fix bug in IncrementalWMethodTestsIterator

single-state automata have no distinguishing suffixes. The iterator wrongfully tried to access an empty suffix list which caused an IndexOutOfBoundsException.

Author: mtf90

util/src/main/java/net/automatalib/util/automata/conformance/IncrementalWMethodTestsIterator.java

@@ -72,7 +72,7 @@ public void update(UniversalDeterministicAutomaton<?, I, ?, ?, ?> automaton) {
             item.maxPrefix = oldNumPrefixes;
             item.suffixIdx = oldNumSuffixes;
             item.minSuffix = oldNumSuffixes;
-            item.middle = startMiddleWord();
+            item.middle = Word.epsilon();
             itemQueue.insert(item);
         }
         // new prefixes with *all* suffixes
@@ -83,15 +83,11 @@ public void update(UniversalDeterministicAutomaton<?, I, ?, ?, ?> automaton) {
             item.maxPrefix = prefixes.size();
             item.suffixIdx = 0;
             item.minSuffix = 0;
-            item.middle = startMiddleWord();
+            item.middle = Word.epsilon();
             itemQueue.insert(item);
         }
     }
 
-    private Word<I> startMiddleWord() {
-        return Word.epsilon();
-    }
-
     @Override
     public boolean hasNext() {
         return !itemQueue.isEmpty();
@@ -111,8 +107,10 @@ public Word<I> next() {
 
     private Word<I> assembleWord(Item<I> item) {
         Word<I> prefix = prefixes.get(item.prefixIdx);
-        Word<I> suffix = suffixes.get(item.suffixIdx);
-        return prefix.concat(item.middle, suffix);
+        if (suffixes.isEmpty()) {
+            return prefix.concat(item.middle);
+        }
+        return prefix.concat(item.middle, suffixes.get(item.suffixIdx));
     }
 
     private @Nullable Item<I> increment(Item<I> item) {

util/src/test/java/net/automatalib/util/automata/conformance/IncrementalWMethodTestsIteratorTest.java

@@ -21,6 +21,7 @@
 
 import com.google.common.collect.Sets;
 import com.google.common.collect.Streams;
+import net.automatalib.automata.fsa.impl.compact.CompactDFA;
 import net.automatalib.automata.transducers.impl.compact.CompactMealy;
 import net.automatalib.commons.util.collections.CollectionsUtil;
 import net.automatalib.util.automata.Automata;
@@ -98,6 +99,27 @@ public void testIncrementalCover() {
         Assert.assertEquals(wMethodTests, iteratorTests);
     }
 
+    @Test
+    public void testSingleStateAutomaton() {
+
+        final CompactDFA<Character> dfa = new CompactDFA<>(alphabet);
+        final int init = dfa.addIntInitialState(false);
+
+        for (Character c : alphabet) {
+            dfa.setTransition(init, c, init, null);
+        }
+
+        final IncrementalWMethodTestsIterator<Character> iter = new IncrementalWMethodTestsIterator<>(alphabet);
+        iter.setMaxDepth(MAX_DEPTH);
+        iter.update(dfa);
+
+        final Set<Word<Character>> tests = Streams.stream(iter).collect(Collectors.toSet());
+
+        for (Character c : alphabet) {
+            Assert.assertTrue(tests.contains(Word.fromLetter(c)));
+        }
+    }
+
     private Set<Word<Character>> computeWMethodTests() {
 
         final List<Word<Character>> characterizingSet = Automata.characterizingSet(mealy, alphabet);