[Mulgara-svn] r183 - trunk/src/jar/tuples/java/org/mulgara/store/tuples

[pag at mulgara.org]

Author: pag
Date: 2007-02-24 13:26:41 -0600 (Sat, 24 Feb 2007)
New Revision: 183

Modified:
   trunk/src/jar/tuples/java/org/mulgara/store/tuples/TuplesOperations.java
Log:
Extra test in subtraction to prevent the operation from proceeding on an empty minuend or subtrahend.

Modified: trunk/src/jar/tuples/java/org/mulgara/store/tuples/TuplesOperations.java
===================================================================
--- trunk/src/jar/tuples/java/org/mulgara/store/tuples/TuplesOperations.java	2007-02-24 19:25:01 UTC (rev 182)
+++ trunk/src/jar/tuples/java/org/mulgara/store/tuples/TuplesOperations.java	2007-02-24 19:26:41 UTC (rev 183)
@@ -292,7 +292,13 @@
 
 
   /**
-   * This is approximately a subtraction.
+   * This is approximately a subtraction.  The subtrahend is matched against the minuend in the same
+   * way as a conjunction, and the matching lines removed from the minuend.  The remaining lines in
+   * the minuend are the result.
+   * @param minuend The tuples to subtract from.
+   * @param subtrahend The tuples to match against the minuend for removal.
+   * @return The contents from the minuend, excluding those rows which match against the subtrahend.
+   * @throws TuplesException If there are no matching variables between the minuend and subtrahend.
    */
   public static Tuples subtract(Tuples minuend, Tuples subtrahend) throws TuplesException {
     try {
@@ -309,6 +315,11 @@
         }
         throw new TuplesException("Unable to subtract: no common variables.");
       }
+      // double check that the variables are not equal
+      if (subtrahend.getRowCardinality() == Cursor.ZERO || minuend.getRowCardinality() == Cursor.ZERO) {
+        logger.warn("Found an empty Tuples with bound variables");
+        return (Tuples)minuend.clone();
+      }
       // reorder the subtrahend as necessary
       Tuples sortedSubtrahend;
       // check if there are variables which should not be considered when sorting