fac

fac n = fac' n 1
fac' n acc =
  guard
    | n == 0 -> acc
    | n < 3  -> n * acc
    | else   -> fac' (n - 1) (acc * n)

fac-rec.dart

int fac(int n) => fac_(n, 1);

int fac_(int n, int acc) {
  if (n == 0) return acc;
  if (n < 3) return n * acc;
  return fac_(n - 1, acc * n);
}

void main() {
  print(fac(5));
}

fac.dart

int fac(int n) => fac_(n, 1);
int fac_(int n, int acc) {
  late int ans; // answer
  var n_ = n;
  var acc_ = acc;

 while (true) {
    if (n_ == 0) {
       ans = acc_;
       break;
    }
    if (n_ < 3) {
       ans = n_ * acc_;
       break;
    }
    var n__ = n_ - 1;
    var acc__ = acc_ * n_;
    n_ = n__;
    acc_ = acc__;
 }
 
  return ans;
}

void main() {
  print(fac(5));
  print(fac(2));
  print(fac(1));
  print(fac(0));
}

gistfile1.txt

-- BASIC MATH
-- 0! = 1
-- 1! = 1
-- 2! = 2 * 1
-- n! = n * n - 1 * .. * 2 * 1
-- fac n = n!