Properly link flecs library

engine/libs/flecs/examples/c/systems/system_ctx/src/main.c

@@ -0,0 +1,106 @@
+#include <system_ctx.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+
+// Applications can pass context data to a system. A common use case where this
+// comes in handy is when a system needs to iterate more than one query. The
+// following example shows how to pass a custom query into a system for a simple
+// collision detection example.
+
+typedef struct {
+    double x, y;
+} Position;
+
+typedef struct {
+    double value;
+} Radius;
+
+double sqr(double value) {
+    return value * value;
+}
+
+double distance_sqr(const Position *p1, const Position *p2) {
+    return sqr(p2->x - p1->x) + sqr(p2->y - p1->y);
+}
+
+void Collide(ecs_iter_t *it) {
+    ecs_query_t *q_collide = it->ctx; // Get query from system context
+    const Position *p1 = ecs_field(it, Position, 1);
+    const Radius *r1 = ecs_field(it, Radius, 2);
+
+    for (int i = 0; i < it->count; i ++) {
+        ecs_entity_t e1 = it->entities[i];
+
+        // For each matching entity, iterate the query
+        ecs_iter_t qit = ecs_query_iter(it->world, q_collide);
+        while (ecs_query_next(&qit)) {
+            const Position *p2 = ecs_field(&qit, Position, 1);
+            const Radius *r2 = ecs_field(&qit, Radius, 2);
+            for (int j = 0; j < qit.count; j ++) {
+                ecs_entity_t e2 = qit.entities[j];
+                if (e1 == e2) {
+                    // don't collide with self
+                    continue;
+                }
+
+                if (e2 > e1) {
+                    // Simple trick to prevent collisions from being detected
+                    // twice with the entities reversed.
+                    continue;
+                }
+
+                // Check for collision
+                double d_sqr = distance_sqr(&p1[i], &p2[j]);
+                double r_sqr = sqr(r1[i].value + r2[j].value);
+                if (r_sqr > d_sqr) {
+                    printf("%u and %u collided!\n", (uint32_t)e1, (uint32_t)e2);
+                }
+            }
+        }
+    }
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Position);
+    ECS_COMPONENT(ecs, Radius);
+
+    // Create a query for Position that we can use inside the collide system to
+    // check each entity with each other entity.
+    ecs_query_t  *q_position = ecs_query(ecs, {
+        .filter.terms = {
+            { ecs_id(Position), .inout = EcsIn },
+            { ecs_id(Radius), .inout = EcsIn }
+        }
+    });
+
+    // Create collide system that passes query as context
+    ecs_entity_t collide = ecs_system(ecs, {
+        .query.filter.terms = {
+            { .id = ecs_id(Position), .inout = EcsIn },
+            { ecs_id(Radius), .inout = EcsIn }
+        },
+        .callback = Collide,
+        .ctx = q_position
+    });
+
+    // Create a few test entities
+    for (int i = 0; i < 10; i ++) {
+        ecs_entity_t e = ecs_new_id(ecs);
+        ecs_set(ecs, e, Position, { .x = rand() % 100, .y = rand() % 100 });
+        ecs_set(ecs, e, Radius, { rand() % 10 + 1 });
+    }
+
+    // Run the system
+    ecs_run(ecs, collide, 0.0f, NULL);
+
+    return ecs_fini(ecs);
+
+    // Output
+    //  508 and 505 collided!
+    //  510 and 508 collided!
+    //  513 and 506 collided!
+    //  514 and 512 collided!
+}