Add implementation of sterlings approximation

src/lib.rs

@@ -3,6 +3,9 @@
 //! Playin' with Numerics in Rust
 #![forbid(unsafe_code)]
 
+use std::f64::consts::PI;
+use std::f64::consts::E;
+
 #[cfg_attr(tarpaulin, skip)]
 pub mod bigint;
 pub mod num;
@@ -173,6 +176,13 @@ pub fn factorial(n: usize) -> Option<u128> {
     }
 }
 
+pub fn approx_factorial(n: f64) -> f64 {
+    let power = (n / E).powf(n);
+    let root = (PI * 2.0 * n).sqrt();
+
+    power * root
+}
+
 #[cfg(test)]
 #[cfg_attr(tarpaulin, skip)]
 mod tests {