Fix bug transitions from invalid checks (#11)

* fix root case

* add comment

* simplify

* update

src/trie.rs

@@ -87,7 +87,9 @@ impl<T> Sparse<T> {
         let mut values = vec![wildcard_value];
         let mut is_used = vec![true];
         let mut stack = vec![(0, 0)];
-        let mut used_bases = HashSet::new();
+        // base=0 must be reserved for avoiding invalid transitions.
+        // See https://github.com/daac-tools/trie-match/pull/11.
+        let mut used_bases = HashSet::from([0]);
         let mut search_start = 0;
         while let Some((state_id, da_pos)) = stack.pop() {
             let state = &self.states[state_id];

tests/tests.rs

@@ -125,3 +125,22 @@ fn test_try_base_conflict() {
     assert_eq!(f("\u{2}\u{3}"), 1);
     assert_eq!(f("\u{3}"), 1);
 }
+
+// This test confirms that check[0] does not have an invalid value of zero.
+#[test]
+fn test_invalid_root_check_of_zero() {
+    // [0] -x01-> [1]
+    //    \-x00-> [0] ? If check[0] is 0, such an invalid transition is possible.
+    //
+    //  base: [0, MAX]
+    // check: [0,   1]
+    let f = |text| {
+        trie_match! {
+            match text {
+                "\u{1}" => 1,
+                _ => 0,
+            }
+        }
+    };
+    assert_eq!(f("\u{0}\u{1}"), 0);
+}