feat: Implement median function (closes #1)

src/primary/median.rs

@@ -0,0 +1,29 @@
+/// Returns the median of a sequence of `f64` values.
+///
+/// For an odd number of values, returns the middle value after sorting.
+/// For an even number of values, returns the average of the two middle values.
+/// Empty input returns `None`.
+///
+/// # Example
+/// ```
+/// use rustats::primary::median;
+///
+/// let values = [1.0, 3.0, 2.0];
+/// let m = median(&values);
+/// assert_eq!(m, Some(2.0));
+/// ```
+pub fn median(values: &[f64]) -> Option<f64> {
+      if values.is_empty() {
+                return None;
+      }
+
+    let mut sorted = values.to_vec();
+      sorted.sort_by(|a, b| a.partial_cmp(b).unwrap());
+
+    let len = sorted.len();
+      if len % 2 == 1 {
+                Some(sorted[len / 2])
+      } else {
+                Some((sorted[len / 2 - 1] + sorted[len / 2]) / 2.0)
+      }
+}

src/primary/mod.rs

@@ -1,3 +1,6 @@
 pub mod mean;
 
 pub use mean::mean;
+
+pub mod median;
+pub use median::median;