package zutil.algo.search;

import zutil.algo.sort.sortable.SortableDataList;

/**
 * This algorithm is a modified QuickSort 
 * to find the k smallest or biggest value
 * http://en.wikipedia.org/wiki/Selection_algorithm
 * 
 * @author Ziver
 *
 */
@SuppressWarnings("unchecked")
public class QuickSelect {
	
	public static Object find(SortableDataList list, int k){
		return find(list, k, 0, list.size()-1);
	}
	
	/*
	 function select(list, k, left, right)
	     select a pivot value list[pivotIndex]
	     pivotNewIndex := partition(list, left, right, pivotIndex)
	     if k = pivotNewIndex
	         return list[k]
	     else if k < pivotNewIndex
	         return select(list, k, left, pivotNewIndex-1)
	     else
	         return select(list, k, pivotNewIndex+1, right)
	 */
	public static Object find(SortableDataList list, int k, int left, int right){
		// select a pivot
		int pivot = right/2;
		int newPivot = partition(list, left, right, pivot);
		if(k == newPivot)
			return list.getIndex(k);
		else if(k < newPivot)
			return find(list, k, left, newPivot-1);
		else
			return find(list, k, newPivot+1, right);
	}
	
	/*
	 function partition(list, left, right, pivotIndex)
	     pivotValue := list[pivotIndex]
	     swap list[pivotIndex] and list[right]  // Move pivot to end
	     storeIndex := left
	     for i from left to right-1
	         if list[i] < pivotValue
	             swap list[storeIndex] and list[i]
	             storeIndex := storeIndex + 1
	     swap list[right] and list[storeIndex]  // Move pivot to its final place
	     return storeIndex
	 */
	private static int partition(SortableDataList list, int left, int right, int pivot){
		Object pivotValue = list.getIndex(pivot);
		list.swap(pivot, right);
		int storeIndex = left;
		for(int i=left; i<right ;i++){
			if(list.compare(i, pivotValue) < 0){
				list.swap(storeIndex, i);
				storeIndex = storeIndex+1;
			}
		}
		list.swap(right, storeIndex);
		return storeIndex;
	}
}