Added tetrahedral_number.rs

DIRECTORY.md

@@ -253,6 +253,7 @@
     * [Sum Of Harmonic Series](https://github.com/TheAlgorithms/Rust/blob/master/src/math/sum_of_harmonic_series.rs)
     * [Sylvester Sequence](https://github.com/TheAlgorithms/Rust/blob/master/src/math/sylvester_sequence.rs)
     * [Tanh](https://github.com/TheAlgorithms/Rust/blob/master/src/math/tanh.rs)
+    * [Tetrahedral Number](https://github.com/TheAlgorithms/Rust/blob/master/src/math/tetrahedral_number.rs)
     * [Trapezoidal Integration](https://github.com/TheAlgorithms/Rust/blob/master/src/math/trapezoidal_integration.rs)
     * [Trial Division](https://github.com/TheAlgorithms/Rust/blob/master/src/math/trial_division.rs)
     * [Trig Functions](https://github.com/TheAlgorithms/Rust/blob/master/src/math/trig_functions.rs)

src/math/mod.rs

@@ -76,6 +76,7 @@ mod sum_of_geometric_progression;
 mod sum_of_harmonic_series;
 mod sylvester_sequence;
 mod tanh;
+mod tetrahedral_number;
 mod trapezoidal_integration;
 mod trial_division;
 mod trig_functions;
@@ -168,6 +169,7 @@ pub use self::sum_of_geometric_progression::sum_of_geometric_progression;
 pub use self::sum_of_harmonic_series::sum_of_harmonic_progression;
 pub use self::sylvester_sequence::sylvester;
 pub use self::tanh::tanh;
+pub use self::tetrahedral_number::tetrahedral_number;
 pub use self::trapezoidal_integration::trapezoidal_integral;
 pub use self::trial_division::trial_division;
 pub use self::trig_functions::cosine;

src/math/tetrahedral_number.rs

@@ -0,0 +1,36 @@
+// Tetrahedral Numbers: Function to the Nth Tetrahedral Number
+// Wikipedia Reference : https://en.wikipedia.org/wiki/Tetrahedral_number
+//
+// This program provides a function to calculate the nth triangular
+// number defined by T_n = 1 + 2 + ... + n = (n(n + 1)(n + 2))/6 =
+// (n + 2) choose 3.
+
+pub fn tetrahedral_number(n: u64) -> u64 {
+    ((n) * (n + 1) * (n + 2)) / 6
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    macro_rules! test_tetrahedral_number {
+        ($($name:ident: $inputs:expr,)*) => {
+        $(
+            #[test]
+            fn $name() {
+                let (n, expected) =$inputs;
+                assert_eq!(tetrahedral_number(n), expected);
+            }
+        )*
+        }
+    }
+
+    test_tetrahedral_number! {
+        input_0: (0, 0),
+        input_1: (1, 1),
+        input_2: (6, 56),
+        input_3: (8, 120),
+        input_4: (11, 286),
+        input_5: (34, 7140),
+    }
+}