net.clementlevallois.utils.CombinationGenerator Maven / Gradle / Ivy
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package net.clementlevallois.utils;
//CombinationGenerator.java
//--------------------------------------
// Systematically generate combinations.
// Found on the Internet
//--------------------------------------
import java.math.BigInteger;
/**
*
* @author LEVALLOIS
*/
public class CombinationGenerator {
private int[] a;
private int n;
private int r;
private BigInteger numLeft;
private BigInteger total;
//------------
// 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
//------
/**
*
*/
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
*/
public BigInteger getNumLeft() {
return numLeft;
}
//-----------------------------
// Are there more combinations?
//-----------------------------
/**
*
* @return
*/
public boolean hasMore() {
return numLeft.compareTo(BigInteger.ZERO) == 1;
}
//------------------------------------
// Return total number of combinations
//------------------------------------
/**
*
* @return
*/
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
*/
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;
}
}