[DOXIA-662] Non unique IDs generated by IndexingSink

This closes #41

doxia-core/src/main/java/org/apache/maven/doxia/index/IndexingSink.java

@@ -19,7 +19,10 @@
  * under the License.
  */
 
+import java.util.HashMap;
+import java.util.Map;
 import java.util.Stack;
+import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.maven.doxia.sink.impl.SinkAdapter;
 import org.apache.maven.doxia.util.HtmlTools;
@@ -69,6 +72,12 @@ public class IndexingSink
     /** The stack. */
     private final Stack<IndexEntry> stack;
 
+    /** A map containing all used ids of index entries as key and how often they are used as value
+     * (0-based, i.e. 0 means used 1 time). {@link AtomicInteger} is only used here as it implements
+     * a mutable integer (not for its atomicity).
+     */
+    private final Map<String, AtomicInteger> usedIds;
+
     /**
      * Default constructor.
      *
@@ -78,7 +87,8 @@ public IndexingSink( IndexEntry sectionEntry )
     {
         stack = new Stack<>();
         stack.push( sectionEntry );
-
+        usedIds = new HashMap<>();
+        usedIds.put( sectionEntry.getId(), new AtomicInteger() );
         init();
     }
 
@@ -311,7 +321,7 @@ public void text( String text )
                 title = title.replaceAll( "[\\r\\n]+", "" );
                 entry.setTitle( title );
 
-                entry.setId( HtmlTools.encodeId( title ) );
+                entry.setId( getUniqueId ( HtmlTools.encodeId( title ) ) );
 
                 break;
             // Dunno how to handle these yet
@@ -323,6 +333,28 @@ public void text( String text )
         }
     }
 
+    /**
+     * Converts the given id into a unique one by potentially suffixing it with an index value.
+     *
+     * @param id
+     * @return the unique id
+     */
+    String getUniqueId( String id )
+    {
+        final String uniqueId;
+
+        if ( usedIds.containsKey( id ) )
+        {
+            uniqueId = id + "_" + usedIds.get( id ).incrementAndGet();
+        }
+        else
+        {
+            usedIds.put( id, new AtomicInteger() );
+            uniqueId = id;
+        }
+        return uniqueId;
+    }
+
     /**
      * Creates and pushes a new IndexEntry onto the top of this stack.
      */

doxia-core/src/test/java/org/apache/maven/doxia/index/IndexingSinkTest.java

@@ -0,0 +1,36 @@
+package org.apache.maven.doxia.index;
+
+/*
+ * 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.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class IndexingSinkTest
+{
+    @Test
+    public void testGetUniqueId()
+    {
+        IndexingSink sink = new IndexingSink( new IndexEntry( "root" ) );
+        assertEquals( "root_1", sink.getUniqueId( "root" ) );
+        assertEquals( "root_2", sink.getUniqueId( "root" ) );
+        assertEquals( "newid", sink.getUniqueId( "newid" ) );
+    }
+}