From 4325c8f3c836697d90815ac86e17561c899bae36 Mon Sep 17 00:00:00 2001
From: valadaptive <valadaptive@protonmail.com>
Date: Sun, 7 Sep 2025 21:33:49 -0400
Subject: [PATCH] Optimize glyph allocation cache lookups

---
 crates/epaint/src/text/font.rs | 38 +++++++++++++++++++++++++++-------
 1 file changed, 31 insertions(+), 7 deletions(-)

diff --git a/crates/epaint/src/text/font.rs b/crates/epaint/src/text/font.rs
index 51cd88024..767c22fd7 100644
--- a/crates/epaint/src/text/font.rs
+++ b/crates/epaint/src/text/font.rs
@@ -81,8 +81,7 @@ pub struct FontImpl {
     ab_glyph_font: ab_glyph::FontArc,
     tweak: FontTweak,
     glyph_info_cache: ahash::HashMap<char, GlyphInfo>,
-    glyph_alloc_cache:
-        ahash::HashMap<(ab_glyph::GlyphId, OrderedFloat<f32>, OrderedFloat<f32>), GlyphAllocation>,
+    glyph_alloc_cache: ahash::HashMap<(ab_glyph::GlyphId, u64), GlyphAllocation>,
 }
 
 trait FontExt {
@@ -262,11 +261,10 @@ impl FontImpl {
             return GlyphAllocation::default();
         };
 
-        let entry = match self.glyph_alloc_cache.entry((
-            glyph_id,
-            metrics.px_scale_factor.into(),
-            metrics.pixels_per_point.into(),
-        )) {
+        let entry = match self
+            .glyph_alloc_cache
+            .entry((glyph_id, metrics.glyph_cache_key()))
+        {
             std::collections::hash_map::Entry::Occupied(glyph_alloc) => {
                 return *glyph_alloc.get();
             }
@@ -441,6 +439,32 @@ pub struct ScaledMetrics {
     pub row_height: f32,
 }
 
+impl ScaledMetrics {
+    fn glyph_cache_key(&self) -> u64 {
+        // This is faster than hashing two `OrderedFloat`s. Neither scale factor should ever be negative-zero or NaN.
+        debug_assert!(
+            self.pixels_per_point.is_finite() && self.pixels_per_point.is_sign_positive(),
+            "pixels_per_point can be safely hashed as raw bits"
+        );
+        debug_assert!(
+            self.px_scale_factor.is_finite() && self.px_scale_factor.is_sign_positive(),
+            "px_scale_factor can be safely hashed as raw bits"
+        );
+        let dpi_bits = self.pixels_per_point.to_le_bytes();
+        let scale_bits = self.px_scale_factor.to_le_bytes();
+        u64::from_le_bytes([
+            dpi_bits[0],
+            dpi_bits[1],
+            dpi_bits[2],
+            dpi_bits[3],
+            scale_bits[0],
+            scale_bits[1],
+            scale_bits[2],
+            scale_bits[3],
+        ])
+    }
+}
+
 /// Code points that will always be invisible (zero width).
 ///
 /// See also [`FontImpl::ignore_character`].