Add support for calculating binomial coefficients

CHANGELOG.md

@@ -7,7 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Code for N-choose-k
+
+## [0.1.0] - 2024-09-06
+
 ### Added
 - Added implementations for incomplete gamma functions, converted from Math.Net
 
-[Unreleased]: https://git.chir.rs/ProcyOS/rust-template
+[Unreleased]: https://git.chir.rs/ProcyOS/extra_math/compare/v0.1.0...main
+[0.1.0]: https://git.chir.rs/ProcyOS/extra_math/releases/tag/v0.1.0

README.md

@@ -30,8 +30,7 @@ repository](https://github.com/PurpleBooth/a-good-readme-template/tags).
 
 ## Authors
 
-  - **Billie Thompson** - *Provided README Template* -
-    [PurpleBooth](https://github.com/PurpleBooth)
+- **Charlotte 🦝 Delenk** [DarkKirb](https://lotte.chir.rs)
 
 See also the list of
 [contributors](https://github.com/PurpleBooth/a-good-readme-template/contributors)

src/lib.rs

@@ -2,3 +2,4 @@
 #![no_std]
 
 pub mod gamma;
+pub mod n_choose_k;

src/n_choose_k.rs

@@ -0,0 +1,36 @@
+//! Calculates the binomial coefficient N-choose-K.
+
+use num_traits::{Num, NumCast, PrimInt};
+
+/// Calculates the binomial coefficient
+pub trait BinomialCoefficient: Num + NumCast {
+    /// Calculates the binary coefficient N-choose-K.
+    ///
+    /// # Return
+    /// Returns None if k cannot be represented in the current number type.
+    fn choose<I: PrimInt>(self, k: I) -> Option<Self>;
+}
+
+impl<N: Num + NumCast + Copy> BinomialCoefficient for N {
+    fn choose<I: PrimInt>(self, k: I) -> Option<Self> {
+        let mut num = Self::one();
+        let mut denom = Self::one();
+        let mut i = I::one();
+        while i <= k {
+            num = num * (self + Self::one() - Self::from(i)?);
+            denom = denom * Self::from(i)?;
+            i = i + I::one();
+        }
+        Some(num / denom)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::n_choose_k::BinomialCoefficient;
+
+    #[test]
+    fn test_binomial() {
+        assert_eq!(BinomialCoefficient::choose(4, 2), Some(6));
+    }
+}