Basic subsetting architecture

Constructs a brand new font file where only necessary tables are contained.

1 files changed, 264 insertions, 0 deletions

diff --git a/src/export/subset.rs b/src/export/subset.rs
new file mode 100644
index 00000000..bf62502b
--- /dev/null
+++ b/src/export/subset.rs
@@ -0,0 +1,264 @@
+//! Font subsetting.
+
+use std::borrow::Cow;
+
+use ttf_parser::parser::{FromData, LazyArray16, Offset, Offset32, Stream};
+use ttf_parser::{Face, Tag};
+
+/// Subset a font face.
+///
+/// Returns `None` if the font data is invalid.
+pub fn subset(data: &[u8], index: u32) -> Option<Vec<u8>> {
+    let mut s = Subsetter::new(data, index)?;
+    s.subset()?;
+    Some(s.encode())
+}
+
+struct Subsetter<'a> {
+    face: Face<'a>,
+    magic: Magic,
+    records: LazyArray16<'a, TableRecord>,
+    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) -> Option<Self> {
+        let face = Face::from_slice(data, index).ok()?;
+        let mut s = Stream::new(&data);
+
+        // Parse font collection header if necessary.
+        let mut magic = s.read::<Magic>()?;
+        if magic == Magic::Collection {
+            s.skip::<u32>();
+            let num_faces = s.read::<u32>()?;
+            let offsets = s.read_array32::<Offset32>(num_faces)?;
+            let offset = offsets.get(index)?.to_usize();
+
+            s = Stream::new_at(&data, offset)?;
+            magic = s.read::<Magic>()?;
+            if magic == Magic::Collection {
+                return None;
+            }
+        }
+
+        // Read number of table records.
+        let count = s.read::<u16>()?;
+
+        // Skip boring parts of header.
+        s.skip::<u16>();
+        s.skip::<u16>();
+        s.skip::<u16>();
+
+        // Read the table records.
+        let records = s.read_array16::<TableRecord>(count)?;
+
+        Some(Self { face, magic, records, tables: vec![] })
+    }
+
+    /// Subset, drop and copy tables.
+    fn subset(&mut self) -> Option<()> {
+        for record in self.records {
+            let tag = record.tag;
+            let data = self.face.table_data(tag)?;
+
+            match &tag.to_bytes() {
+                // Glyphs are already mapped.
+                b"cmap" => {}
+
+                // Layout is already finished.
+                b"GPOS" | b"GSUB" | b"BASE" | b"JSTF" | b"MATH" | b"ankr" | b"kern"
+                | b"kerx" | b"mort" | b"morx" | b"trak" | b"bsln" | b"just"
+                | b"feat" | b"prop" => {}
+
+                // TODO: Subset.
+                // b"loca" => {}
+                // b"glyf" => {}
+                // b"sbix" => {}
+                // b"SVG " => {}
+                // b"post" => {}
+
+                // All other tables are simply copied.
+                _ => self.tables.push((tag, Cow::Borrowed(data))),
+            }
+        }
+        Some(())
+    }
+
+    /// Encode the subsetted font file.
+    fn encode(mut self) -> Vec<u8> {
+        let mut w = Vec::new();
+        w.write(self.magic);
+
+        // Write table directory.
+        let count = self.tables.len() as u16;
+        let entry_selector = (count as f32).log2().floor() as u16;
+        let search_range = entry_selector.pow(2) * 16;
+        let range_shift = count * 16 - search_range;
+        w.write(count);
+        w.write(search_range);
+        w.write(entry_selector);
+        w.write(range_shift);
+
+        // Tables shall be sorted by tag.
+        self.tables.sort_by_key(|&(tag, _)| tag);
+
+        // This variable will hold the offset to the checksum adjustment field
+        // in the head table, which we'll have to write in the end (after
+        // checksumming the whole font).
+        let mut checksum_adjustment_offset = None;
+
+        // Write table records.
+        let mut offset = 12 + self.tables.len() * TableRecord::SIZE;
+        for (tag, data) in &mut self.tables {
+            if *tag == Tag::from_bytes(b"head") {
+                // Zero out checksum field in head table.
+                data.to_mut()[8 .. 12].copy_from_slice(&[0; 4]);
+                checksum_adjustment_offset = Some(offset + 8);
+            }
+
+            let len = data.len();
+            w.write(TableRecord {
+                tag: *tag,
+                checksum: checksum(&data),
+                offset: offset as u32,
+                length: len as u32,
+            });
+
+            // Account for the padding to 4 bytes.
+            offset += len + len % 4;
+        }
+
+        // Write tables.
+        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));
+        }
+
+        // Write checksumAdjustment field in head table.
+        if let Some(i) = checksum_adjustment_offset {
+            let sum = checksum(&w);
+            let val = 0xB1B0AFBA_u32.wrapping_sub(sum);
+            w[i .. i + 4].copy_from_slice(&val.to_be_bytes());
+        }
+
+        w
+    }
+}
+
+/// Calculate a checksum over the sliced data as sum of u32's. The data length
+/// must be a multiple of four.
+fn checksum(data: &[u8]) -> u32 {
+    let mut sum = 0u32;
+    for chunk in data.chunks(4) {
+        let mut bytes = [0; 4];
+        bytes[.. chunk.len()].copy_from_slice(chunk);
+        sum = sum.wrapping_add(u32::from_be_bytes(bytes));
+    }
+    sum
+}
+
+/// Convenience trait for writing into a byte buffer.
+trait BufExt {
+    fn write<T: ToData>(&mut self, v: T);
+}
+
+impl BufExt for Vec<u8> {
+    fn write<T: ToData>(&mut self, v: T) {
+        v.write(self);
+    }
+}
+
+/// A trait for writing raw binary data.
+trait ToData {
+    fn write(&self, data: &mut Vec<u8>);
+}
+
+impl ToData for u8 {
+    fn write(&self, data: &mut Vec<u8>) {
+        data.push(*self);
+    }
+}
+
+impl ToData for u16 {
+    fn write(&self, data: &mut Vec<u8>) {
+        data.extend(&self.to_be_bytes());
+    }
+}
+
+impl ToData for u32 {
+    fn write(&self, data: &mut Vec<u8>) {
+        data.extend(&self.to_be_bytes());
+    }
+}
+
+impl ToData for Tag {
+    fn write(&self, data: &mut Vec<u8>) {
+        self.as_u32().write(data);
+    }
+}
+
+/// Font magic number.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+enum Magic {
+    TrueType,
+    OpenType,
+    Collection,
+}
+
+impl FromData for Magic {
+    const SIZE: usize = 4;
+
+    fn parse(data: &[u8]) -> Option<Self> {
+        match u32::parse(data)? {
+            0x00010000 | 0x74727565 => Some(Magic::TrueType),
+            0x4F54544F => Some(Magic::OpenType),
+            0x74746366 => Some(Magic::Collection),
+            _ => None,
+        }
+    }
+}
+
+impl ToData for Magic {
+    fn write(&self, data: &mut Vec<u8>) {
+        let value: u32 = match self {
+            Magic::TrueType => 0x00010000,
+            Magic::OpenType => 0x4F54544F,
+            Magic::Collection => 0x74746366,
+        };
+        value.write(data);
+    }
+}
+
+/// Locates a table in the font file.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+struct TableRecord {
+    tag: Tag,
+    checksum: u32,
+    offset: u32,
+    length: u32,
+}
+
+impl FromData for TableRecord {
+    const SIZE: usize = 16;
+
+    fn parse(data: &[u8]) -> Option<Self> {
+        let mut s = Stream::new(data);
+        Some(TableRecord {
+            tag: s.read::<Tag>()?,
+            checksum: s.read::<u32>()?,
+            offset: s.read::<u32>()?,
+            length: s.read::<u32>()?,
+        })
+    }
+}
+
+impl ToData for TableRecord {
+    fn write(&self, data: &mut Vec<u8>) {
+        self.tag.write(data);
+        self.checksum.write(data);
+        self.offset.write(data);
+        self.length.write(data);
+    }
+}