Spelling: remember failed compound tails

crates/engine/src/spell.rs

@@ -304,7 +304,12 @@ impl Dictionary {
 
     /// `w` made of dictionary parts allowed in compounds (`COMPOUNDFLAG`, or the begin, middle and
     /// end flags), each at least `COMPOUNDMIN` characters long.
-    fn compounded(&self, w: &[char], index: usize) -> bool {
+    fn compounded(&self, w: &[char], index: usize, failed: &mut HashSet<(usize, usize)>) -> bool {
+        // `w` is always a tail of the checked word, so its length names it: a (tail, part index)
+        // that failed once fails again however the earlier parts were split.
+        if failed.contains(&(w.len(), index)) {
+            return false;
+        }
         if self.compound.is_none() && self.compound_end.is_none() {
             return false;
         }
@@ -322,10 +327,11 @@ impl Dictionary {
             if [self.compound, self.compound_end].into_iter().flatten().any(|f| self.compound_part(rest, f, true)) {
                 return true;
             }
-            if self.compounded(rest, index + 1) {
+            if self.compounded(rest, index + 1, failed) {
                 return true;
             }
         }
+        failed.insert((w.len(), index));
         false
     }
 
@@ -352,7 +358,7 @@ impl Dictionary {
                 return true;
             }
         }
-        self.compounded(&w, 0)
+        self.compounded(&w, 0, &mut HashSet::new())
     }
 
     /// Is `word` spelt correctly? Capitalised and upper-case forms of a lower-case word pass.
@@ -931,6 +937,15 @@ SFX D Y 1\nSFX D 0 ed/S .\n";
         }
     }
 
+    #[test]
+    fn overlapping_compound_roots_with_a_failing_ending_stay_cheap() {
+        // Every split of "aaa…ab" reaches the same (tail, part) states; unmemoized this is
+        // ~C(99, 7) searches.
+        let d = Dictionary::parse(b"COMPOUNDFLAG X\nCOMPOUNDMIN 1\n".as_slice(), b"1\na/X\n".as_slice());
+        assert!(!d.check(&format!("{}b", "a".repeat(99))));
+        assert!(d.check(&"a".repeat(5)));
+    }
+
     #[test]
     fn runs_give_words_their_language() {
         use deckcraft_model::{Paragraph, Run, RunProps};