Properly link flecs library

engine/libs/flecs/examples/cpp/rules/cyclic_variables/src/main.cpp

@@ -0,0 +1,67 @@
+#include <cyclic_variables.h>
+#include <iostream>
+
+// This example shows how a rule may have terms with cyclic dependencies on
+// variables.
+
+struct Likes { };
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    flecs::entity bob = ecs.entity("Bob");
+    flecs::entity alice = ecs.entity("Alice");
+    flecs::entity john = ecs.entity("John");
+    flecs::entity jane = ecs.entity("Jane");
+
+    bob.add<Likes>(alice);
+    alice.add<Likes>(bob);
+    john.add<Likes>(jane);
+    jane.add<Likes>(john);
+    bob.add<Likes>(jane); // inserting a bit of drama
+
+    // The following rule will only return entities that have a cyclic Likes
+    // relationship- that is they must both like each other.
+    //
+    // The equivalent query in the DSL is:
+    //   Likes($X, $Y), Likes($Y, $X)
+    //
+    // This is also an example of a query where all sources are variables. By
+    // default queries use the builtin "This" variable as subject, which is what
+    // populates the entities array in the query result (accessed by the
+    // iter::entity function). 
+    //
+    // Because this query does not use This at all, the entities array will not
+    // be populated, and it.count() will always be 0.
+    flecs::rule<> r = ecs.rule_builder()
+        .with<Likes>("$Y").src("$X")
+        .with<Likes>("$X").src("$Y")
+        .build();
+
+    // Lookup the index of the variables. This will let us quickly lookup their
+    // values while we're iterating.
+    int x_var = r.find_var("X");
+    int y_var = r.find_var("Y");
+
+    // Because the query doesn't use the This variable we cannot use "each"
+    // which iterates the entities array. Instead we can use iter like this:
+    r.iter([&](flecs::iter& it) {
+        flecs::entity x = it.get_var(x_var);
+        flecs::entity y = it.get_var(y_var);
+        std::cout << x.name() << " likes " << y.name() << "\n";
+    });
+
+    // Output
+    //  Alice likes Bob
+    //  Bob likes Alice
+    //  Jane likes John
+    //  John likes Jane
+
+    // Note that the rule returns each pair twice. The reason for this is that
+    // the goal of the rule engine is to return all "facts" that are true
+    // within the given constraints. Since we did not give it any constraints
+    // that would favor a person being matched by X or Y, the rule engine
+    // returns both.
+
+    r.destruct();
+}