/**
* Computes the factorial of a number (n!).
*
* @param n Number.
* @return Result.
*/
public static double Factorial(int n) {
if (fcache == null) {
// Initialize factorial cache
fcache = new double[33];
fcache[0] = 1;
fcache[1] = 1;
fcache[2] = 2;
fcache[3] = 6;
fcache[4] = 24;
ftop = 4;
}
if (n < 0) {
try {
throw new ArithmeticException("Argument cannot be negative.");
} catch (Exception e) {
e.printStackTrace();
}
}
if (n > 32) {
// Return Gamma approximation using exp(gammaln(n+1)),
// which for some reason is faster than gamma(n+1).
return Math.exp(Gamma.Log(n + 1.0));
} else {
// Compute in the standard way, but use the
// factorial cache to speed up computations.
while (ftop < n) {
int j = ftop++;
fcache[ftop] = fcache[j] * ftop;
}
return fcache[n];
}
}
/**
* Returns the log factorial of a number (ln(n!))
*
* @param n Number.
* @return Result.
*/
public static double LogFactorial(int n) {
if (lnfcache == null) lnfcache = new double[101];
if (n < 0) {
// Factorial is not defined for negative numbers.
try {
throw new ArithmeticException("Argument cannot be negative.");
} catch (Exception e) {
e.printStackTrace();
}
}
if (n <= 1) {
// Factorial for n between 0 and 1 is 1, so log(factorial(n)) is 0.
return 0.0;
}
if (n <= 100) {
// Compute the factorial using ln(gamma(n)) approximation, using the cache
// if the value has been previously computed.
return (lnfcache[n] > 0) ? lnfcache[n] : (lnfcache[n] = Gamma.Log(n + 1.0));
} else {
// Just compute the factorial using ln(gamma(n)) approximation.
return Gamma.Log(n + 1.0);
}
}
