fix(fonts): subset added TrueType fonts to used glyphs on document save

crates/engine/src/lib.rs

@@ -4484,12 +4484,14 @@ impl Session {
         // file so that revision stays in the chain. A full rewrite (redaction, new encryption,
         // a reconstructed file) still writes the current graph on its own.
         let rewrite = editor.cos.full_save_required() || editor.cos.encryption_changed() || editor.cos.revisions().is_empty();
+        let mut cos = editor.cos.clone();
+        pdfcraft_edit::text::subset_document_fonts(&mut cos)?;
         guard(|| {
             if on_saved || rewrite {
-                write_incremental(&editor.cos, &opts)
+                write_incremental(&cos, &opts)
             } else {
                 let mut base = pdfcraft_cos::Document::open_with_password(editor.saved.clone(), editor.saved_keys.reopen.as_deref())?;
-                base.overlay_changes_from(&editor.cos)?;
+                base.overlay_changes_from(&cos)?;
                 write_incremental(&base, &opts)
             }
         })
@@ -4506,7 +4508,9 @@ impl Session {
         }
         let editor = doc.editor.as_ref().ok_or_else(|| EditError::ReadOnly(doc.read_only_reason.clone().unwrap_or_default()))?;
         let opts = SaveOptions { mod_date: self.now().map(pdfcraft_cos::pdf_date), ..SaveOptions::default() };
-        guard(|| write_full(&editor.cos, &opts)).map_err(EditError::Write)?.map(Arc::new).map_err(|e| EditError::Write(e.to_string()))
+        let mut cos = editor.cos.clone();
+        pdfcraft_edit::text::subset_document_fonts(&mut cos)?;
+        guard(|| write_full(&cos, &opts)).map_err(EditError::Write)?.map(Arc::new).map_err(|e| EditError::Write(e.to_string()))
     }
 
     /// Check that `bytes`, from [`Self::save_bytes`] or [`Self::save_full_bytes`], reopen as this

crates/engine/tests/it/document_fonts.rs

@@ -96,3 +96,50 @@ fn contextual_arabic_marks_ligatures_and_mixed_bidi_keep_logical_text_after_reop
     }
     exercise(font.bytes.to_vec(), "سَلَام 123", "NotoSansArabic-Regular");
 }
+
+#[test]
+fn added_font_is_subset_to_used_glyphs_on_save() {
+    let font_bytes = std::fs::read(std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("../../assets/fonts/Inter-Regular.ttf")).unwrap();
+    let font_len = font_bytes.len();
+    assert!(font_len > 300_000, "source font is full sized");
+
+    let mut s = Session::new();
+    let id = s.open("source.pdf", None, document(), None).unwrap();
+    s.apply(id, Edit::RegisterTextFont { page: 0, bytes: Arc::new(font_bytes) }).unwrap();
+    let resource = s.get(id).unwrap().text_editor_fonts(0).into_iter().find(|f| f.face == "Inter-Regular").unwrap().resource;
+    let style = BlockStyle { resource: Some(resource), width: Some(240.0), ..Default::default() };
+    let text = "Three words here";
+    s.apply(id, Edit::EditTextBlock { page: 0, block: 0, text: text.into(), style }).unwrap();
+
+    let saved = s.save_bytes(id).unwrap();
+    assert!(saved.len() < 50_000, "saved PDF with 3 words must be subset and small, was {} bytes", saved.len());
+
+    let saved_doc = pdfcraft_cos::Document::open(saved.clone()).unwrap();
+    let mut subset_found = false;
+    for (_, obj) in saved_doc.scan_objects() {
+        if let pdfcraft_cos::Object::Dict(d) = obj.as_ref()
+            && d.name(b"Type") == Some(b"Font")
+            && d.name(b"Subtype") == Some(b"Type0")
+            && let Some(base) = d.name(b"BaseFont")
+        {
+            let base_str = String::from_utf8_lossy(base);
+            if base_str.ends_with("Inter-Regular") {
+                assert!(base_str.len() >= 7 && base_str.as_bytes()[6] == b'+', "font base name {base_str} must have subset prefix");
+                subset_found = true;
+            }
+        }
+        if let pdfcraft_cos::Object::Stream(stream) = obj.as_ref()
+            && stream.dict.get(b"PCFullFont").is_some()
+        {
+            panic!("saved PDF must not retain PCFullFont flag");
+        }
+    }
+    assert!(subset_found, "Type0 subset font must be present in saved document");
+
+    // Also verify reopening and rendering matches:
+    let re = s.open("saved.pdf", None, saved, None).unwrap();
+    assert_eq!(s.get(re).unwrap().text_blocks(0)[0].editor_text, text);
+    let re_pixels = pixels(s.get(re).unwrap().display.clone());
+    let orig_pixels = pixels(s.get(id).unwrap().display.clone());
+    assert_eq!(re_pixels, orig_pixels, "reopened subset font renders identically to original");
+}