add merge sort

src/user.c

@@ -429,6 +429,22 @@ INTERNAL void debug_draw_movement(struct entity *ent)
     draw_solid_arrow_ray(G.viewport_canvas, pos, vel_ray, thickness, arrow_len, color_vel);
 }
 
+/* ========================== *
+ * Sort entities
+ * ========================== */
+
+INTERNAL SORT_COMPARE_FUNC_DEF(sort_entities, arg_a, arg_b, udata)
+{
+    (UNUSED)udata;
+    struct entity *a = *(struct entity **)arg_a;
+    struct entity *b = *(struct entity **)arg_b;
+
+    u64 a_index = a->handle.idx;
+    u64 b_index = b->handle.idx;
+
+    return (a_index < b_index) - (a_index > b_index);
+}
+
 /* ========================== *
  * Update
  * ========================== */
@@ -872,13 +888,32 @@ INTERNAL void user_update(void)
     /* ---------------------------------------------------------------------- */
     /* ---------------------------------------------------------------------- */
 
+    /* ========================== *
+     * Sort entities
+     * ========================== */
+
+    struct entity **sorted = arena_dry_push(scratch.arena, struct entity *);
+    u64 sorted_count = 0;
+    {
+        /* Copy valid entities */
+        for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
+            struct entity *ent = &store->entities[entity_index];
+            if (entity_is_valid_and_active(ent)) {
+                *arena_push(scratch.arena, struct entity *) = ent;
+                ++sorted_count;
+            }
+        }
+        /* Sort */
+        merge_sort(sorted, sorted_count, sizeof(*sorted), sort_entities, NULL);
+    }
+
     /* ========================== *
      * Draw entities
      * ========================== */
 
-    for (u64 entity_index = 0; entity_index < store->reserved; ++entity_index) {
+    for (u64 sorted_index = 0; sorted_index < sorted_count; ++sorted_index) {
         __profscope(user_entity_iter);
-        struct entity *ent = &store->entities[entity_index];
+        struct entity *ent = sorted[sorted_index];
         if (!entity_is_valid_and_active(ent)) continue;
         //if (sprite_tag_is_nil(ent->sprite)) continue;
 

src/util.h

@@ -7,6 +7,7 @@
 #include "arena.h"
 #include "atomic.h"
 #include "math.h"
+#include "scratch.h"
 
 /* Utility functions and stuff that don't have a home :( */
 
@@ -42,6 +43,77 @@ INLINE u128 hash_fnv128(u128 seed, struct buffer buff)
     return hash;
 }
 
+/* ========================== *
+ * Merge sort
+ * ========================== */
+
+/* Compare functions should
+ * return an int < 0 if a <  b
+ * return an int = 0 if a == b
+ * return an int > 0 if a > b
+ */
+#define SORT_COMPARE_FUNC_DEF(name, arg_a, arg_b, arg_udata) i32 name(void *arg_a, void *arg_b, void *arg_udata)
+typedef SORT_COMPARE_FUNC_DEF(sort_compare_func, a, b, udata);
+
+INLINE void merge_sort_internal(void *left, void *right, void *items, u64 left_count, u64 right_count, u64 item_size, sort_compare_func *callback, void *udata)
+{
+    u64 i = 0;
+    u64 l = 0;
+    u64 r = 0;
+    while (l < left_count && r < right_count) {
+        u8 *dst = ((u8 *)items) + (i * item_size);
+        u8 *left_item = ((u8 *)left) + (l * item_size);
+        u8 *right_item = ((u8 *)right) + (r * item_size);
+        ++i;
+        if (callback(left_item, right_item, udata) > 0) {
+            MEMCPY(dst, left_item, item_size);
+            ++l;
+        } else {
+            MEMCPY(dst, right_item, item_size);
+            ++r;
+        }
+    }
+    /* Copy remaining */
+    u64 left_remaining_bytes = (left_count - l) * item_size;
+    u64 right_remaining_bytes = (right_count - r) * item_size;
+    if (left_remaining_bytes > 0) {
+        u8 *dst = ((u8 *)items) + (i * item_size);
+        u8 *src = ((u8 *)left) + (l * item_size);
+        MEMCPY(dst, src, left_remaining_bytes);
+        i += left_count - l;
+        l = left_remaining_bytes;
+    }
+    if (right_remaining_bytes > 0) {
+        u8 *dst = ((u8 *)items) + (i * item_size);
+        u8 *src = ((u8 *)right) + (r * item_size);
+        MEMCPY(dst, src, right_remaining_bytes);
+        i += right_count - r;
+        l = right_remaining_bytes;
+    }
+}
+
+INLINE void merge_sort(void *items, u64 item_count, u64 item_size, sort_compare_func *callback, void *udata)
+{
+    if (item_count > 1) {
+        struct temp_arena scratch = scratch_begin_no_conflict();
+        u64 left_count = item_count / 2;
+        u64 right_count = item_count - left_count;
+
+        u64 left_size = left_count * item_size;
+        u64 right_size = right_count * item_size;
+
+        u8 *left = arena_push_array(scratch.arena, u8, left_size);
+        u8 *right = arena_push_array(scratch.arena, u8, right_size);
+        MEMCPY(left, items, left_size);
+        MEMCPY(right, (u8 *)items + left_size, right_size);
+
+        merge_sort(left, left_count, item_size, callback, udata);
+        merge_sort(right, right_count, item_size, callback, udata);
+        merge_sort_internal(left, right, items, left_count, right_count, item_size, callback, udata);
+        scratch_end(scratch);
+    }
+}
+
 /* ========================== *
  * Fixed Dict
  *