From ebdf70c95dceb666269343b86ac9efbc097e51a6 Mon Sep 17 00:00:00 2001
From: Behdad Esfahbod <behdad@behdad.org>
Date: Sat, 10 Oct 2026 00:28:32 -0600
Subject: [PATCH] [skera] Support identity CFF charsets

---
 skera/src/cff/subset.rs |  8 +++++
 skera/src/lib.rs        |  4 +++
 skera/src/main.rs       |  7 +++++
 skera/tests/cff_test.rs | 69 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 88 insertions(+)

diff --git a/skera/src/cff/subset.rs b/skera/src/cff/subset.rs
index 3ceaf8b67..621b48060 100644
--- a/skera/src/cff/subset.rs
+++ b/skera/src/cff/subset.rs
@@ -397,6 +397,10 @@ pub(super) fn assemble<F: CffFlavor>(
     let mut strings = Vec::new();
     let mut sid_map = BTreeMap::new();
     let mut codes = vec![0];
+    let identity_charset = source.cid
+        && plan
+            .subset_flags
+            .contains(SubsetFlags::SUBSET_FLAGS_CFF_IDENTITY_CHARSET);
     if !F::CFF2 {
         let cs = Charset::new(
             FontData::new(source.font.data()),
@@ -405,6 +409,10 @@ pub(super) fn assemble<F: CffFlavor>(
         )
         .ok_or(Error)?;
         for &old in old_gids.iter().skip(1) {
+            if identity_charset {
+                codes.push(codes.len());
+                continue;
+            }
             let code = cs
                 .string_id(GlyphId::new(old as u32))
                 .ok_or(Error)?
diff --git a/skera/src/lib.rs b/skera/src/lib.rs
index 9714e3f22..d1650f308 100644
--- a/skera/src/lib.rs
+++ b/skera/src/lib.rs
@@ -278,6 +278,10 @@ impl SubsetFlags {
     /// If set do not pull mirrored versions of input codepoints into the subset.
     pub const SUBSET_FLAGS_NO_BIDI_CLOSURE: Self = Self(0x0800);
 
+    /// If set CID-keyed CFF charsets use sequential CIDs equal to the output
+    /// glyph IDs. Name-keyed CFF and CFF2 fonts are unaffected.
+    pub const SUBSET_FLAGS_CFF_IDENTITY_CHARSET: Self = Self(0x8000);
+
     /// Returns `true` if all of the flags in `other` are contained within `self`.
     #[inline]
     pub const fn contains(&self, other: Self) -> bool {
diff --git a/skera/src/main.rs b/skera/src/main.rs
index ded50210b..5496be0b5 100644
--- a/skera/src/main.rs
+++ b/skera/src/main.rs
@@ -110,6 +110,10 @@ struct Args {
     #[arg(long)]
     optimize: bool,
 
+    /// Emit an identity CFF charset (CID = output GID) for CID-keyed CFF
+    #[arg(long)]
+    cff_identity_charset: bool,
+
     ///run subsetter N times
     #[arg(short, long)]
     num_iterations: Option<u32>,
@@ -342,5 +346,8 @@ fn parse_subset_flags(args: &Args) -> SubsetFlags {
     if args.optimize {
         flags |= SubsetFlags::SUBSET_FLAGS_OPTIMIZE_IUP_DELTAS;
     }
+    if args.cff_identity_charset {
+        flags |= SubsetFlags::SUBSET_FLAGS_CFF_IDENTITY_CHARSET;
+    }
     flags
 }
diff --git a/skera/tests/cff_test.rs b/skera/tests/cff_test.rs
index 4bd1830c0..9a1372ec3 100644
--- a/skera/tests/cff_test.rs
+++ b/skera/tests/cff_test.rs
@@ -123,6 +123,75 @@ fn retained_gid_holes_and_notdef_are_empty() {
     }
 }
 
+#[test]
+fn identity_charsets_use_output_gids_only_for_cid_keyed_cff() {
+    for file in [
+        "SourceHanSans-Regular_subset.otf",
+        "SourceSansPro-Regular.otf",
+        "AdobeVFPrototype.otf",
+    ] {
+        let source = std::fs::read(format!("test-data/fonts/{file}")).unwrap();
+        let font = FontRef::new(&source).unwrap();
+        for retain in [false, true] {
+            let flags = SubsetFlags::SUBSET_FLAGS_NOTDEF_OUTLINE
+                | if retain {
+                    SubsetFlags::SUBSET_FLAGS_RETAIN_GIDS
+                } else {
+                    SubsetFlags::default()
+                };
+            let dense_plan = plan(&font, flags, "U+0041,U+0043,U+0061,U+4E00");
+            let identity_plan = plan(
+                &font,
+                flags | SubsetFlags::SUBSET_FLAGS_CFF_IDENTITY_CHARSET,
+                "U+0041,U+0043,U+0061,U+4E00",
+            );
+            let ordinary = subset_font(&font, &dense_plan).unwrap();
+            let identity = subset_font(&font, &identity_plan).unwrap();
+            let ordinary = FontRef::new(&ordinary).unwrap();
+            let identity = FontRef::new(&identity).unwrap();
+            let tag = if identity.cff2().is_ok() {
+                Tag::new(b"CFF2")
+            } else {
+                Tag::new(b"CFF ")
+            };
+            let a_data = ordinary.data_for_tag(tag).unwrap().as_bytes();
+            let b_data = identity.data_for_tag(tag).unwrap().as_bytes();
+            let a = CffFontRef::new(a_data, 0, None).unwrap();
+            let b = CffFontRef::new(b_data, 0, None).unwrap();
+            assert_eq!(a.num_glyphs(), b.num_glyphs());
+            if tag == Tag::new(b"CFF ") && b.is_cid() {
+                for gid in 0..b.num_glyphs() {
+                    let gid = GlyphId::new(gid);
+                    assert_eq!(
+                        b.charset().unwrap().string_id(gid).unwrap().to_u16() as u32,
+                        gid.to_u32()
+                    );
+                    assert_eq!(
+                        a.charstrings().get(gid.to_u32() as usize),
+                        b.charstrings().get(gid.to_u32() as usize)
+                    );
+                    assert_eq!(a.subfont_index(gid), b.subfont_index(gid));
+                }
+                assert!(
+                    identity_plan
+                        .old_to_new_glyph_mapping()
+                        .any(|(old, new)| old != new)
+                        || retain
+                );
+            } else {
+                assert_eq!(a_data, b_data, "{file}");
+            }
+            for tag in [b"cmap", b"hmtx", b"GPOS", b"GSUB"] {
+                let tag = Tag::new(tag);
+                assert_eq!(
+                    ordinary.data_for_tag(tag).map(|d| d.as_bytes()),
+                    identity.data_for_tag(tag).map(|d| d.as_bytes())
+                );
+            }
+        }
+    }
+}
+
 #[test]
 fn cff2_subsetting_preserves_variation_locations() {
     use write_fonts::types::F2Dot14;