[pull] master from TheAlgorithms:master

src/math/area_under_curve.rs

@@ -0,0 +1,53 @@
+fn area_under_curve(start: f64, end: f64, func: fn(f64) -> f64, step_count: usize) -> f64 {
+    assert!(step_count > 0);
+
+    let (start, end) = if start > end {
+        (end, start)
+    } else {
+        (start, end)
+    }; //swap if bounds reversed
+
+    let step_length: f64 = (end - start) / step_count as f64;
+    let mut area: f64 = 0f64;
+    let mut fx1 = func(start);
+    let mut fx2: f64;
+
+    for eval_point in (1..step_count + 1).map(|x| (x as f64 * step_length) + start) {
+        fx2 = func(eval_point);
+        area += (fx2 + fx1).abs() * step_length * 0.5;
+        fx1 = fx2;
+    }
+
+    area
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_linear_func() {
+        assert_eq!(area_under_curve(1f64, 2f64, |x| x, 10), 1.5000000000000002);
+    }
+
+    #[test]
+    fn test_quadratic_func() {
+        assert_eq!(
+            area_under_curve(1f64, 2f64, |x| x * x, 1000),
+            2.333333500000005
+        );
+    }
+
+    #[test]
+    fn test_zero_length() {
+        assert_eq!(area_under_curve(0f64, 0f64, |x| x * x, 1000), 0.0);
+    }
+
+    #[test]
+    fn test_reverse() {
+        assert_eq!(
+            area_under_curve(1f64, 2f64, |x| x, 10),
+            area_under_curve(2f64, 1f64, |x| x, 10)
+        );
+    }
+}