Properly link flecs library

engine/libs/flecs/examples/c/rules/basics/src/main.c

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+#include <basics.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_TAG(ecs, Eats);
+    ECS_TAG(ecs, Healthy);
+
+    ecs_entity_t Apples = ecs_new_entity(ecs, "Apples");
+    ecs_entity_t Salad = ecs_new_entity(ecs, "Salad");
+    ecs_entity_t Burgers = ecs_new_entity(ecs, "Burgers");
+    ecs_entity_t Pizza = ecs_new_entity(ecs, "Pizza");
+    ecs_entity_t Chocolate = ecs_new_entity(ecs, "Chocolate");
+
+    ecs_add(ecs, Apples, Healthy);
+    ecs_add(ecs, Salad, Healthy);
+
+    ecs_entity_t bob = ecs_new_entity(ecs, "Bob");
+    ecs_add_pair(ecs, bob, Eats, Apples);
+    ecs_add_pair(ecs, bob, Eats, Burgers);
+    ecs_add_pair(ecs, bob, Eats, Pizza);
+
+    ecs_entity_t alice = ecs_new_entity(ecs, "Alice");
+    ecs_add_pair(ecs, alice, Eats, Salad);
+    ecs_add_pair(ecs, alice, Eats, Chocolate);
+    ecs_add_pair(ecs, alice, Eats, Apples);
+
+    // Here we're creating a rule that in the query DSL would look like this:
+    //   Eats($This, $Food), Healthy($Food)
+    //
+    // Rules are similar to queries, but support more advanced features. This
+    // example shows how the basics of how to use rules & variables.
+    ecs_rule_t *r = ecs_rule(ecs, {
+        .terms = {
+            // Query variables are like wildcards, but enforce that the entity
+            // substituted by the wildcard is the same across terms.
+            //
+            // For example, in this query it is not guaranteed that the entity
+            // substituted by the * for Eats is the same as for Healthy:
+            //   (Eats, *), Healthy(*)
+            //
+            // By replacing * with _Food, both terms are constrained to use the
+            // same entity.
+            { .first.id = Eats, .second = { 
+                .name = "Food", .flags = EcsIsVariable },
+            },
+            { .first.id = Healthy, .src = {
+                .name = "Food", .flags = EcsIsVariable }
+            }
+        }
+    });
+
+    // Lookup the index of the variable. This will let us quickly lookup its
+    // value while we're iterating.
+    int food_var = ecs_rule_find_var(r, "Food");
+
+    // Iterate the rule
+    ecs_iter_t it = ecs_rule_iter(ecs, r);
+    while (ecs_rule_next(&it)) {
+        ecs_entity_t food = ecs_iter_get_var(&it, food_var);
+        for (int i = 0; i < it.count; i ++) {
+            printf("%s eats %s\n", ecs_get_name(ecs, it.entities[i]),
+                ecs_get_name(ecs, food));
+        }
+    }
+
+    // Rules need to be explicitly deleted
+    ecs_rule_fini(r);
+
+    // Output
+    //  Bob eats Apples
+    //  Alice eats Apples
+    //  Alice eats Salad
+
+    return ecs_fini(ecs);
+}