M10: a swapped post 2.0 table's glyph names serve /Differences (#2946)

Review follow-up on #2949.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv2WwdgjmBz33GWkwgNn3Q

crates/render/src/raster.rs

@@ -4708,7 +4708,7 @@ trailer << /Root 1 0 R >>
             .find(|e| &e[..4] == b"post")
             .map(|e| u32::from_be_bytes([e[8], e[9], e[10], e[11]]) as usize)
             .expect("the test font has a post table");
-        let render = |version: [u8; 4]| -> Vec<u8> {
+        let render_with = |version: [u8; 4], encoding: &str| -> Vec<u8> {
             let mut program = font.to_vec();
             program[post..post + 4].copy_from_slice(&version);
             let content = b"BT /F1 28 Tf 10 40 Td (Hamburg 0123) Tj ET";
@@ -4718,7 +4718,7 @@ trailer << /Root 1 0 R >>
                 b"<< /Type /Pages /Kids [3 0 R] /Count 1 >>".to_vec(),
                 b"<< /Type /Page /Parent 2 0 R /MediaBox [0 0 300 80] /Contents 4 0 R /Resources << /Font << /F1 5 0 R >> >> >>".to_vec(),
                 [format!("<< /Length {} >>\nstream\n", content.len()).into_bytes(), content.to_vec(), b"\nendstream".to_vec()].concat(),
-                format!("<< /Type /Font /Subtype /TrueType /BaseFont /JetBrainsMono-Regular /FirstChar 32 /LastChar 126 /Widths [{widths}] /FontDescriptor 6 0 R /Encoding /WinAnsiEncoding >>").into_bytes(),
+                format!("<< /Type /Font /Subtype /TrueType /BaseFont /JetBrainsMono-Regular /FirstChar 32 /LastChar 126 /Widths [{widths}] /FontDescriptor 6 0 R /Encoding {encoding} >>").into_bytes(),
                 b"<< /Type /FontDescriptor /FontName /JetBrainsMono-Regular /Flags 32 /FontBBox [-1000 -300 1000 1000] /ItalicAngle 0 /Ascent 1020 /Descent -300 /CapHeight 730 /StemV 80 /FontFile2 7 0 R >>".to_vec(),
                 [format!("<< /Length {} >>\nstream\n", program.len()).into_bytes(), program, b"\nendstream".to_vec()].concat(),
             ];
@@ -4747,10 +4747,16 @@ trailer << /Root 1 0 R >>
             assert!(page.error.is_none(), "{:?}", page.error);
             page.rgba.to_vec()
         };
+        let render = |version: [u8; 4]| render_with(version, "/WinAnsiEncoding");
         let correct = render([0, 3, 0, 0]);
         assert_eq!(render([0, 0, 0, 3]), correct, "3.0 with swapped halves draws with the embedded font");
         assert_eq!(render([0, 0, 0, 2]), render([0, 2, 0, 0]), "2.0 with swapped halves too");
         assert_ne!(render([0, 9, 0, 0]), correct, "an unknown version is still refused");
+        // A /Differences name only the font's own post table knows (W's alternate, `W.cv07`)
+        // is found through a swapped 2.0 table too (review on #2949).
+        let alternate = |version: [u8; 4]| render_with(version, "<< /BaseEncoding /WinAnsiEncoding /Differences [72 /W.cv07] >>");
+        assert_ne!(alternate([0, 2, 0, 0]), render([0, 2, 0, 0]), "the alternate W draws instead of H");
+        assert_eq!(alternate([0, 0, 0, 2]), alternate([0, 2, 0, 0]), "and also with swapped halves");
     }
 
     /// A page whose content expands far beyond its compressed stream stops at the expansion budget,

vendor/hayro-interpret/src/font/blob.rs

@@ -256,6 +256,27 @@ impl OpenTypeFontBlob {
             return glyph_names;
         };
 
+        // PdfCraft patch: a version written with its halves swapped (`00 00 00 02`, accepted in
+        // `new`) is read as the version it means, from a copy of the table with the version
+        // written correctly: read-fonts reads a 2.0 table's name index only under version 2.0,
+        // so otherwise its glyph names were never found (#2946).
+        let repaired = match post.version().to_be_bytes() {
+            [0, 0, 0, n @ 1..=3] => self
+                .font_ref()
+                .table_data(skrifa::raw::types::Tag::new(b"post"))
+                .map(|data| {
+                    let mut bytes = data.as_bytes().to_vec();
+                    if let Some(version) = bytes.get_mut(..4) {
+                        version.copy_from_slice(&[0, n, 0, 0]);
+                    }
+                    bytes
+                }),
+            _ => None,
+        };
+        let repaired_post = repaired.as_deref().and_then(|b| {
+            skrifa::raw::tables::post::Post::read(ReadFontData::new(b)).ok()
+        });
+        let post = repaired_post.as_ref().unwrap_or(&post);
         match post.version().to_major_minor() {
             (1, 0) => {
                 for (gid, name) in DEFAULT_GLYPH_NAMES.iter().enumerate() {