PPTX export: bound the retained glyph-to-character maps

* PPTX export: keep only the last four faces' glyph-to-character maps

crates/pptx/src/fonts.rs

@@ -15,6 +15,9 @@ pub struct RunFont {
     pub italic: bool,
 }
 
+/// Faces whose glyph-to-character map is kept at once.
+const MAX_REVERSE_MAPS: usize = 4;
+
 #[derive(Default)]
 pub struct Fonts {
     names: HashMap<u32, RunFont>,
@@ -116,6 +119,11 @@ impl Fonts {
 
     /// The character a glyph shows (the lowest code point mapping to it).
     pub fn char_of(&mut self, face: &FontFace, gid: u32) -> Option<char> {
+        // A face's reverse map can hold over a million entries; keeping one per face a bundle uses
+        // is unbounded, so only the last few stay (a rebuilt map answers the same).
+        if !self.reverse.contains_key(&face.id()) && self.reverse.len() >= MAX_REVERSE_MAPS {
+            self.reverse.clear();
+        }
         let map = self.reverse.entry(face.id()).or_insert_with(|| {
             let mut m = HashMap::new();
             for (c, g) in face.chars() {
@@ -216,3 +224,26 @@ pub fn eot(face: &FontFace) -> Result<Vec<u8>, NoEmbed> {
     h.extend_from_slice(data);
     Ok(h)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn only_a_few_reverse_maps_are_kept_and_answers_do_not_change() {
+        let db = designcraft_fonts::FontDb::global();
+        let mut fonts = Fonts::default();
+        let mut last = None;
+        for style in ["Regular", "Bold", "Italic", "Bold Italic", "Semibold", "Light", "Black", "ExtraLight", "Medium", "SemiBold Italic"] {
+            let face = db.face("Source Serif 4", style);
+            let gid = face.glyph_for('A');
+            assert_eq!(fonts.char_of(&face, gid), Some('A'), "{style}");
+            assert!(fonts.reverse.len() <= MAX_REVERSE_MAPS);
+            last = Some((face, gid));
+        }
+        // The map evicted earlier is rebuilt on demand with the same answer.
+        if let Some((face, gid)) = last {
+            assert_eq!(fonts.char_of(&face, gid), Some('A'));
+        }
+    }
+}