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Java/LoopDetector.java

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+/*   
+LoopDetector.java
+ *    
+ *    Copyright (c) 2000, Ted Nelson and Tuomas Lukka
+ *
+ *    You may use and distribute under the terms of either the GNU Lesser
+ *    General Public License, either version 2 of the license or,
+ *    at your choice, any later version. Alternatively, you may use and
+ *    distribute under the terms of the XPL.
+ *
+ *    See the LICENSE.lgpl and LICENSE.xpl files for the specific terms of 
+ *    the licenses.
+ *
+ *    This software is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the README
+ *    file for more details.
+ *
+ */
+/*
+ * Written by Tuomas Lukka
+ */
+package org.gzigzag;
+import java.util.*;
+
+/** A loop detector.
+ * Uses a simple constant-space algorithm that will detect any loop before 
+ * it's cycled through 3 times, except very small loops.  
+ * Example use:
+ * <pre>
+ * 	LoopDetector l = new LoopDetector();
+ *	while((x = iter(x)) != null) {
+ *		l.detect(x);
+ *		// ... do something with x.
+ *	}
+ * </pre>
+ * Note: this only works for deterministic loops that always have the same
+ * elements and length.
+ * <p>
+ * The Object.equals method is used for the comparisons.
+ */
+
+public final class LoopDetector {
+public static final String rcsid = "$Id: LoopDetector.java,v 1.6 2000/11/02 13:03:08 tjl Exp $";
+    Object obj = this;
+    int count = 0;
+    int nth = 10;
+    public void reset() { count = 0; obj = null; }
+    /** Throws an error if the parameter is a part of a looping sequence.
+     */
+    public void detect(Object o) {
+	if(isLooping(o))
+		throw new ZZInfiniteLoop("Infinite loop detected");
+    }
+    /** Returns true if the parameter is a part of a looping sequence.
+     */
+    public boolean isLooping(Object o) {
+	if(o.equals(obj)) return true;
+	if(count-- < 0) {
+	    nth *= 2;
+	    count = nth;
+	    obj = o;
+	}
+	return false;
+    }
+}