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Add compile-time Factorial operation #6

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Add compile-time Factorial operation #6

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29 changes: 29 additions & 0 deletions src/main.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down Expand Up @@ -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);
Expand Down