fix(extractor): document bfrange remap truncation and assert visit count

The 65,536 cap counts overwrites so repeated ranges cannot keep expanding.
The test now checks the assignment count, not just HashMap size (u16 keys
are always ≤ 65,536).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/tounicode.rs

@@ -540,18 +540,14 @@ impl ToUnicodeCMap {
 
     /// Remap a CMap that references pre-subsetting GIDs to sequential post-subsetting GIDs.
     /// Collects all source CIDs, sorts them, and reassigns to 1, 2, 3, ...
+    ///
+    /// Range expansion stops after `MAX_CID_W_EXPANSION` CID visits, counting
+    /// overwrites, so repeated full-width `bfrange`s cannot re-expand the
+    /// 16-bit domain. Later overlapping ranges that would have introduced new
+    /// CIDs after that many visits are truncated.
     pub fn remap_to_sequential(&self) -> ToUnicodeCMap {
         let mut cid_to_unicode: HashMap<u16, String> = HashMap::new();
-
-        // Expand ranges first
-        for &(start, end, base) in &self.ranges {
-            for cid in start..=end {
-                let unicode_cp = base + (cid - start) as u32;
-                if let Some(ch) = char::from_u32(unicode_cp) {
-                    cid_to_unicode.insert(cid, ch.to_string());
-                }
-            }
-        }
+        expand_bfranges_for_remap(&self.ranges, &mut cid_to_unicode, MAX_CID_W_EXPANSION);
 
         // char_map entries override range entries
         for (&cid, unicode) in &self.char_map {
@@ -576,6 +572,33 @@ impl ToUnicodeCMap {
     }
 }
 
+/// Expand `bfrange` entries into individual CID→Unicode inserts.
+/// Returns how many CIDs were visited. Counts overwrites so a repeated
+/// full-width range cannot keep working after `max_assignments`.
+fn expand_bfranges_for_remap(
+    ranges: &[(u16, u16, u32)],
+    cid_to_unicode: &mut HashMap<u16, String>,
+    max_assignments: usize,
+) -> usize {
+    let mut assigned = 0usize;
+    'ranges: for &(start, end, base) in ranges {
+        if start > end {
+            continue;
+        }
+        for cid in start..=end {
+            if assigned >= max_assignments {
+                break 'ranges;
+            }
+            assigned += 1;
+            let unicode_cp = base + (cid - start) as u32;
+            if let Some(ch) = char::from_u32(unicode_cp) {
+                cid_to_unicode.insert(cid, ch.to_string());
+            }
+        }
+    }
+    assigned
+}
+
 /// Parse a hex string to u16
 fn parse_hex_u16(hex: &str) -> Option<u16> {
     u16::from_str_radix(hex.trim(), 16).ok()
@@ -1868,10 +1891,10 @@ fn merge_cmaps(mut base: ToUnicodeCMap, overlay: ToUnicodeCMap) -> ToUnicodeCMap
 }
 
 /// Shared 16-bit CID expansion cap (65,536).
-/// Encoding `begincidrange` and `/W` width assignment count every insert,
-/// including overwrites, so a repeated full-width range cannot keep working
-/// after the domain is filled. The `/W` unicode heuristic caps unique CIDs
-/// with the same number.
+/// Encoding `begincidrange`, `/W` width assignment, and ToUnicode sequential
+/// remap count every insert, including overwrites, so a repeated full-width
+/// range cannot keep working after the domain is filled. The `/W` unicode
+/// heuristic caps unique CIDs with the same number.
 pub(crate) const MAX_CID_W_EXPANSION: usize = 65_536;
 
 /// Check if a CIDFont's /W (widths) array contains CID values that look like
@@ -2924,6 +2947,33 @@ endbfrange
         assert!(remapped.ranges.is_empty());
     }
 
+    #[test]
+    fn remap_to_sequential_repeated_full_bfranges_stay_bounded() {
+        // 5,000 copies of `<0003> <ffff>` must stop after 65,536 CID visits,
+        // not 5,000 × ~65,533 expansions.
+        let ranges = vec![(3u16, 65535u16, 0x41u32); 5_000];
+        let mut map = std::collections::HashMap::new();
+        let assigned = expand_bfranges_for_remap(&ranges, &mut map, MAX_CID_W_EXPANSION);
+        assert_eq!(assigned, MAX_CID_W_EXPANSION);
+        assert!(map.len() <= MAX_CID_W_EXPANSION);
+
+        let mut body = String::new();
+        let mut remaining = 5_000usize;
+        while remaining > 0 {
+            let n = remaining.min(100);
+            body.push_str(&format!("{n} beginbfrange\n"));
+            for _ in 0..n {
+                body.push_str("<0003> <ffff> <0041>\n");
+            }
+            body.push_str("endbfrange\n");
+            remaining -= n;
+        }
+        let data = format!("1 begincodespacerange\n<0000> <ffff>\nendcodespacerange\n{body}");
+        let cmap = ToUnicodeCMap::parse(data.as_bytes()).unwrap();
+        let remapped = cmap.remap_to_sequential();
+        assert_eq!(remapped.lookup(1), Some("A".to_string()));
+    }
+
     #[test]
     fn test_min_source_cid() {
         let cmap_content = r#"