add letter spacing

src/attrs.rs

@@ -5,6 +5,7 @@ use alloc::vec::Vec;
 use core::ops::Range;
 use rangemap::RangeMap;
 use smol_str::SmolStr;
+use std::hash::{Hash, Hasher};
 
 use crate::{CacheKeyFlags, Metrics};
 
@@ -125,6 +126,37 @@ impl From<CacheMetrics> for Metrics {
     }
 }
 
+/// A wrapper for letter spacing to get around that f32 doesn't implement Eq and Hash
+#[derive(Clone, Copy, Debug)]
+pub struct LetterSpacing(pub f32);
+
+impl PartialEq for LetterSpacing {
+    fn eq(&self, other: &Self) -> bool {
+        if self.0.is_nan() {
+            other.0.is_nan()
+        } else {
+            self.0 == other.0
+        }
+    }
+}
+
+impl Eq for LetterSpacing {}
+
+impl Hash for LetterSpacing {
+    fn hash<H: Hasher>(&self, hasher: &mut H) {
+        const CANONICAL_NAN_BITS: u32 = 0x7fc0_0000;
+
+        let bits = if self.0.is_nan() {
+            CANONICAL_NAN_BITS
+        } else {
+            // Add +0.0 to canonicalize -0.0 to +0.0
+            (self.0 + 0.0).to_bits()
+        };
+
+        bits.hash(hasher);
+    }
+}
+
 /// Text attributes
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct Attrs<'a> {
@@ -137,6 +169,8 @@ pub struct Attrs<'a> {
     pub metadata: usize,
     pub cache_key_flags: CacheKeyFlags,
     pub metrics_opt: Option<CacheMetrics>,
+    /// Letter spacing (tracking) in pixels
+    pub letter_spacing_opt: Option<LetterSpacing>,
 }
 
 impl<'a> Attrs<'a> {
@@ -153,6 +187,7 @@ impl<'a> Attrs<'a> {
             metadata: 0,
             cache_key_flags: CacheKeyFlags::empty(),
             metrics_opt: None,
+            letter_spacing_opt: None,
         }
     }
 
@@ -204,6 +239,12 @@ impl<'a> Attrs<'a> {
         self
     }
 
+    /// Set letter spacing (tracking) in EM
+    pub fn letter_spacing(mut self, letter_spacing: f32) -> Self {
+        self.letter_spacing_opt = Some(LetterSpacing(letter_spacing));
+        self
+    }
+
     /// Check if font matches
     pub fn matches(&self, face: &fontdb::FaceInfo) -> bool {
         //TODO: smarter way of including emoji
@@ -252,6 +293,8 @@ pub struct AttrsOwned {
     pub metadata: usize,
     pub cache_key_flags: CacheKeyFlags,
     pub metrics_opt: Option<CacheMetrics>,
+    /// Letter spacing (tracking) in EM
+    pub letter_spacing_opt: Option<LetterSpacing>,
 }
 
 impl AttrsOwned {
@@ -265,6 +308,7 @@ impl AttrsOwned {
             metadata: attrs.metadata,
             cache_key_flags: attrs.cache_key_flags,
             metrics_opt: attrs.metrics_opt,
+            letter_spacing_opt: attrs.letter_spacing_opt,
         }
     }
 
@@ -278,6 +322,7 @@ impl AttrsOwned {
             metadata: self.metadata,
             cache_key_flags: self.cache_key_flags,
             metrics_opt: self.metrics_opt,
+            letter_spacing_opt: self.letter_spacing_opt,
         }
     }
 }

src/shape.rs

@@ -155,11 +155,6 @@ fn shape_fallback(
     glyphs.reserve(glyph_infos.len());
     let glyph_start = glyphs.len();
     for (info, pos) in glyph_infos.iter().zip(glyph_positions.iter()) {
-        let x_advance = pos.x_advance as f32 / font_scale;
-        let y_advance = pos.y_advance as f32 / font_scale;
-        let x_offset = pos.x_offset as f32 / font_scale;
-        let y_offset = pos.y_offset as f32 / font_scale;
-
         let start_glyph = start_run + info.cluster as usize;
 
         if info.glyph_id == 0 {
@@ -167,6 +162,12 @@ fn shape_fallback(
         }
 
         let attrs = attrs_list.get_span(start_glyph);
+        let x_advance = pos.x_advance as f32 / font_scale
+            + attrs.letter_spacing_opt.map_or(0.0, |spacing| spacing.0);
+        let y_advance = pos.y_advance as f32 / font_scale;
+        let x_offset = pos.x_offset as f32 / font_scale;
+        let y_offset = pos.y_offset as f32 / font_scale;
+
         glyphs.push(ShapeGlyph {
             start: start_glyph,
             end: end_run, // Set later
@@ -453,7 +454,8 @@ fn shape_skip(
             .char_indices()
             .map(|(chr_idx, codepoint)| {
                 let glyph_id = charmap.map(codepoint);
-                let x_advance = glyph_metrics.advance_width(glyph_id);
+                let x_advance = glyph_metrics.advance_width(glyph_id)
+                    + attrs.letter_spacing_opt.map_or(0.0, |spacing| spacing.0);
                 let attrs = attrs_list.get_span(start_run + chr_idx);
 
                 ShapeGlyph {