From 272b7360f50f1283def4a3681223804f2a5e0439 Mon Sep 17 00:00:00 2001
From: Timothy Warren <twarren@nabancard.com>
Date: Wed, 4 Mar 2020 16:53:31 -0500
Subject: [PATCH] Add implementation of sterlings approximation

---
 src/lib.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/src/lib.rs b/src/lib.rs
index 5d7c946..46f9c62 100644
--- a/src/lib.rs
+++ b/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 {