Fixing ConflictCause construction in ArrayAssignment, adding related …

…solver test.

src/main/java/at/ac/tuwien/kr/alpha/solver/ArrayAssignment.java

@@ -225,14 +225,16 @@ private ConflictCause assignWithDecisionLevel(int atom, ThriceTruth value, NoGoo
 
 		// Check consistency.
 		if (!assignmentsConsistent(current, value)) {
-			ConflictCause conflictCause = null;
+			ConflictCause conflictCause = new ConflictCause(impliedBy);
 			// Assignments are inconsistent, prepare the reason.
 			NoGood violated = impliedBy;
 			if (decisionLevel < current.getWeakDecisionLevel()) {
 				// New assignment is lower than the current one, hence cause is the reason for the (higher) current one.
 				violated = current.getPrevious() == null ? current.getImpliedBy() : current.getPrevious().getImpliedBy();	// take MBT reason if it exists.
 				if (violated == null) {
 					conflictCause = new ConflictCause(current);
+				} else {
+					conflictCause = new ConflictCause(violated);
 				}
 				// The lower assignment takes precedence over the current value, overwrite it and adjust mbtCounter.
 				if (current.getTruth() == MBT) {
@@ -244,7 +246,6 @@ private ConflictCause assignWithDecisionLevel(int atom, ThriceTruth value, NoGoo
 				}
 
 			}
-			conflictCause = new ConflictCause(impliedBy);
 			return conflictCause;
 		}
 

src/test/java/at/ac/tuwien/kr/alpha/solver/SolverTests.java

@@ -862,4 +862,30 @@ public void noPositiveCycleSelfFoundingGuess() throws IOException {
 		Set<AnswerSet> answerSets = solver.collectSet();
 		assertTrue(answerSets.isEmpty());
 	}
+
+	@Test
+	public void conflictFromUnaryNoGood() throws IOException {
+		String program =
+			"d(b).\n" +
+			"sel(X) :- not nsel(X), d(X).\n" +
+			"nsel(X) :- not sel(X), d(X).\n" +
+			"t(a) :- sel(b).\n" +
+			":- t(X).\n";
+
+		ParsedProgram parsedProgram = parseVisit(program);
+		NaiveGrounder grounder = new NaiveGrounder(parsedProgram);
+		Solver solver = getInstance(grounder);
+
+		Set<AnswerSet> expected = new HashSet<>(Collections.singletonList(
+				new BasicAnswerSet.Builder()
+						.predicate("d")
+						.instance("b")
+						.predicate("nsel")
+						.instance("b")
+						.build()
+		));
+
+		Set<AnswerSet> answerSets = solver.collectSet();
+		assertEquals(expected, answerSets);
+	}
 }