From e59d19dbf06b91752413e648f33b994f9883c8d5 Mon Sep 17 00:00:00 2001
From: Ludi Rehak <ludi317@gmail.com>
Date: Sat, 17 Feb 2024 21:36:14 -0800
Subject: [PATCH] Add compile-time Factorial operation

---
 src/main.rs | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/src/main.rs b/src/main.rs
index 58bed50..10e0ee5 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -130,6 +130,26 @@ where
     type Product = <(Succ<U>, <(T, Succ<U>) as Mul>::Product) as Add>::Sum;
 }
 
+// Factorial
+
+trait Factorial {
+    type Result;
+}
+
+// Base case: 0! = 1
+impl Factorial for Zero {
+    type Result = Succ<Zero>;
+}
+
+impl<T> Factorial for Succ<T>
+where
+    T: Factorial,
+    (Succ<T>, T::Result): Mul,
+{
+    // n! = n * (n-1)!
+    type Result = <(Succ<T>, T::Result) as Mul>::Product;
+}
+
 // Division
 
 trait GreaterThanEq {
@@ -229,6 +249,15 @@ mod tests {
         assert_eq!(<(encode!(**), encode!(***)) as Mul>::Product::VALUE, 6);
     }
 
+    #[test]
+    fn factorial() {
+        assert_eq!(<encode!() as Factorial>::Result::VALUE, 1);
+        assert_eq!(<encode!(*) as Factorial>::Result::VALUE, 1);
+        assert_eq!(<encode!(**) as Factorial>::Result::VALUE, 2);
+        assert_eq!(<encode!(***) as Factorial>::Result::VALUE, 6);
+        assert_eq!(<encode!(****) as Factorial>::Result::VALUE, 24);
+    }
+
     #[test]
     fn greater_than_eq() {
         assert_eq!(<(encode!(), encode!()) as GreaterThanEq>::Greater::VALUE, 1);