Fix least-free-vars SIPS metric (#2587)

Author: jjjxia

src/ast/tests/CMakeLists.txt

@@ -13,3 +13,4 @@ souffle_add_binary_test(ast_utils_test ast)
 souffle_add_binary_test(type_system_test ast)
 souffle_add_binary_test(constraints_test ast)
 souffle_add_binary_test(ast_recursive_clauses_test ast)
+souffle_add_binary_test(sips_metric_test ast)

src/ast/tests/sips_metric_test.cpp

@@ -0,0 +1,57 @@
+/*
+ * Souffle - A Datalog Compiler
+ * Copyright (c) 2026 The Souffle Developers. All rights reserved
+ * Licensed under the Universal Permissive License v 1.0 as shown at:
+ * - https://opensource.org/licenses/UPL
+ * - <souffle root>/licenses/SOUFFLE-UPL.txt
+ */
+
+/************************************************************************
+ *
+ * @file sips_metric_test.cpp
+ *
+ * Tests souffle's SIPS cost metrics.
+ *
+ ***********************************************************************/
+
+#include "tests/test.h"
+
+#include "Global.h"
+#include "ast/Program.h"
+#include "ast/QualifiedName.h"
+#include "ast/TranslationUnit.h"
+#include "ast2ram/utility/SipsMetric.h"
+#include "parser/ParserDriver.h"
+#include "reports/DebugReport.h"
+#include "reports/ErrorReport.h"
+#include <string>
+#include <vector>
+
+namespace souffle::ast {
+
+namespace test {
+
+TEST(SipsMetric, LeastFreeVarsCountsUnboundVariables) {
+    Global glb;
+    ErrorReport e;
+    DebugReport d(glb);
+
+    Own<TranslationUnit> tu = ParserDriver::parseTranslationUnit(glb,
+            R"(
+                .decl seed(x:number)
+                .decl low(z:number, w:number)
+                .decl high(x:number, y:number)
+                .decl result(y:number, z:number, w:number)
+
+                result(y, z, w) :- seed(x), low(z, w), high(x, y).
+            )",
+            e, d);
+
+    auto* clause = tu->getProgram().getClauses(QualifiedName::fromString("result"))[0];
+    LeastFreeVarsSips sips(*tu);
+
+    EXPECT_EQ(std::vector<std::size_t>({0, 2, 1}), sips.getReordering(clause, {"seed", "low", "high"}));
+}
+
+}  // namespace test
+}  // namespace souffle::ast

src/ast2ram/utility/SipsMetric.cpp

@@ -478,7 +478,7 @@ std::vector<double> LeastFreeVarsSips::evaluateCosts(const std::vector<Atom*> at
         // use a set to hold all free variables to avoid double-counting
         std::set<std::string> freeVars;
         visit(*atom, [&](const Variable& var) {
-            if (bindingStore.isBound(var.getName())) {
+            if (!bindingStore.isBound(var.getName())) {
                 freeVars.insert(var.getName());
             }
         });