger a change in the comparator's ordering
* for that object, but should NOT change the equality of that
* object.
*
* @author Dan Klein
* @author Christopher Manning
* @version 1.2, 07/31/02
*/
public class ArrayHeap extends AbstractSet implements Heap {
/**
* A HeapEntry stores an object in the heap along with
* its current location (array position) in the heap.
*
* @author Dan Klein
* @version 1.2
*/
private static final class HeapEntry {
public E object;
public int index;
}
/**
* indexToEntry maps linear array locations (not
* priorities) to heap entries.
*/
private ArrayList> indexToEntry;
/**
* objectToEntry maps heap objects to their heap
* entries.
*/
private Map> objectToEntry;
/**
* cmp is the comparator passed on construction.
*/
private Comparator cmp;
// Primitive Heap Operations
private static int parent(final int index) {
return (index - 1) / 2;
}
private HeapEntry parent(HeapEntry entry) {
int index = entry.index;
return (index > 0 ? indexToEntry.get((index - 1) / 2) : null);
}
private HeapEntry leftChild(HeapEntry entry) {
int index = entry.index;
int leftIndex = index * 2 + 1;
return (leftIndex < size() ? indexToEntry.get(leftIndex) : null);
}
private HeapEntry rightChild(HeapEntry entry) {
int index = entry.index;
int rightIndex = index * 2 + 2;
return (rightIndex < size() ? indexToEntry.get(rightIndex) : null);
}
private int compare(HeapEntry entryA, HeapEntry entryB) {
return cmp.compare(entryA.object, entryB.object);
}
private void swap(HeapEntry entryA, HeapEntry entryB) {
int indexA = entryA.index;
int indexB = entryB.index;
entryA.index = indexB;
entryB.index = indexA;
indexToEntry.set(indexA, entryB);
indexToEntry.set(indexB, entryA);
}
/**
* Remove the last element of the heap (last in the index array).
* Do not call this on other entries; the last entry is only passed
* in for efficiency.
*
* @param entry the last entry in the array
*/
private void removeLast(HeapEntry entry) {
indexToEntry.remove(entry.index);
objectToEntry.remove(entry.object);
}
private HeapEntry getEntry(E o) {
HeapEntry entry = objectToEntry.get(o);
if (entry == null) {
entry = new HeapEntry();
entry.index = size();
entry.object = o;
indexToEntry.add(entry);
objectToEntry.put(o, entry);
}
return entry;
}
/**
* iterative heapify up: move item o at index up until correctly placed
*/
private int heapifyUp(HeapEntry entry) {
int numSwaps = 0;
while (true) {
if (entry.index == 0) {
break;
}
HeapEntry parentEntry = parent(entry);
if (compare(entry, parentEntry) >= 0) {
break;
}
numSwaps++;
swap(entry, parentEntry);
}
return numSwaps;
}
/**
* On the assumption that
* leftChild(entry) and rightChild(entry) satisfy the heap property,
* make sure that the heap at entry satisfies this property by possibly
* percolating the element o downwards. I've replaced the obvious
* recursive formulation with an iterative one to gain (marginal) speed
*/
private void heapifyDown(HeapEntry entry) {
// int size = size();
HeapEntry minEntry; // = null;
do {
minEntry = entry;
HeapEntry leftEntry = leftChild(entry);
if (leftEntry != null) {
if (compare(minEntry, leftEntry) > 0) {
minEntry = leftEntry;
}
}
HeapEntry rightEntry = rightChild(entry);
if (rightEntry != null) {
if (compare(minEntry, rightEntry) > 0) {
minEntry = rightEntry;
}
}
if (minEntry != entry) {
// Swap min and current
swap(minEntry, entry);
// at start of next loop, we set currentIndex to largestIndex
// this indexation now holds current, so it is unchanged
}
} while (minEntry != entry);
// System.err.println("Done with heapify down");
// verify();
}
/**
* Finds the object with the minimum key, removes it from the heap,
* and returns it.
*
* @return The object with minimum key
*/
@Override
public E extractMin() {
if (isEmpty()) {
throw new NoSuchElementException();
}
HeapEntry minEntry = indexToEntry.get(0);
int lastIndex = size() - 1;
if (lastIndex > 0) {
HeapEntry lastEntry = indexToEntry.get(lastIndex);
swap(lastEntry, minEntry);
removeLast(minEntry);
heapifyDown(lastEntry);
} else {
removeLast(minEntry);
}
return minEntry.object;
}
/**
* Finds the object with the minimum key and returns it, without
* modifying the heap.
*
* @return The object with minimum key
*/
@Override
public E min() {
HeapEntry minEntry = indexToEntry.get(0);
return minEntry.object;
}
/**
* Adds an object to the heap. If the object is already in the heap
* with worse score, this acts as a decrease key. If the object is
* already present, with better score, it will NOT cause an
* "increase key".
*
* @param o an Object value
*/
@Override
public boolean add(E o) {
decreaseKey(o);
return true;
}
/**
* Changes the position of an element o in the heap based on a
* change in the ordering of o. If o's key has actually increased,
* it will do nothing, particularly not an "increase key".
*
* @param o An Object value
* @return The number of swaps done on decrease.
*/
@Override
public int decreaseKey(E o) {
HeapEntry entry = getEntry(o);
if (o != entry.object) {
if (cmp.compare(o, entry.object) < 0) {
entry.object = o;
}
}
return heapifyUp(entry);
}
/**
* Checks if the heap is empty.
*
* @return a boolean value
*/
@Override
public boolean isEmpty() {
return indexToEntry.isEmpty();
}
/**
* Get the number of elements in the heap.
*
* @return an int value
*/
@Override
public int size() {
return indexToEntry.size();
}
@Override
public Iterator iterator() {
Heap tempHeap = new ArrayHeap(cmp, size());
List tempList = new ArrayList(size());
for (E obj : objectToEntry.keySet()) {
tempHeap.add(obj);
}
while (!tempHeap.isEmpty()) {
tempList.add(tempHeap.extractMin());
}
return tempList.iterator();
}
/**
* Clears the heap. Equivalent to calling extractMin repeatedly
* (but faster).
*/
@Override
public void clear() {
indexToEntry.clear();
objectToEntry.clear();
}
public void dump() {
for (int j = 0; j < indexToEntry.size(); j++) {
System.err.println(" " + j + " " + ((Scored) indexToEntry.get(j).object).score());
}
}
public void verify() {
for (int i = 0; i < indexToEntry.size(); i++) {
if (i != 0) {
// check ordering
if (compare(indexToEntry.get(i), indexToEntry.get(parent(i))) < 0) {
System.err.println("Error in the ordering of the heap! (" + i + ")");
dump();
System.exit(0);
}
}
// check placement
if (i != indexToEntry.get(i).index) {
System.err.println("Error in placement in the heap!");
}
}
}
/** Create an ArrayHeap.
*
* @param cmp The objects added will be ordered using the Comparator.
*/
public ArrayHeap(Comparator cmp) {
this.cmp = cmp;
indexToEntry = new ArrayList>();
objectToEntry = Generics.newHashMap();
}
public ArrayHeap(Comparator cmp, int initCapacity) {
this.cmp = cmp;
indexToEntry = new ArrayList>(initCapacity);
objectToEntry = Generics.newHashMap(initCapacity);
}
public List asList() {
return new LinkedList(this);
}
/**
* Prints the array entries in sorted comparator order.
* @return The array entries in sorted comparator order.
*/
@Override
public String toString() {
ArrayList result = new ArrayList();
for(E key : objectToEntry.keySet())
result.add(key);
Collections.sort(result,cmp);
return result.toString();
}
}
