fonts-build: read each shared cmap subtable once

A cmap can list 65535 encoding records that all name the same subtable,
and Cmap::ranges expanded that subtable's spans once per record before
merging them. Records naming a subtable already read are now skipped,
and at most 16 distinct Unicode subtables are read from one face.

crates/fonts-build/src/cmap.rs

@@ -1,6 +1,11 @@
 //! Just enough of an OpenType reader to ask which characters a face maps: the `cmap` table's
 //! Unicode subtables in formats 4 and 12, in a single font or one face of a collection.
 
+/// The most distinct Unicode subtables read from one face. Real fonts have one to three; a cmap
+/// may list 65535 encoding records, and each would make [`Cmap::ranges`] and [`Cmap::covers`] walk
+/// its subtable again.
+const MAX_SUBTABLES: usize = 16;
+
 /// The Unicode character maps of one face.
 pub(crate) struct Cmap<'a> {
     subtables: Vec<&'a [u8]>,
@@ -38,8 +43,15 @@ impl<'a> Cmap<'a> {
                 Some(12) => u32_at(rest, 4).and_then(|len| usize::try_from(len).ok()),
                 _ => None,
             };
-            if let Some(subtable) = len.and_then(|len| rest.get(..len)) {
+            // Encoding records often share one subtable (Unicode and Windows naming the same
+            // format 4 table); each is read once.
+            if let Some(subtable) = len.and_then(|len| rest.get(..len))
+                && !subtables.iter().any(|t: &&[u8]| t.as_ptr() == subtable.as_ptr() && t.len() == subtable.len())
+            {
                 subtables.push(subtable);
+                if subtables.len() == MAX_SUBTABLES {
+                    break;
+                }
             }
         }
         (!subtables.is_empty()).then_some(Self { subtables })

crates/fonts-build/src/tests.rs

@@ -384,6 +384,62 @@ fn cmap_reads_each_subtable_within_its_declared_length() {
     assert!(cmap.covers('A') && !cmap.covers('\u{20000}'));
 }
 
+/// A font whose cmap has one Windows full-repertoire encoding record per item of `records`, each
+/// naming that index of `subtables`: format 12 subtables of `(first, last)` groups.
+fn shared_cmap_font(records: &[usize], subtables: &[&[(u32, u32)]]) -> Vec<u8> {
+    let be32 = |v: &mut Vec<u8>, x: u32| v.extend_from_slice(&x.to_be_bytes());
+    let mut tables: Vec<Vec<u8>> = Vec::new();
+    for groups in subtables {
+        let mut t = vec![0, 12, 0, 0];
+        be32(&mut t, 16 + 12 * u32::try_from(groups.len()).unwrap());
+        be32(&mut t, 0);
+        be32(&mut t, u32::try_from(groups.len()).unwrap());
+        for &(first, last) in *groups {
+            [first, last, 1].into_iter().for_each(|x| be32(&mut t, x));
+        }
+        tables.push(t);
+    }
+    let mut cmap = vec![0, 0];
+    cmap.extend_from_slice(&u16::try_from(records.len()).unwrap().to_be_bytes());
+    let mut offsets = Vec::new();
+    let mut at = 4 + 8 * u32::try_from(records.len()).unwrap();
+    for t in &tables {
+        offsets.push(at);
+        at += u32::try_from(t.len()).unwrap();
+    }
+    for &r in records {
+        cmap.extend_from_slice(&[0, 3, 0, 10]);
+        be32(&mut cmap, offsets[r]);
+    }
+    tables.iter().for_each(|t| cmap.extend(t));
+    let mut font = vec![0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0];
+    font.extend_from_slice(b"cmap");
+    for x in [0, 28, u32::try_from(cmap.len()).unwrap()] {
+        be32(&mut font, x);
+    }
+    font.extend(cmap);
+    font
+}
+
+#[test]
+fn cmap_reads_a_shared_subtable_once() {
+    // 65535 encoding records naming one subtable of 2000 one-character groups: read once, not
+    // 131 million spans.
+    let groups: Vec<(u32, u32)> = (0..2000).map(|i| (0x1_0000 + 2 * i, 0x1_0000 + 2 * i)).collect();
+    let font = shared_cmap_font(&vec![0; usize::from(u16::MAX)], &[&groups]);
+    let started = std::time::Instant::now();
+    let cmap = Cmap::read(&font, 0).unwrap();
+    assert_eq!(cmap.ranges(), groups);
+    assert!(cmap.covers('\u{10000}') && !cmap.covers('\u{10001}'));
+    assert!(started.elapsed() < std::time::Duration::from_secs(10), "{:?}", started.elapsed());
+
+    // Past MAX_SUBTABLES distinct subtables, the rest are not read.
+    let one: Vec<[(u32, u32); 1]> = (0..20).map(|i| [(0x4E00 + i, 0x4E00 + i)]).collect();
+    let subtables: Vec<&[(u32, u32)]> = one.iter().map(|g| &g[..]).collect();
+    let font = shared_cmap_font(&(0..20).collect::<Vec<_>>(), &subtables);
+    assert_eq!(Cmap::read(&font, 0).unwrap().ranges(), [(0x4E00, 0x4E0F)]);
+}
+
 /// A craft-fonts checkout in `scratch` with these `(family, scripts, role, font)` faces, all
 /// Regular.
 fn checkout(scratch: &Scratch, faces: &[(&str, &str, &str, Vec<u8>)]) -> PathBuf {