More lerp tests, altering lerp docs

library/std/src/f32.rs

@@ -879,19 +879,27 @@ impl f32 {
 
     /// Linear interpolation between `start` and `end`.
     ///
-    /// This enables the calculation of a "smooth" transition between `start` and `end`,
-    /// where start is represented by `self == 0.0` and `end` is represented by `self == 1.0`.
-    ///
-    /// Values below 0.0 or above 1.0 are allowed, and in general this function closely
-    /// resembles the value of `start + self * (end - start)`, plus additional guarantees.
-    ///
-    /// Those guarantees are, assuming that all values are [`finite`]:
-    ///
-    /// * The value at 0.0 is always `start` and the value at 1.0 is always `end` (exactness)
-    /// * If `start == end`, the value at any point will always be `start == end` (consistency)
-    /// * The values will always move in the direction from `start` to `end` (monotonicity)
-    ///
-    /// [`finite`]: #method.is_finite
+    /// This enables linear interpolation between `start` and `end`, where start is represented by
+    /// `self == 0.0` and `end` is represented by `self == 1.0`. This is the basis of all
+    /// "transition", "easing", or "step" functions; if you change `self` from 0.0 to 1.0
+    /// at a given rate, the result will change from `start` to `end` at a similar rate.
+    ///
+    /// Values below 0.0 or above 1.0 are allowed, allowing you to extrapolate values outside the
+    /// range from `start` to `end`. This also is useful for transition functions which might
+    /// move slightly past the end or start for a desired effect. Mathematically, the values
+    /// returned are equivalent to `start + self * (end - start)`, although we make a few specific
+    /// guarantees that are useful specifically to linear interpolation.
+    ///
+    /// These guarantees are:
+    ///
+    /// * If `start` and `end` are [finite], the value at 0.0 is always `start` and the
+    ///   value at 1.0 is always `end`. (exactness)
+    /// * If `start` and `end` are [finite], the values will always move in the direction from
+    ///   `start` to `end` (monotonicity)
+    /// * If `self` is [finite] and `start == end`, the value at any point will always be
+    ///   `start == end`. (consistency)
+    ///
+    /// [finite]: #method.is_finite
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[unstable(feature = "float_interpolation", issue = "71015")]
     pub fn lerp(self, start: f32, end: f32) -> f32 {

library/std/src/f32/tests.rs

@@ -760,8 +760,11 @@ fn test_total_cmp() {
 
 #[test]
 fn test_lerp_exact() {
+    // simple values
     assert_eq!(f32::lerp(0.0, 2.0, 4.0), 2.0);
     assert_eq!(f32::lerp(1.0, 2.0, 4.0), 4.0);
+
+    // boundary values
     assert_eq!(f32::lerp(0.0, f32::MIN, f32::MAX), f32::MIN);
     assert_eq!(f32::lerp(1.0, f32::MIN, f32::MAX), f32::MAX);
 }
@@ -770,11 +773,50 @@ fn test_lerp_exact() {
 fn test_lerp_consistent() {
     assert_eq!(f32::lerp(f32::MAX, f32::MIN, f32::MIN), f32::MIN);
     assert_eq!(f32::lerp(f32::MIN, f32::MAX, f32::MAX), f32::MAX);
+
+    // as long as t is finite, a/b can be infinite
+    assert_eq!(f32::lerp(f32::MAX, f32::NEG_INFINITY, f32::NEG_INFINITY), f32::NEG_INFINITY);
+    assert_eq!(f32::lerp(f32::MIN, f32::INFINITY, f32::INFINITY), f32::INFINITY);
+}
+
+#[test]
+fn test_lerp_nan_infinite() {
+    // non-finite t is not NaN if a/b different
+    assert!(!f32::lerp(f32::INFINITY, f32::MIN, f32::MAX).is_nan());
+    assert!(!f32::lerp(f32::NEG_INFINITY, f32::MIN, f32::MAX).is_nan());
 }
 
 #[test]
 fn test_lerp_values() {
+    // just a few basic values
     assert_eq!(f32::lerp(0.25, 1.0, 2.0), 1.25);
     assert_eq!(f32::lerp(0.50, 1.0, 2.0), 1.50);
     assert_eq!(f32::lerp(0.75, 1.0, 2.0), 1.75);
 }
+
+#[test]
+fn test_lerp_monotonic() {
+    // near 0
+    let below_zero = f32::lerp(-f32::EPSILON, f32::MIN, f32::MAX);
+    let zero = f32::lerp(0.0, f32::MIN, f32::MAX);
+    let above_zero = f32::lerp(f32::EPSILON, f32::MIN, f32::MAX);
+    assert!(below_zero <= zero);
+    assert!(zero <= above_zero);
+    assert!(below_zero <= above_zero);
+
+    // near 0.5
+    let below_half = f32::lerp(0.5 - f32::EPSILON, f32::MIN, f32::MAX);
+    let half = f32::lerp(0.5, f32::MIN, f32::MAX);
+    let above_half = f32::lerp(0.5 + f32::EPSILON, f32::MIN, f32::MAX);
+    assert!(below_half <= half);
+    assert!(half <= above_half);
+    assert!(below_half <= above_half);
+
+    // near 1
+    let below_one = f32::lerp(1.0 - f32::EPSILON, f32::MIN, f32::MAX);
+    let one = f32::lerp(1.0, f32::MIN, f32::MAX);
+    let above_one = f32::lerp(1.0 + f32::EPSILON, f32::MIN, f32::MAX);
+    assert!(below_one <= one);
+    assert!(one <= above_one);
+    assert!(below_one <= above_one);
+}

library/std/src/f64.rs

@@ -881,19 +881,27 @@ impl f64 {
 
     /// Linear interpolation between `start` and `end`.
     ///
-    /// This enables the calculation of a "smooth" transition between `start` and `end`,
-    /// where start is represented by `self == 0.0` and `end` is represented by `self == 1.0`.
-    ///
-    /// Values below 0.0 or above 1.0 are allowed, and in general this function closely
-    /// resembles the value of `start + self * (end - start)`, plus additional guarantees.
-    ///
-    /// Those guarantees are, assuming that all values are [`finite`]:
-    ///
-    /// * The value at 0.0 is always `start` and the value at 1.0 is always `end` (exactness)
-    /// * If `start == end`, the value at any point will always be `start == end` (consistency)
-    /// * The values will always move in the direction from `start` to `end` (monotonicity)
-    ///
-    /// [`finite`]: #method.is_finite
+    /// This enables linear interpolation between `start` and `end`, where start is represented by
+    /// `self == 0.0` and `end` is represented by `self == 1.0`. This is the basis of all
+    /// "transition", "easing", or "step" functions; if you change `self` from 0.0 to 1.0
+    /// at a given rate, the result will change from `start` to `end` at a similar rate.
+    ///
+    /// Values below 0.0 or above 1.0 are allowed, allowing you to extrapolate values outside the
+    /// range from `start` to `end`. This also is useful for transition functions which might
+    /// move slightly past the end or start for a desired effect. Mathematically, the values
+    /// returned are equivalent to `start + self * (end - start)`, although we make a few specific
+    /// guarantees that are useful specifically to linear interpolation.
+    ///
+    /// These guarantees are:
+    ///
+    /// * If `start` and `end` are [finite], the value at 0.0 is always `start` and the
+    ///   value at 1.0 is always `end`. (exactness)
+    /// * If `start` and `end` are [finite], the values will always move in the direction from
+    ///   `start` to `end` (monotonicity)
+    /// * If `self` is [finite] and `start == end`, the value at any point will always be
+    ///   `start == end`. (consistency)
+    ///
+    /// [finite]: #method.is_finite
     #[must_use = "method returns a new number and does not mutate the original value"]
     #[unstable(feature = "float_interpolation", issue = "71015")]
     pub fn lerp(self, start: f64, end: f64) -> f64 {

library/std/src/f64/tests.rs

@@ -756,8 +756,11 @@ fn test_total_cmp() {
 
 #[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);
 }
@@ -766,11 +769,42 @@ fn test_lerp_exact() {
 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);
+}