Properly link flecs library

engine/libs/flecs/examples/c/entities/hierarchy/src/main.c

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+#include <hierarchy.h>
+#include <stdio.h>
+
+typedef struct {
+    double x, y;
+} Position;
+
+// Forward declare component so we can use it from functions other than main
+ECS_COMPONENT_DECLARE(Position);
+
+void iterate_tree(ecs_world_t *ecs, ecs_entity_t e, Position p_parent) {
+    // Print hierarchical name of entity & the entity type
+    char *path_str = ecs_get_fullpath(ecs, e);
+    char *type_str = ecs_type_str(ecs, ecs_get_type(ecs, e));
+    printf("%s [%s]\n", path_str, type_str);
+    ecs_os_free(type_str);
+    ecs_os_free(path_str);
+
+    // Get entity position
+    const Position *ptr = ecs_get(ecs, e, Position);
+    
+    // Calculate actual position
+    Position p_actual = {ptr->x + p_parent.x, ptr->y + p_parent.y};
+    printf("{%f, %f}\n\n", p_actual.x, p_actual.y);
+
+    // Iterate children recursively
+    ecs_iter_t it = ecs_children(ecs, e);
+    while (ecs_children_next(&it)) {
+        for (int i = 0; i < it.count; i ++) {
+            iterate_tree(ecs, it.entities[i], p_actual);
+        }
+    }
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT_DEFINE(ecs, Position);
+    ECS_TAG(ecs, Star);
+    ECS_TAG(ecs, Planet);
+    ECS_TAG(ecs, Moon);
+
+    // Create a simple hierarchy.
+    // Hierarchies use ECS relationships and the builtin flecs::ChildOf relationship to
+    // create entities as children of other entities.
+
+    ecs_entity_t sun = ecs_new_entity(ecs, "Sun");
+    ecs_add(ecs, sun, Star);
+    ecs_set(ecs, sun, Position, {1, 1});
+
+        ecs_entity_t mercury = ecs_new_entity(ecs, "Mercury");
+        ecs_add_pair(ecs, mercury, EcsChildOf, sun);
+        ecs_add(ecs, mercury, Planet);
+        ecs_set(ecs, mercury, Position, {1, 1});
+
+        ecs_entity_t venus = ecs_new_entity(ecs, "Venus");
+        ecs_add_pair(ecs, venus, EcsChildOf, sun);
+        ecs_add(ecs, venus, Planet);
+        ecs_set(ecs, venus, Position, {2, 2});
+
+        ecs_entity_t earth = ecs_new_entity(ecs, "Earth");
+        ecs_add_pair(ecs, earth, EcsChildOf, sun);
+        ecs_add(ecs, earth, Planet);
+        ecs_set(ecs, earth, Position, {3, 3});
+
+            ecs_entity_t moon = ecs_new_entity(ecs, "Moon");
+            ecs_add_pair(ecs, moon, EcsChildOf, earth);
+            ecs_add(ecs, moon, Moon);
+            ecs_set(ecs, moon, Position, {0.1, 0.1});
+
+    // Is the Moon a child of Earth?
+    printf("Child of Earth? %d\n", ecs_has_pair(ecs, moon, EcsChildOf, earth));
+
+    // Do a depth-first walk of the tree
+    iterate_tree(ecs, sun, (Position){0, 0});
+
+    return ecs_fini(ecs);
+}