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/**
 *    Copyright 2011 Peter Murray-Rust et. al.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */

package org.xmlcml.util;

//--------------------------------------
//Systematically generate combinations.
//--------------------------------------

import java.math.BigInteger;

/** generate combinations.
 * 
 */
public class CombinationGenerator {

	private int[] a;
	private int n;
	private int r;
	private BigInteger numLeft;
	private BigInteger total;

	//------------
	// Constructor
	//------------

	/** constructor.
	 * @param n
	 * @param r
	 */
	public CombinationGenerator (int n, int r) {
		if (r > n) {
			throw new IllegalArgumentException ();
		}
		if (n < 1) {
			throw new IllegalArgumentException ();
		}
		this.n = n;
		this.r = r;
		a = new int[r];
		BigInteger nFact = getFactorial (n);
		BigInteger rFact = getFactorial (r);
		BigInteger nminusrFact = getFactorial (n - r);
		total = nFact.divide (rFact.multiply (nminusrFact));
		reset ();
	}

	//------
	// Reset
	//------

	/** reset.
	 * 
	 */
	public void reset () {
		for (int i = 0; i < a.length; i++) {
			a[i] = i;
		}
		numLeft = new BigInteger (total.toString ());
	}

	//------------------------------------------------
	// Return number of combinations not yet generated
	//------------------------------------------------

	/** 
	 * @return big int
	 */
	public BigInteger getNumLeft () {
		return numLeft;
	}

	//-----------------------------
	// Are there more combinations?
	//-----------------------------

	/**
	 * @return has more
	 */
	public boolean hasMore () {
		return numLeft.compareTo (BigInteger.ZERO) == 1;
	}

	/**
	* Return total number of combinations
	 * @return big int
	 */
	public BigInteger getTotal () {
		return total;
	}

	//------------------
	// Compute factorial
	//------------------

	private static BigInteger getFactorial (int n) {
		BigInteger fact = BigInteger.ONE;
		for (int i = n; i > 1; i--) {
			fact = fact.multiply (new BigInteger (Integer.toString (i)));
		}
		return fact;
	}

	/** Generate next combination (algorithm from Rosen p. 286)
	 * 
	 * @return combinations
	 */

	public int[] getNext () {

		if (numLeft.equals (total)) {
			numLeft = numLeft.subtract (BigInteger.ONE);
			return a;
		}

		int i = r - 1;
		while (a[i] == n - r + i) {
			i--;
		}
		a[i] = a[i] + 1;
		for (int j = i + 1; j < r; j++) {
			a[j] = a[i] + j - i;
		}

		numLeft = numLeft.subtract (BigInteger.ONE);
		return a;

	}
}