1 files changed, 55 insertions, 0 deletions
diff --git a/library/std/src/f64/tests.rs b/library/std/src/f64/tests.rs
index 5c163cfe90e..04cb0109261 100644
--- a/library/std/src/f64/tests.rs
+++ b/library/std/src/f64/tests.rs
@@ -753,3 +753,58 @@ fn test_total_cmp() {
     assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&f64::INFINITY));
     assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&s_nan()));
 }
+
+#[test]
+fn test_lerp_exact() {
+    // simple values
+    assert_eq!(f64::lerp(0.0, 2.0, 4.0), 2.0);
+    assert_eq!(f64::lerp(1.0, 2.0, 4.0), 4.0);
+
+    // boundary values
+    assert_eq!(f64::lerp(0.0, f64::MIN, f64::MAX), f64::MIN);
+    assert_eq!(f64::lerp(1.0, f64::MIN, f64::MAX), f64::MAX);
+}
+
+#[test]
+fn test_lerp_consistent() {
+    assert_eq!(f64::lerp(f64::MAX, f64::MIN, f64::MIN), f64::MIN);
+    assert_eq!(f64::lerp(f64::MIN, f64::MAX, f64::MAX), f64::MAX);
+
+    // as long as t is finite, a/b can be infinite
+    assert_eq!(f64::lerp(f64::MAX, f64::NEG_INFINITY, f64::NEG_INFINITY), f64::NEG_INFINITY);
+    assert_eq!(f64::lerp(f64::MIN, f64::INFINITY, f64::INFINITY), f64::INFINITY);
+}
+
+#[test]
+fn test_lerp_nan_infinite() {
+    // non-finite t is not NaN if a/b different
+    assert!(!f64::lerp(f64::INFINITY, f64::MIN, f64::MAX).is_nan());
+    assert!(!f64::lerp(f64::NEG_INFINITY, f64::MIN, f64::MAX).is_nan());
+}
+
+#[test]
+fn test_lerp_values() {
+    // just a few basic values
+    assert_eq!(f64::lerp(0.25, 1.0, 2.0), 1.25);
+    assert_eq!(f64::lerp(0.50, 1.0, 2.0), 1.50);
+    assert_eq!(f64::lerp(0.75, 1.0, 2.0), 1.75);
+}
+
+#[test]
+fn test_lerp_monotonic() {
+    // near 0
+    let below_zero = f64::lerp(-f64::EPSILON, f64::MIN, f64::MAX);
+    let zero = f64::lerp(0.0, f64::MIN, f64::MAX);
+    let above_zero = f64::lerp(f64::EPSILON, f64::MIN, f64::MAX);
+    assert!(below_zero <= zero);
+    assert!(zero <= above_zero);
+    assert!(below_zero <= above_zero);
+
+    // near 1
+    let below_one = f64::lerp(1.0 - f64::EPSILON, f64::MIN, f64::MAX);
+    let one = f64::lerp(1.0, f64::MIN, f64::MAX);
+    let above_one = f64::lerp(1.0 + f64::EPSILON, f64::MIN, f64::MAX);
+    assert!(below_one <= one);
+    assert!(one <= above_one);
+    assert!(below_one <= above_one);
+}