Primitive CFF1 subsetting

The subsetting strategy is somewhat crazy for now: Simply zero the glyph data for all unused glyphs. While the CFF table itself doesn't shrink, the actual embedded font is compressed and greatly benefits from the repeated zeros.

This already compresses the fonts a lot (~90% for NotoSerifCJK), but they are still quite large.

Therefore, the plan of action:
- First, find more data that can be zeroed out.
- Then _maybe_ see whether we can instead properly rebuild the subsetted font.

src/export/pdf.rs

@@ -402,7 +402,7 @@ impl<'a> PdfExporter<'a> {
 
             // Subset and write the face's bytes.
             let buffer = face.buffer();
-            let subsetted = subset(buffer, face.index(), glyphs.iter().copied());
+            let subsetted = subset(buffer, face.index(), glyphs);
             let data = subsetted.as_deref().unwrap_or(buffer);
             self.writer
                 .stream(refs.data, &deflate(data))

src/export/subset.rs

@@ -1,42 +1,39 @@
-//! Font subsetting.
+//! OpenType font subsetting.
 
 use std::borrow::Cow;
 use std::collections::HashSet;
-use std::convert::TryInto;
+use std::convert::{TryFrom, TryInto};
+use std::iter;
 
 use ttf_parser::parser::{
-    FromData, LazyArray16, LazyArray32, Offset16, Offset32, Stream, F2DOT14,
+    FromData, LazyArray16, LazyArray32, Offset, Offset16, Offset32, Stream, F2DOT14,
 };
 use ttf_parser::Tag;
 
 /// Subset a font face for PDF embedding.
 ///
 /// This will remove the outlines of all glyphs that are not part of the given
-/// iterator. Furthmore, all character mapping and layout tables are dropped as
+/// slice. Furthmore, all character mapping and layout tables are dropped as
 /// shaping has already happened.
 ///
-/// Returns `None` if the font data is fatally invalid (in which case
+/// Returns `None` if the font data is fatally broken (in which case
 /// `ttf-parser` would probably already have rejected the font, so this
 /// shouldn't happen if the font data has already passed through `ttf-parser`).
-pub fn subset<I>(data: &[u8], index: u32, glyphs: I) -> Option<Vec<u8>>
-where
-    I: IntoIterator<Item = u16>,
-{
-    let glyphs = glyphs.into_iter().collect();
+pub fn subset(data: &[u8], index: u32, glyphs: &HashSet<u16>) -> Option<Vec<u8>> {
     Some(Subsetter::new(data, index, glyphs)?.subset())
 }
 
 struct Subsetter<'a> {
     data: &'a [u8],
+    glyphs: &'a HashSet<u16>,
     magic: Magic,
     records: LazyArray16<'a, TableRecord>,
-    glyphs: Vec<u16>,
     tables: Vec<(Tag, Cow<'a, [u8]>)>,
 }
 
 impl<'a> Subsetter<'a> {
     /// Parse the font header and create a new subsetter.
-    fn new(data: &'a [u8], index: u32, glyphs: Vec<u16>) -> Option<Self> {
+    fn new(data: &'a [u8], index: u32, glyphs: &'a HashSet<u16>) -> Option<Self> {
         let mut s = Stream::new(&data);
 
         let mut magic = s.read::<Magic>()?;
@@ -125,7 +122,7 @@ impl<'a> Subsetter<'a> {
         for (_, data) in &self.tables {
             // Write data plus padding zeros to align to 4 bytes.
             w.extend(data.as_ref());
-            w.extend(std::iter::repeat(0).take(data.len() % 4));
+            w.extend(iter::repeat(0).take(data.len() % 4));
         }
 
         // Write checksumAdjustment field in head table.
@@ -141,10 +138,11 @@ impl<'a> Subsetter<'a> {
     /// Subset, drop and copy tables.
     fn subset_tables(&mut self) {
         // Remove unnecessary name information.
-        let handled_post = self.subset_post().is_some();
+        let handled_post = post::subset(self).is_some();
 
         // Remove unnecessary glyph outlines.
-        let handled_glyf_loca = self.subset_glyf_loca().is_some();
+        let handled_glyf_loca = glyf::subset(self).is_some();
+        let handled_cff1 = cff::subset_v1(self).is_some();
 
         for record in self.records {
             // If `handled` is true, we don't take any further action, if it's
@@ -169,6 +167,7 @@ impl<'a> Subsetter<'a> {
                 // failed, we simply copy the affected table(s).
                 b"post" => handled_post,
                 b"loca" | b"glyf" => handled_glyf_loca,
+                b"CFF " => handled_cff1,
 
                 // Copy: All other tables are simply copied.
                 _ => false,
@@ -202,6 +201,7 @@ const MAXP: Tag = Tag::from_bytes(b"maxp");
 const POST: Tag = Tag::from_bytes(b"post");
 const LOCA: Tag = Tag::from_bytes(b"loca");
 const GLYF: Tag = Tag::from_bytes(b"glyf");
+const CFF1: Tag = Tag::from_bytes(b"CFF ");
 
 /// Calculate a checksum over the sliced data as sum of u32's. The data length
 /// must be a multiple of four.
@@ -331,57 +331,67 @@ impl ToData for TableRecord {
     }
 }
 
-impl Subsetter<'_> {
+mod post {
+    use super::*;
+
     /// Subset the post table by removing the name information.
-    fn subset_post(&mut self) -> Option<()> {
-        // Set version to 3.0.
-        let post = self.table_data(POST)?;
+    pub(super) fn subset(subsetter: &mut Subsetter) -> Option<()> {
+        // Table version three is the one without names.
         let mut new = 0x00030000_u32.to_be_bytes().to_vec();
-        new.extend(post.get(4 .. 32)?);
-        self.push_table(POST, new);
+        new.extend(subsetter.table_data(POST)?.get(4 .. 32)?);
+        subsetter.push_table(POST, new);
         Some(())
     }
 }
 
-impl Subsetter<'_> {
-    /// Subset the glyf and loca tables by clearing out glyph data for unused
-    /// glyphs.
-    fn subset_glyf_loca(&mut self) -> Option<()> {
-        let head = self.table_data(HEAD)?;
+mod glyf {
+    use super::*;
+
+    /// Subset the glyf and loca tables by clearing out glyph data for
+    /// unused glyphs.
+    pub(super) fn subset(subsetter: &mut Subsetter) -> Option<()> {
+        let head = subsetter.table_data(HEAD)?;
         let short = Stream::read_at::<i16>(head, 50)? == 0;
         if short {
-            self.subset_glyf_loca_impl::<Offset16>()
+            subset_impl::<Offset16>(subsetter)
         } else {
-            self.subset_glyf_loca_impl::<Offset32>()
+            subset_impl::<Offset32>(subsetter)
         }
     }
 
-    fn subset_glyf_loca_impl<T>(&mut self) -> Option<()>
+    fn subset_impl<T>(subsetter: &mut Subsetter) -> Option<()>
     where
         T: LocaOffset,
     {
-        let loca = self.table_data(LOCA)?;
-        let glyf = self.table_data(GLYF)?;
+        let loca = subsetter.table_data(LOCA)?;
+        let glyf = subsetter.table_data(GLYF)?;
+        let maxp = subsetter.table_data(MAXP)?;
+
+        // Find out number of glyphs.
+        let num_glyphs = Stream::read_at::<u16>(maxp, 4)?;
 
         let offsets = LazyArray32::<T>::new(loca);
-        let slice = |id: u16| {
-            let from = offsets.get(u32::from(id))?.to_usize();
-            let to = offsets.get(u32::from(id) + 1)?.to_usize();
+        let glyph_data = |id: u16| {
+            let from = offsets.get(u32::from(id))?.loca_to_usize();
+            let to = offsets.get(u32::from(id) + 1)?.loca_to_usize();
             glyf.get(from .. to)
         };
 
-        // To compute the set of all glyphs we want to keep, we use a work stack
-        // containing glyphs whose components we still need to consider.
-        let mut glyphs = HashSet::new();
-        let mut work: Vec<u16> = std::mem::take(&mut self.glyphs);
+        // The set of all glyphs we will include in the subset.
+        let mut subset = HashSet::new();
 
-        // Always include the notdef glyph.
-        work.push(0);
+        // Because glyphs may depend on other glyphs as components (also with
+        // multiple layers of nesting), we have to process all glyphs to find
+        // their components. For notdef and all requested glyphs we simply use
+        // an iterator, but to track other glyphs that need processing we create
+        // a work stack.
+        let mut iter = iter::once(0).chain(subsetter.glyphs.iter().copied());
+        let mut work = vec![];
 
         // Find composite glyph descriptions.
-        while let Some(id) = work.pop() {
-            if glyphs.insert(id) {
-                let mut s = Stream::new(slice(id)?);
+        while let Some(id) = work.pop().or_else(|| iter.next()) {
+            if subset.insert(id) {
+                let mut s = Stream::new(glyph_data(id)?);
                 if let Some(num_contours) = s.read::<i16>() {
                     // Negative means this is a composite glyph.
                     if num_contours < 0 {
@@ -401,97 +411,295 @@ impl Subsetter<'_> {
         let mut sub_loca = vec![];
         let mut sub_glyf = vec![];
 
-        // Find out number of glyphs.
-        let maxp = self.table_data(MAXP)?;
-        let num_glyphs = Stream::read_at::<u16>(maxp, 4)?;
-
         for id in 0 .. num_glyphs {
-            sub_loca.write(T::from_usize(sub_glyf.len())?);
-
-            // If the glyph shouldn't be contained in the subset, it will still
-            // get a loca entry, but the glyf data is simply empty.
-            if glyphs.contains(&id) {
-                sub_glyf.extend(slice(id)?);
+            // If the glyph shouldn't be contained in the subset, it will
+            // still get a loca entry, but the glyf data is simply empty.
+            sub_loca.write(T::usize_to_loca(sub_glyf.len())?);
+            if subset.contains(&id) {
+                sub_glyf.extend(glyph_data(id)?);
             }
         }
 
-        sub_loca.write(T::from_usize(sub_glyf.len())?);
+        sub_loca.write(T::usize_to_loca(sub_glyf.len())?);
 
-        self.push_table(LOCA, sub_loca);
-        self.push_table(GLYF, sub_glyf);
+        subsetter.push_table(LOCA, sub_loca);
+        subsetter.push_table(GLYF, sub_glyf);
 
         Some(())
     }
-}
 
-/// Offsets for loca table.
-trait LocaOffset: Sized + FromData + ToData {
-    fn to_usize(self) -> usize;
-    fn from_usize(offset: usize) -> Option<Self>;
-}
-
-impl LocaOffset for Offset16 {
-    fn to_usize(self) -> usize {
-        2 * usize::from(self.0)
+    trait LocaOffset: Sized + FromData + ToData {
+        fn loca_to_usize(self) -> usize;
+        fn usize_to_loca(offset: usize) -> Option<Self>;
     }
 
-    fn from_usize(offset: usize) -> Option<Self> {
-        if offset % 2 == 0 {
-            (offset / 2).try_into().ok().map(Self)
-        } else {
-            None
+    impl LocaOffset for Offset16 {
+        fn loca_to_usize(self) -> usize {
+            2 * usize::from(self.0)
+        }
+
+        fn usize_to_loca(offset: usize) -> Option<Self> {
+            if offset % 2 == 0 {
+                (offset / 2).try_into().ok().map(Self)
+            } else {
+                None
+            }
         }
     }
-}
 
-impl LocaOffset for Offset32 {
-    fn to_usize(self) -> usize {
-        self.0 as usize
+    impl LocaOffset for Offset32 {
+        fn loca_to_usize(self) -> usize {
+            self.0 as usize
+        }
+
+        fn usize_to_loca(offset: usize) -> Option<Self> {
+            offset.try_into().ok().map(Self)
+        }
     }
 
-    fn from_usize(offset: usize) -> Option<Self> {
-        offset.try_into().ok().map(Self)
+    /// Returns an iterator over the component glyphs referenced by the given
+    /// `glyf` table composite glyph description.
+    fn component_glyphs(mut s: Stream) -> impl Iterator<Item = u16> + '_ {
+        const ARG_1_AND_2_ARE_WORDS: u16 = 0x0001;
+        const WE_HAVE_A_SCALE: u16 = 0x0008;
+        const MORE_COMPONENTS: u16 = 0x0020;
+        const WE_HAVE_AN_X_AND_Y_SCALE: u16 = 0x0040;
+        const WE_HAVE_A_TWO_BY_TWO: u16 = 0x0080;
+
+        let mut done = false;
+        iter::from_fn(move || {
+            if done {
+                return None;
+            }
+
+            let flags = s.read::<u16>()?;
+            let component = s.read::<u16>()?;
+
+            if flags & ARG_1_AND_2_ARE_WORDS != 0 {
+                s.skip::<i16>();
+                s.skip::<i16>();
+            } else {
+                s.skip::<u16>();
+            }
+
+            if flags & WE_HAVE_A_SCALE != 0 {
+                s.skip::<F2DOT14>();
+            } else if flags & WE_HAVE_AN_X_AND_Y_SCALE != 0 {
+                s.skip::<F2DOT14>();
+                s.skip::<F2DOT14>();
+            } else if flags & WE_HAVE_A_TWO_BY_TWO != 0 {
+                s.skip::<F2DOT14>();
+                s.skip::<F2DOT14>();
+                s.skip::<F2DOT14>();
+                s.skip::<F2DOT14>();
+            }
+
+            done = flags & MORE_COMPONENTS == 0;
+            Some(component)
+        })
     }
 }
 
-/// Returns an iterator over the component glyphs referenced by the given
-/// `glyf` table composite glyph description.
-fn component_glyphs(mut s: Stream) -> impl Iterator<Item = u16> + '_ {
-    const ARG_1_AND_2_ARE_WORDS: u16 = 0x0001;
-    const WE_HAVE_A_SCALE: u16 = 0x0008;
-    const MORE_COMPONENTS: u16 = 0x0020;
-    const WE_HAVE_AN_X_AND_Y_SCALE: u16 = 0x0040;
-    const WE_HAVE_A_TWO_BY_TWO: u16 = 0x0080;
+mod cff {
+    use super::*;
 
-    let mut done = false;
-    std::iter::from_fn(move || {
-        if done {
+    /// Subset the CFF table by zeroing glyph data for unused glyphs.
+    pub(super) fn subset_v1(subsetter: &mut Subsetter) -> Option<()> {
+        let cff = subsetter.table_data(CFF1)?;
+        let mut s = Stream::new(cff);
+
+        let (major, _) = (s.read::<u8>()?, s.skip::<u8>());
+        if major != 1 {
             return None;
         }
 
-        let flags = s.read::<u16>()?;
-        let component = s.read::<u16>()?;
+        let header_size = s.read::<u8>()?;
+        s = Stream::new_at(cff, usize::from(header_size))?;
 
-        if flags & ARG_1_AND_2_ARE_WORDS != 0 {
-            s.skip::<i16>();
-            s.skip::<i16>();
-        } else {
-            s.skip::<u16>();
+        // Skip the name index.
+        Index::parse(&mut s);
+
+        // Read the top dict.
+        let top_dict_index = Index::parse(&mut s)?;
+        let top_dict = Dict::parse(top_dict_index.get(0)?);
+
+        let mut sub_cff = cff.to_vec();
+
+        // Because completely rebuilding the CFF structure would be pretty
+        // complex, for now, we employ a peculiar strategy for CFF subsetting:
+        // We simply fill the data for all unused glyphs with zeros. This way,
+        // the font structure and offsets can stay the same. And while the CFF
+        // table itself doesn't shrink, the actual embedded font is compressed
+        // and greatly benefits from the repeated zeros.
+        if let Some(index_offset) = top_dict.get_offset(Op::CHAR_STRINGS) {
+            let index_data = cff.get(index_offset ..)?;
+            let index = Index::parse(&mut Stream::new(index_data))?;
+
+            let mut start = index_offset + index.data_offset;
+            for (id, data) in index.items.iter().enumerate() {
+                let end = start + data.len();
+                if !subsetter.glyphs.contains(&(id as u16)) {
+                    memzero(sub_cff.get_mut(start .. end)?);
+                }
+                start = end;
+            }
         }
 
-        if flags & WE_HAVE_A_SCALE != 0 {
-            s.skip::<F2DOT14>();
-        } else if flags & WE_HAVE_AN_X_AND_Y_SCALE != 0 {
-            s.skip::<F2DOT14>();
-            s.skip::<F2DOT14>();
-        } else if flags & WE_HAVE_A_TWO_BY_TWO != 0 {
-            s.skip::<F2DOT14>();
-            s.skip::<F2DOT14>();
-            s.skip::<F2DOT14>();
-            s.skip::<F2DOT14>();
+        subsetter.push_table(CFF1, sub_cff);
+
+        Some(())
+    }
+
+    /// Zero all bytes in a slice.
+    fn memzero(slice: &mut [u8]) {
+        for byte in slice {
+            *byte = 0;
+        }
+    }
+
+    /// A CFF1 INDEX structure.
+    struct Index<'a> {
+        /// The offset of the data from the start of the index.
+        data_offset: usize,
+        /// The data for the actual items.
+        items: Vec<&'a [u8]>,
+    }
+
+    impl<'a> Index<'a> {
+        fn parse(s: &mut Stream<'a>) -> Option<Self> {
+            let data = s.tail()?;
+            let count = usize::from(s.read::<u16>()?);
+
+            let mut data_offset = 2;
+            let mut items = Vec::with_capacity(count);
+
+            if count > 0 {
+                let offsize = usize::from(s.read::<u8>()?);
+                if offsize < 1 || offsize > 4 {
+                    return None;
+                }
+
+                // The data starts right behind the offsets.
+                data_offset += 1 + offsize * (count + 1);
+
+                // Read an offset and transform it to be relative to the start
+                // of the index.
+                let mut read_offset = || {
+                    let mut bytes = [0u8; 4];
+                    bytes[4 - offsize .. 4].copy_from_slice(s.read_bytes(offsize)?);
+                    Some(data_offset - 1 + u32::from_be_bytes(bytes) as usize)
+                };
+
+                let mut len = 0;
+                let mut last = read_offset()?;
+
+                for _ in 0 .. count {
+                    let offset = read_offset()?;
+                    let item = data.get(last .. offset)?;
+                    items.push(item);
+                    last = offset;
+                    len += item.len();
+                }
+
+                // Advance the stream past the data.
+                s.advance(len);
+            }
+
+            Some(Self { data_offset, items })
         }
 
-        done = flags & MORE_COMPONENTS == 0;
-        Some(component)
-    })
+        fn get(&self, idx: usize) -> Option<&'a [u8]> {
+            self.items.get(idx).copied()
+        }
+    }
+
+    /// A CFF1 DICT structure.
+    struct Dict<'a>(Vec<Pair<'a>>);
+
+    impl<'a> Dict<'a> {
+        fn parse(data: &'a [u8]) -> Self {
+            let mut s = Stream::new(data);
+            Self(iter::from_fn(|| Pair::parse(&mut s)).collect())
+        }
+
+        fn get(&self, op: Op) -> Option<&[Operand<'a>]> {
+            self.0
+                .iter()
+                .find(|pair| pair.op == op)
+                .map(|pair| pair.operands.as_slice())
+        }
+
+        fn get_offset(&self, op: Op) -> Option<usize> {
+            match self.get(op)? {
+                &[Operand::Int(offset)] if offset > 0 => usize::try_from(offset).ok(),
+                _ => None,
+            }
+        }
+    }
+
+    struct Pair<'a> {
+        operands: Vec<Operand<'a>>,
+        op: Op,
+    }
+
+    impl<'a> Pair<'a> {
+        fn parse(s: &mut Stream<'a>) -> Option<Self> {
+            let mut operands = vec![];
+            while s.clone().read::<u8>()? > 21 {
+                operands.push(Operand::parse(s)?);
+            }
+            Some(Self { operands, op: Op::parse(s)? })
+        }
+    }
+
+    #[derive(Eq, PartialEq)]
+    struct Op(u8, u8);
+
+    impl Op {
+        const CHAR_STRINGS: Self = Self(17, 0);
+
+        fn parse(s: &mut Stream) -> Option<Self> {
+            let b0 = s.read::<u8>()?;
+            match b0 {
+                12 => Some(Self(b0, s.read::<u8>()?)),
+                0 ..= 21 => Some(Self(b0, 0)),
+                _ => None,
+            }
+        }
+    }
+
+    enum Operand<'a> {
+        Int(i32),
+        Real(&'a [u8]),
+    }
+
+    impl<'a> Operand<'a> {
+        fn parse(s: &mut Stream<'a>) -> Option<Self> {
+            let b0 = i32::from(s.read::<u8>()?);
+            Some(match b0 {
+                30 => {
+                    let mut len = 0;
+                    for &byte in s.tail()? {
+                        len += 1;
+                        if byte & 0x0f == 0x0f {
+                            break;
+                        }
+                    }
+                    Self::Real(s.read_bytes(len)?)
+                }
+                32 ..= 246 => Self::Int(b0 - 139),
+                247 ..= 250 => {
+                    let b1 = i32::from(s.read::<u8>()?);
+                    Self::Int((b0 - 247) * 256 + b1 + 108)
+                }
+                251 ..= 254 => {
+                    let b1 = i32::from(s.read::<u8>()?);
+                    Self::Int(-(b0 - 251) * 256 - b1 - 108)
+                }
+                28 => Self::Int(i32::from(s.read::<i16>()?)),
+                29 => Self::Int(s.read::<i32>()?),
+                _ => return None,
+            })
+        }
+    }
 }