Issue #96: Fix LevelWeight comparison.

* MinimizeBuilder::CmpWeight failed to handle missing levels correctly.
  In particular, if a tuple R with *negative* weight w at level p was
  compared to a tuple L with missing weight at p, RHS was incorrectly
  considered to be greater than L. Fix this so that now the weight at
  p is compared to 0 (no weight at p).

src/minimize_constraint.cpp

@@ -617,7 +617,9 @@ bool MinimizeBuilder::CmpWeight::operator()(const MLit& lhs, const MLit& rhs) co
 	const SharedData::LevelWeight* wLhs = &(*weights)[lhs.weight];
 	const SharedData::LevelWeight* wRhs = &(*weights)[rhs.weight];
 	for (;; ++wLhs, ++wRhs) {
-		if (wLhs->level != wRhs->level)  { return wLhs->level < wRhs->level; }
+		if (wLhs->level != wRhs->level)  {
+			return wLhs->level < wRhs->level ? wLhs->weight > 0 : 0 > wRhs->weight;
+		}
 		if (wLhs->weight != wRhs->weight){ return wLhs->weight > wRhs->weight; }
 		if (!wLhs->next) { return wRhs->next && (++wRhs)->weight < 0; }
 		if (!wRhs->next) { ++wLhs; break; }

tests/minimize_test.cpp

@@ -247,6 +247,38 @@ TEST_CASE("Model-guided minimize", "[constraint][asp]") {
 		REQUIRE((newMin->shared()->weights[0].weight == 1));
 		REQUIRE((newMin->shared()->weights[1].weight == -5));
 	}
+	SECTION("testSparseCompare") {
+		mb.add(0, WeightLiteral(b, 1));
+		mb.add(1, WeightLiteral(a, 1));
+		mb.add(2, WeightLiteral(c, -1));
+		mb.add(2, WeightLiteral(b, -2));
+		mb.add(2, WeightLiteral(a, -2));
+
+		newMin = test.buildAndAttach(mb);
+		MinimizeConstraint::SharedDataP shared = newMin->shared();
+
+#define CHECK_LEVEL_WEIGHT(idx, w, l, n) \
+	CHECK(shared->weights.at(idx).weight == w); \
+	CHECK(shared->weights.at(idx).level == l); \
+	CHECK(shared->weights.at(idx).next == n)
+
+		REQUIRE(test.countMinLits() == 3);
+
+		CHECK(shared->lits[0] == WeightLiteral(~b, 0));
+		CHECK_LEVEL_WEIGHT(0, 2, 0, 1);
+		CHECK_LEVEL_WEIGHT(1, -1, 2, 0);
+
+		CHECK(shared->lits[1] == WeightLiteral(~a, 2));
+		CHECK_LEVEL_WEIGHT(2, 2, 0, 1);
+		CHECK_LEVEL_WEIGHT(3, -1, 1, 0);
+
+		CHECK(shared->lits[2] == WeightLiteral(~c, 4));
+		CHECK_LEVEL_WEIGHT(4, 1, 0, 0);
+
+		CHECK(shared->adjust(0) == -5);
+		CHECK(shared->adjust(1) == 1);
+		CHECK(shared->adjust(2) == 1);
+	}
 	SECTION("testInitFromOther") {
 		min.push_back( WeightLiteral(~a, 2) );
 		min.push_back( WeightLiteral(~a, -3));