Properly link flecs library

engine/libs/flecs/examples/c/queries/group_by/src/main.c

@@ -0,0 +1,126 @@
+#include <group_by.h>
+#include <stdio.h>
+
+// Group by is a feature of cached queries that allows applications to assign a
+// group id to each matched table. Tables that are assigned the same group id
+// are stored together in "groups". This ensures that when a query is iterated,
+// tables that share a group are iterated together.
+//
+// Groups in the cache are ordered by group id, which ensures that tables with
+// lower ids are iterated before table with higher ids. This is the same 
+// mechanism that is used by the cascade feature, which groups tables by depth
+// in a relationship hierarchy.
+//
+// This makes groups a more efficient, though less granular mechanism for 
+// ordering entities. Order is maintained at the group level, which means that
+// once a group is created, tables can get added and removed to the group
+// with is an O(1) operation.
+//
+// Groups can also be used as an efficient filtering mechanism. See the 
+// group_iter example for more details.
+
+typedef struct {
+    double x, y;
+} Position;
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Position);
+    ECS_TAG(ecs, Tag); // Dummy tag to put entities in different tables
+
+    // Create a relationship to use for the group_by function. Tables will
+    // be assigned the relationship target as group id
+    ECS_TAG(ecs, Group);
+
+    // Targets for the relationship, which will be used as group ids.
+    ECS_TAG(ecs, First);
+    ECS_TAG(ecs, Second);
+    ECS_TAG(ecs, Third);
+
+    // Grouped query
+    ecs_query_t *q = ecs_query(ecs, {
+        .filter.terms = {
+            { .id = ecs_id(Position) }
+        },
+        // Group tables by the relationship target of the "Group" relationship.
+        // A custom group_by function can be provided to derive a group id from
+        // a table, see the group_by_custom example for more details.
+        .group_by_id = Group
+    });
+
+    // Create entities in 6 different tables with 3 group ids
+    ecs_entity_t e1 = ecs_new_w_pair(ecs, Group, Third);
+    ecs_set(ecs, e1, Position, {1, 1});
+
+    ecs_entity_t e2 = ecs_new_w_pair(ecs, Group, Second);
+    ecs_set(ecs, e2, Position, {2, 2});
+
+    ecs_entity_t e3 = ecs_new_w_pair(ecs, Group, First);
+    ecs_set(ecs, e3, Position, {3, 3});
+
+    ecs_entity_t e4 = ecs_new_w_pair(ecs, Group, Third);
+    ecs_set(ecs, e4, Position, {4, 4});
+    ecs_add(ecs, e4, Tag);
+
+    ecs_entity_t e5 = ecs_new_w_pair(ecs, Group, Second);
+    ecs_set(ecs, e5, Position, {5, 5});
+    ecs_add(ecs, e5, Tag);
+
+    ecs_entity_t e6 = ecs_new_w_pair(ecs, Group, First);
+    ecs_set(ecs, e6, Position, {6, 6});
+    ecs_add(ecs, e6, Tag);
+
+    // The query cache now looks like this:
+    //  - group First:
+    //     - table [Position, (Group, First)]
+    //     - table [Postion, Tag, (Group, First)]
+    //
+    //  - group Second:
+    //     - table [Position, (Group, Second)]
+    //     - table [Postion, Tag, (Group, Second)]
+    //  
+    //  - group Third:
+    //     - table [Position, (Group, Third)]
+    //     - table [Postion, Tag, (Group, Third)]
+    //
+
+    // Iterate query, print position & table components
+    ecs_iter_t it = ecs_query_iter(ecs, q);
+    while (ecs_query_next(&it)) {
+        Position *p = ecs_field(&it, Position, 1);
+        char *table_str = ecs_table_str(ecs, it.table);
+        char *group_str = ecs_get_fullpath(ecs, it.group_id);
+
+        printf(" - group %s: table [%s]\n", group_str, table_str);
+
+        for (int i = 0; i < it.count; i ++) {
+            printf("     {%f, %f}\n", p[i].x, p[i].y);
+        }
+
+        ecs_os_free(group_str);
+        ecs_os_free(table_str);
+        printf("\n");
+    }
+
+    // Output:
+    //  - group First: table [Position, (Group,First)]
+    //      {3.000000, 3.000000}
+    // 
+    //  - group First: table [Position, Tag, (Group,First)]
+    //      {6.000000, 6.000000}
+    // 
+    //  - group Second: table [Position, (Group,Second)]
+    //      {2.000000, 2.000000}
+    // 
+    //  - group Second: table [Position, Tag, (Group,Second)]
+    //      {5.000000, 5.000000}
+    // 
+    //  - group Third: table [Position, (Group,Third)]
+    //      {1.000000, 1.000000}
+    // 
+    //  - group Third: table [Position, Tag, (Group,Third)]
+    //      {4.000000, 4.000000}
+
+    return ecs_fini(ecs);
+}