org.nd4j.linalg.util.Factorial Maven / Gradle / Ivy
package org.nd4j.linalg.util;
/**
*
* @author dmtrl
*/
import java.math.*;
import java.util.Vector;
/** Factorials.
*/
class Factorial {
/** The list of all factorials as a vector.
*/
static Vector a = new Vector();
/** ctor().
* Initialize the vector of the factorials with 0!=1 and 1!=1.
*/
public Factorial() {
if (a.size() == 0) {
a.add(BigInteger.ONE);
a.add(BigInteger.ONE);
}
}
/** Compute the factorial of the non-negative integer.
* @param n the argument to the factorial, non-negative.
* @return the factorial of n.
*/
public BigInteger at(int n) {
while (a.size() <= n) {
final int lastn = a.size() - 1;
final BigInteger nextn = new BigInteger("" + (lastn + 1));
a.add(a.elementAt(lastn).multiply(nextn));
}
return a.elementAt(n);
}
} /* Factorial */