Add emath::inverse_lerp

crates/emath/src/lib.rs

@@ -87,6 +87,14 @@ impl Real for f64 {}
 // ----------------------------------------------------------------------------
 
 /// Linear interpolation.
+///
+/// ```
+/// # use emath::lerp;
+/// assert_eq!(lerp(1.0..=5.0, 0.0), 1.0);
+/// assert_eq!(lerp(1.0..=5.0, 0.5), 3.0);
+/// assert_eq!(lerp(1.0..=5.0, 1.0), 5.0);
+/// assert_eq!(lerp(1.0..=5.0, 2.0), 9.0);
+/// ```
 #[inline(always)]
 pub fn lerp<R, T>(range: RangeInclusive<R>, t: T) -> R
 where
@@ -96,6 +104,34 @@ where
     (T::one() - t) * *range.start() + t * *range.end()
 }
 
+/// Where in the range is this value? Returns 0-1 if within the range.
+///
+/// Returns <0 if before and >1 if after.
+///
+/// Returns `None` if the input range is zero-width.
+///
+/// ```
+/// # use emath::inverse_lerp;
+/// assert_eq!(inverse_lerp(1.0..=5.0, 1.0), Some(0.0));
+/// assert_eq!(inverse_lerp(1.0..=5.0, 3.0), Some(0.5));
+/// assert_eq!(inverse_lerp(1.0..=5.0, 5.0), Some(1.0));
+/// assert_eq!(inverse_lerp(1.0..=5.0, 9.0), Some(2.0));
+/// assert_eq!(inverse_lerp(1.0..=1.0, 3.0), None);
+/// ```
+#[inline]
+pub fn inverse_lerp<R>(range: RangeInclusive<R>, value: R) -> Option<R>
+where
+    R: Copy + PartialEq + Sub<R, Output = R> + Div<R, Output = R>,
+{
+    let min = *range.start();
+    let max = *range.end();
+    if min == max {
+        None
+    } else {
+        Some((value - min) / (max - min))
+    }
+}
+
 /// Linearly remap a value from one range to another,
 /// so that when `x == from.start()` returns `to.start()`
 /// and when `x == from.end()` returns `to.end()`.