text: the CJK system-font search no longer keeps every probed font file in memory (#769)

crates/text/src/fonts.rs

@@ -215,6 +215,18 @@ impl Face {
     pub fn has_char(&self, c: char) -> bool {
         self.glyph(c).is_some()
     }
+    /// Whether the face covers every character of `text`. Unlike [`Face::has_char`], a face whose
+    /// file is not loaded yet is read into a temporary buffer that is dropped afterwards, so
+    /// probing many system faces does not keep every font file in memory.
+    pub fn covers_text(&self, text: &str) -> bool {
+        if let (FaceData::File(p), None) = (&self.info.data, self.bytes.get()) {
+            let Ok(data) = std::fs::read(p) else { return false };
+            let Ok(font) = FontRef::from_index(&data, self.info.index) else { return false };
+            let charmap = font.charmap();
+            return text.chars().all(|c| charmap.map(c).is_some_and(|g| g.to_u32() != 0));
+        }
+        text.chars().all(|c| self.has_char(c))
+    }
     /// Height of the glyph's vertical origin above its baseline, in pixels.
     /// OpenType VORG takes precedence; TrueType uses the ink top plus vmtx's top bearing.
     pub(crate) fn vertical_origin(&self, gid: u32, px: f32) -> Option<f32> {
@@ -569,4 +581,26 @@ mod tests {
         assert!(!f.has_char('\u{5d0}'), "Inter has no Hebrew");
         assert!(f.has_feature(b"liga") || f.has_feature(b"calt"));
     }
+
+    #[test]
+    fn covers_text_does_not_keep_file_bytes() {
+        let path = std::env::temp_dir().join(format!("filmcraft-covers-text-{}.ttf", std::process::id()));
+        std::fs::write(&path, INTER_REGULAR).expect("write temp font");
+        let info = FaceInfo {
+            family: "Inter".into(),
+            style: "Regular".into(),
+            weight: 400,
+            italic: false,
+            index: 0,
+            data: FaceData::File(path.clone()),
+            origin: "system",
+        };
+        let f = Face { id: 0, info, bytes: OnceLock::new(), shaper: OnceLock::new() };
+        assert!(f.covers_text("A\u{416}"));
+        assert!(!f.covers_text("A\u{5d0}"));
+        assert!(f.bytes.get().is_none(), "probing coverage must not cache the file");
+        assert!(f.data().is_some());
+        assert!(f.bytes.get().is_some());
+        let _ = std::fs::remove_file(&path);
+    }
 }

crates/ui-egui/src/cjk.rs

@@ -33,7 +33,7 @@ fn system_font() -> Option<Arc<FontData>> {
         }
         filmcraft_text::fonts::scan_system();
         let faces: Vec<_> = filmcraft_text::fonts::all_faces().into_iter().filter(|f| f.info.origin == "system" && !f.info.italic).collect();
-        let covers = |f: &filmcraft_text::fonts::Face| SAMPLE.chars().all(|c| f.has_char(c));
+        let covers = |f: &filmcraft_text::fonts::Face| f.covers_text(SAMPLE);
         let by_weight = |f: &&Arc<filmcraft_text::fonts::Face>| f.info.weight.abs_diff(400);
         let preferred = PREFERRED.iter().find_map(|name| faces.iter().filter(|f| f.info.family.eq_ignore_ascii_case(name) && covers(f)).min_by_key(by_weight));
         let face = preferred.or_else(|| faces.iter().filter(|f| covers(f)).min_by_key(by_weight))?;

crates/ui-egui/src/i18n/mod.rs

@@ -247,7 +247,7 @@ pub fn system_japanese_font() -> Option<Arc<egui::FontData>> {
     FONT.get_or_init(|| {
         filmcraft_text::fonts::scan_system();
         let faces: Vec<_> = filmcraft_text::fonts::all_faces().into_iter().filter(|f| f.info.origin == "system" && !f.info.italic).collect();
-        let covers = |f: &filmcraft_text::fonts::Face| JAPANESE_SAMPLE.chars().all(|c| f.has_char(c));
+        let covers = |f: &filmcraft_text::fonts::Face| f.covers_text(JAPANESE_SAMPLE);
         // within a family, the face closest to regular weight
         let by_weight = |f: &&Arc<filmcraft_text::fonts::Face>| f.info.weight.abs_diff(400);
         let preferred =