handle edge case when building dfa from a string, where an exception was raised when our search regex contains an alternation where the second alternation is a substring of the first alternation, and add a test for it to make sure that it passes

fcore/search-list/dfa-gen.sml

@@ -729,13 +729,21 @@ struct
           in
             Vector.concat [dtran, el]
           end
-        else
+        else if dStateIdx < Vector.length dtran then
           let
             val el = Vector.sub (dtran, dStateIdx)
             val el = Set.insertOrReplace (char, toStateIdx, el)
           in
             Vector.update (dtran, dStateIdx, el)
           end
+        else
+          let
+            val appendLength = dStateIdx - Vector.length dtran
+            val appendVecs = Vector.tabulate (appendLength, fn _ => Set.LEAF)
+            val dtran = Vector.concat [dtran, appendVecs]
+          in
+            insert (dStateIdx, char, toStateIdx, dtran)
+          end
     end
 
     fun convertChar

test/regression.sml

@@ -67,6 +67,15 @@ struct
            in
              Expect.isTrue true
            end)
+    , test
+        "DfaGen does not cause exception \
+        \when parsing alternation that contains a char \
+        \from the previous alternation (1)"
+        (fn _ =>
+           (let val dfa = CaseSensitiveDfa.fromString "str|s"
+            in Expect.isTrue true
+            end)
+           handle _ => Expect.isTrue false)
     ]
 
   val tests = [charEventTests]