From 34ccbafa9221066cf0ebcad8596404bbc2048302 Mon Sep 17 00:00:00 2001
From: quou <quou@disroot.org>
Date: Fri, 28 Apr 2023 22:24:08 +1000
Subject: Improve the C++ ECS demo by making it respect alignment with the
 allocation.

---
 ecs2.cpp | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/ecs2.cpp b/ecs2.cpp
index 2b2ac66..28f88ae 100644
--- a/ecs2.cpp
+++ b/ecs2.cpp
@@ -2,15 +2,12 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdint.h>
+#include <stddef.h>
 #include <assert.h>
 
 /* So it's like the other ECS example with a bunch of academic masturbation
- * See https://git.quou.xyz/samples/tree/ecs.c
- *
- * Areas for improvement:
- * - Should really respect alignment.
- * - Entity destruction
- */
+ * See https://git.quou.xyz/samples/tree/ecs.c */
 
 #define max_entities 256
 /* If you want more than 32 component types then you'll have to
@@ -22,6 +19,20 @@ typedef int Entity;
 
 unsigned get_new_component_id();
 
+void* aligned_malloc(size_t size, size_t alignment) {
+	size_t offset = alignment - 1 + sizeof(void*);
+	void* p1 = malloc(size + offset);
+	void** p2 = (void**)(((uintptr_t)(p1) + offset) & ~(alignment - 1));
+	p2[-1] = p1;
+	return p2;
+}
+
+void aligned_free(void* ptr) {
+	if (ptr) {
+		free(((void**)ptr)[-1]);
+	}
+}
+
 /* DO NOT make either of these functions static. That will break stuff. */
 template <typename T>
 unsigned get_component_id() {
@@ -45,13 +56,13 @@ struct Component_Pool {
 
 	Component_Pool(): data(0) {}
 
-	void init(int es) {
+	void init(int es, int alignment) {
 		element_size = es;
-		data = malloc(element_size * max_entities);
+		data = aligned_malloc(element_size * max_entities, alignment);
 	}
 
 	void deinit() {
-		free(data);
+		aligned_free(data);
 	}
 
 	bool isinit() {
@@ -61,10 +72,9 @@ struct Component_Pool {
 
 struct World {
 	Component_Pool pools[max_component_types];
+	unsigned bitset[max_entities];
 	int entity_count;
 
-	int bitset[max_entities];
-
 	World(): entity_count(0) {}
 
 	void deinit() {
@@ -77,7 +87,7 @@ struct World {
 
 	template <typename T>
 	void register_component_type() {
-		pools[get_component_id<T>()].init(sizeof(T));
+		pools[get_component_id<T>()].init(sizeof(T), alignof(T));
 	}
 
 	Entity create_entity() {
-- 
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