package Sequenic.T2.Engines;

import java.math.BigInteger;

/**
 * This implements a generator producing all sequences of length
 * r whose elements are to be chosen from a set of n elements. 
 * In other words, it produces all possible cartesian product over
 * n elements.
 *
 * @author Wishnu Prasetya
 */
public class SequenceGenerator extends CombinationGenerator {
	
    public SequenceGenerator(int n, int r) {
    	assert n>0 && r>=0  ;
    	this.n = n ;
    	this.r = r ;
        a = new int[r];
        try { 
        	long total = power(n,r) ; 
        	if (total <= (long) Integer.MAX_VALUE) this.total = (int) total ;
        	else throw new Error("The number of combinations is too large.") ;
        }
        catch (ArithmeticException e) {
        	throw new Error("The number of combinations is too large.",e) ;
        }
        reset();
    }
    
    /**
     * Calculate n^r.
     */
    static protected long power(int n, int r) {
    	long x = 1 ;
    	long n_ = n ;
    	while (r>0) {
    		x = n_ * x ; 
    		r-- ;
    	}
    	return x ;
    }
    
    //------
    // Reset
    //------
    @Override
    public void reset() {
        numLeft = total ;
    }


    /**
     * Generate next sequence.
     */
    @Override
    public void  getNext(int[] res) {

    	assert res.length == a.length ;
    	assert numLeft > 0 ;
 
    	numLeft-- ;
    	
        int x = numLeft ;
        int i = 0 ;
        while (i<r) { a[i] = 0 ; i++ ; }
        i=0 ;
        // System.out.println(">>> a.size = " + a.length) ;
        while (x > 0) {
        	a[i] =  x % n  ; 
        	x = x / n ;
        	i++ ;
        }
        copyArray(a,res) ;
        return ;
    }

}