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Implementation of the downhill collider algorithm.

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package fr.vergne.downhill;

import java.util.Collection;

/**
 * A {@link DownhillCollider} aims at merging elements together depending on
 * some similarity measures. A close description can be found with Snowball Sampling,
 * yet this is not exactly what we do here. However, the metaphor is the same:
 * little snowballs are rolling downhill and merge together when they
 * collide (touch each other). At the end, one or several big snowballs
 * remain, which are completely separated and so could not collide.

 * 

 * It is not a clustering algorithm, in the sense that it does not group
 * the elements in several sets, but merge them together. It can be used
 * as such if the balls are actually sets, which means that these sets
 * will be merged together, producing a clustering. But it is a specific
 * algorithm which can manage only certain types of clustering, not all of them.

 * 
 * @author Matthieu Vergne 
 * 
 * @param 
 */
public interface DownhillCollider {

	/**
	 * 
	 * @param balls
	 *            the balls to roll downhill
	 * @param colliders
	 *            the set of collision rules
	 * @return the set of balls remaining when no merging possibility remains
	 */
	public Collection rolls(Collection balls,
			Collider... colliders);

	/**
	 * A {@link Collider} describe how collision are managed.
	 * 
	 * @author Matthieu Vergne 
	 * 
	 * @param 
	 */
	public static interface Collider {
		/**
		 * 
		 * @param b1
		 *            a ball
		 * @param b2
		 *            another ball
		 * @return true if the balls are colliding,
		 *         false otherwise
		 */
		public boolean areColliding(Ball b1, Ball b2);

		/**
		 * 
		 * @param b1
		 *            a ball
		 * @param b2
		 *            another ball
		 * @return the merging of the two balls in a single one
		 */
		public Ball collide(Ball b1, Ball b2);
	}
}