PairLoop.java

package oripa.paint.util;

import java.util.Iterator;

public class PairLoop {

	/**
	 * loop of (begin, begin+1), ... , (n-2, n-1), (n-1, 0).
	 * @param begin
	 * @param elements Loop target
	 * @param block What to do for the pairs
	 * @return The first element of the last pair, in other word, i of (i, i+1).
	 *         null if {@code block} never made a break by returning false.
	 */
	public static <Element> Element iterateFrom(Iterator<Element> begin,
			Iterable<Element> elements, Block<Element> block){

        Iterator<Element> iterator = begin;
		Element e1 = iterator.next();
        
        while(iterator.hasNext()) {
			Element e2 = iterator.next();

			if(block.yield(e1, e2) == false){
				return e1;
			}
			
			e1 = e2;
		}
        
        if(block.yield(e1, elements.iterator().next()) == false){
        	return e1;
        }
        
        return null;
		
	}
	
	/**
	 * loop of (0, 1), (1, 2) ... (n-2, n-1), (n-1, 0) for n elements.
	 * @param elements Loop target
	 * @param block What to do for the pairs
	 * @return The first element of the last pair, in other word, i of (i, i+1).
	 *         null if {@code block} never made a break by returning false.
	 */
	public static <Element> Element iterateAll(
			Iterable<Element> elements, Block<Element> block){
		return iterateFrom(elements.iterator(), elements, block);
	}

	/**
	 * loop of first {@code count} pairs.
	 * This method does not iterate to (n-1, 0) pair.
	 * @param elements Loop target
	 * @param count the times of iteration
	 * @param block What to do for the pairs
	 * @return The first element of the last pair, in other word, i of (i, i+1).
	 *         null if {@code block} never made a break by returning false.
	 */
	
	public static <Element> Element iterateWithCount(
			Iterable<Element> elements, int count, Block<Element> block){

		if(count <= 0){
			throw new IllegalArgumentException("count should be larger than 0");
		}
		
		Iterator<Element> iterator = elements.iterator();
		Element e1 = iterator.next();
        
        while(iterator.hasNext() && count > 0) {
			Element e2 = iterator.next();

			if(block.yield(e1, e2) == false){
				return e1;
			}
			
			e1 = e2;
			count--;
		}
        
        
        return null;
				
	}
	
	public interface Block<Element>{
		/**
		 * this method must return true to go next.
		 * @param element
		 * @param nextElement
		 * @return true if the loop continues, otherwise false.
		 */
		public boolean yield(Element element, Element nextElement);
	}

}