Add emath::fast_midpoint (#7435)

Author: emilk

Cargo.toml

@@ -300,7 +300,7 @@ zero_sized_map_values = "warn"
 # doc_comment_double_space_linebreaks = "warn"
 # elidable_lifetime_names = "warn"
 # ignore_without_reason = "warn"
-# manual_midpoint = "warn"
+# manual_midpoint = "warn" # NOTE `midpoint` is often a lot slower for floats, so we have our own `emath::fast_midpoint` function.
 # non_std_lazy_statics = "warn"
 # precedence_bits = "warn"
 # return_and_then = "warn"

crates/emath/src/align.rs

@@ -134,7 +134,7 @@ impl Align {
                 if size == f32::INFINITY {
                     Rangef::new(f32::NEG_INFINITY, f32::INFINITY)
                 } else {
-                    let left = f32::midpoint(min, max) - size / 2.0;
+                    let left = crate::fast_midpoint(min, max) - size / 2.0;
                     Rangef::new(left, left + size)
                 }
             }

crates/emath/src/lib.rs

@@ -112,6 +112,21 @@ where
     (T::ONE - t) * *range.start() + t * *range.end()
 }
 
+/// This is a faster version of [`f32::midpoint`] which doesn't handle overflow.
+///
+/// ```
+/// # use emath::fast_midpoint;
+/// assert_eq!(fast_midpoint(1.0, 5.0), 3.0);
+/// ```
+#[inline(always)]
+pub fn fast_midpoint<R>(a: R, b: R) -> R
+where
+    R: Copy + Add<R, Output = R> + Div<R, Output = R> + One,
+{
+    let two = R::ONE + R::ONE;
+    (a + b) / two
+}
+
 /// Where in the range is this value? Returns 0-1 if within the range.
 ///
 /// Returns <0 if before and >1 if after.

crates/emath/src/rect.rs

@@ -1,6 +1,6 @@
 use std::fmt;
 
-use crate::{Div, Mul, NumExt as _, Pos2, Rangef, Rot2, Vec2, lerp, pos2, vec2};
+use crate::{Div, Mul, NumExt as _, Pos2, Rangef, Rot2, Vec2, fast_midpoint, lerp, pos2, vec2};
 use std::ops::{BitOr, BitOrAssign};
 
 /// A rectangular region of space.
@@ -331,8 +331,8 @@ impl Rect {
     #[inline(always)]
     pub fn center(&self) -> Pos2 {
         Pos2 {
-            x: f32::midpoint(self.min.x, self.max.x),
-            y: f32::midpoint(self.min.y, self.max.y),
+            x: fast_midpoint(self.min.x, self.max.x),
+            y: fast_midpoint(self.min.y, self.max.y),
         }
     }
 

crates/emath/src/smart_aim.rs

@@ -1,5 +1,7 @@
 //! Find "simple" numbers is some range. Used by sliders.
 
+use crate::fast_midpoint;
+
 const NUM_DECIMALS: usize = 15;
 
 /// Find the "simplest" number in a closed range [min, max], i.e. the one with the fewest decimal digits.
@@ -43,7 +45,7 @@ pub fn best_in_range_f64(min: f64, max: f64) -> f64 {
 
     if min_exponent.floor() != max_exponent.floor() {
         // pick the geometric center of the two:
-        let exponent = f64::midpoint(min_exponent, max_exponent);
+        let exponent = fast_midpoint(min_exponent, max_exponent);
         return 10.0_f64.powi(exponent.round() as i32);
     }