Index: lucene/src/test/org/apache/lucene/util/automaton/TestMinimize.java
===================================================================
--- lucene/src/test/org/apache/lucene/util/automaton/TestMinimize.java	(revision 0)
+++ lucene/src/test/org/apache/lucene/util/automaton/TestMinimize.java	(revision 0)
@@ -0,0 +1,54 @@
+package org.apache.lucene.util.automaton;
+
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import org.apache.lucene.util.LuceneTestCase;
+
+/** 
+ * This test builds some randomish NFA/DFA and minimizes them.
+ * the minimal and non-minimal are compared to ensure they are the same.
+ */
+public class TestMinimize extends LuceneTestCase {
+  public void test() {
+    int num = 10000 * RANDOM_MULTIPLIER;
+    for (int i = 0; i < num; i++) {
+      Automaton a = randomishAutomaton();
+      Automaton b = a.clone();
+      MinimizationOperations.minimize(b);
+      assertTrue(BasicOperations.sameLanguage(a, b));
+    }
+  }
+  
+  private Automaton randomishAutomaton() {
+    // get two random Automata from regexps
+    Automaton a1 = AutomatonTestUtil.randomRegexp(random).toAutomaton();
+    if (random.nextBoolean())
+      a1 = BasicOperations.complement(a1);
+    
+    Automaton a2 = AutomatonTestUtil.randomRegexp(random).toAutomaton();
+    if (random.nextBoolean()) 
+      a2 = BasicOperations.complement(a2);
+    
+    // combine them in random ways
+    switch(random.nextInt(3)) {
+      case 0: return BasicOperations.concatenate(a1, a2);
+      case 1: return BasicOperations.union(a1, a2);
+      default: return BasicOperations.minus(a1, a2);
+    }
+  }
+}

Property changes on: lucene\src\test\org\apache\lucene\util\automaton\TestMinimize.java
___________________________________________________________________
Added: svn:eol-style
   + native

Index: lucene/src/java/org/apache/lucene/util/automaton/BasicOperations.java
===================================================================
--- lucene/src/java/org/apache/lucene/util/automaton/BasicOperations.java	(revision 1026095)
+++ lucene/src/java/org/apache/lucene/util/automaton/BasicOperations.java	(working copy)
@@ -61,6 +61,8 @@
   static public Automaton concatenate(Automaton a1, Automaton a2) {
     if (a1.isSingleton() && a2.isSingleton()) return BasicAutomata
         .makeString(a1.singleton + a2.singleton);
+    if (isEmpty(a1) || isEmpty(a2))
+      return BasicAutomata.makeEmpty();
     // adding epsilon transitions with the NFA concatenation algorithm
     // in this case always produces a resulting DFA, preventing expensive
     // redundant determinize() calls for this common case.