[slider] only show full precision if significant

egui/src/math.rs

@@ -86,7 +86,8 @@ pub fn round_to_precision(value: f32, decimal_places: usize) -> f32 {
 
 pub fn format_with_minimum_precision(value: f32, precision: usize) -> String {
     let text = format!("{:.*}", precision, value);
-    if (text.parse::<f32>().unwrap() - value).abs() <= std::f32::EPSILON {
+    let epsilon = 16.0 * f32::EPSILON; // margin large enough to handle most peoples round-tripping needs
+    if almost_equal(text.parse::<f32>().unwrap(), value, epsilon) {
         // Enough precision to show the value accurately - good!
         text
     } else {
@@ -96,3 +97,52 @@ pub fn format_with_minimum_precision(value: f32, precision: usize) -> String {
         value.to_string()
     }
 }
+
+/// Should return true when arguments are the same within some rounding error.
+/// For instance `almost_equal(x, x.to_degrees().to_radians(), f32::EPSILON)` should hold true for all x.
+/// The `epsilon`  can be `f32::EPSILON` to handle simple transforms (like degrees -> radians)
+/// but should be higher to handle more complex transformations.
+pub fn almost_equal(a: f32, b: f32, epsilon: f32) -> bool {
+    #![allow(clippy::float_cmp)]
+
+    if a == b {
+        true // handle infinites
+    } else {
+        let abs_max = a.abs().max(b.abs());
+        abs_max <= epsilon || ((a - b).abs() / abs_max) <= epsilon
+    }
+}
+
+#[test]
+fn test_almost_equal() {
+    for &x in &[
+        0.0_f32,
+        f32::MIN_POSITIVE,
+        1e-20,
+        1e-10,
+        f32::EPSILON,
+        0.1,
+        0.99,
+        1.0,
+        1.001,
+        1e10,
+        f32::MAX / 100.0,
+        // f32::MAX, // overflows in rad<->deg test
+        f32::INFINITY,
+    ] {
+        for &x in &[-x, x] {
+            for roundtrip in &[
+                |x: f32| x.to_degrees().to_radians(),
+                |x: f32| x.to_radians().to_degrees(),
+            ] {
+                let epsilon = f32::EPSILON;
+                assert!(
+                    almost_equal(x, roundtrip(x), epsilon),
+                    "{} vs {}",
+                    x,
+                    roundtrip(x)
+                );
+            }
+        }
+    }
+}