From 7c9c45845cfaeae56f0112a2c4eb151571385d9f Mon Sep 17 00:00:00 2001
From: quou <quou@disroot.org>
Date: Fri, 28 Apr 2023 20:45:18 +1000
Subject: Add a generic ECS.

---
 ecs2.cpp | 212 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 212 insertions(+)
 create mode 100644 ecs2.cpp

diff --git a/ecs2.cpp b/ecs2.cpp
new file mode 100644
index 0000000..dfc992b
--- /dev/null
+++ b/ecs2.cpp
@@ -0,0 +1,212 @@
+#include <utility>
+
+#include <stdio.h>
+#include <stdlib.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 */
+
+#define max_entities 256
+/* If you want more than 32 component types then you'll have to
+ * use an unsigned long instead of an int for the component type
+ * IDs. */
+#define max_component_types 32
+
+typedef int Entity;
+
+unsigned get_new_component_id();
+
+/* DO NOT make either of these functions static. That will break stuff. */
+template <typename T>
+unsigned get_component_id() {
+  static unsigned id = get_new_component_id();
+  return id;
+}
+unsigned get_new_component_id() {
+  static unsigned id = 0;
+  return id++;
+}
+
+struct Component_Pool {
+	void* data;
+	int element_size;
+
+	template <typename T>
+	T& get(int idx) {
+		assert(sizeof(T) == element_size);
+		return ((T*)data)[idx];
+	}
+
+	Component_Pool(): data(0) {}
+
+	void init(int es) {
+		element_size = es;
+		data = malloc(element_size * max_entities);
+	}
+
+	void deinit() {
+		free(data);
+	}
+
+	bool isinit() {
+		return data != nullptr;
+	}
+};
+
+struct World {
+	Component_Pool pools[max_component_types];
+	int entity_count;
+
+	int bitset[max_entities];
+
+	World(): entity_count(0) {}
+
+	void deinit() {
+		for (int i = 0; i < max_component_types; i++) {
+			if (pools[i].isinit()) {
+				pools[i].deinit();
+			}
+		}
+	}
+
+	template <typename T>
+	void register_component_type() {
+		pools[get_component_id<T>()].init(sizeof(T));
+	}
+
+	Entity create_entity() {
+		auto e = entity_count++;
+		bitset[e] = 0;
+		return e;
+	}
+
+	template <typename T>
+	T& add_component(Entity e, T&& c) {
+		unsigned id = get_component_id<T>();
+		Component_Pool& pool = pools[id];
+		T& el = pool.get<T>(e);
+		bitset[e] |= (1 << id);
+		el = std::move(c);
+		return el;
+	}
+
+	template <typename T>
+	void remove_component(Entity e) {
+		unsigned id = get_component_id<T>();
+		bitset[e] &= ~(1 << id);
+	}
+
+	template <typename T>
+	T& get_component(Entity e) {
+		unsigned id = get_component_id<T>();
+		Component_Pool& pool = pools[id];
+		return pool.get<T>(e);
+	}
+
+	template <typename Fn>
+	void iterate_with_mask(unsigned mask, Fn fn) {
+		for (int i = 0; i < entity_count; i++) {
+			if (bitset[i] & mask) {
+				fn(i);
+			}
+		}
+	}
+
+	template <typename A, typename Fn>
+	void iterate_with(Fn fn) {
+		unsigned mask_a = 1 << get_component_id<A>();
+		unsigned bits;
+		for (int i = 0; i < entity_count; i++) {
+			bits = bitset[i];
+			if (bits & mask_a) {
+				fn(i);
+			}
+		}
+	}
+
+	/* You need to make an overload for every number of components that you want to
+	 * iterate at once. Just making two for example.
+	 *
+	 * There's probably a modern C++ way to not have to do this, but I don't know it.
+	 * Please tell me if you know. */
+	template <typename A, typename B, typename Fn>
+	void iterate_with(Fn fn) {
+		unsigned mask_a = 1 << get_component_id<A>();
+		unsigned mask_b = 1 << get_component_id<B>();
+		unsigned bits;
+		for (int i = 0; i < entity_count; i++) {
+			bits = bitset[i];
+			if ((bits & mask_a) && (bits & mask_b)) {
+				fn(i);
+			}
+		}
+	}
+};
+
+/* Example */
+struct Transform {
+	int x, y;
+	int scale_x, scale_y;
+};
+
+struct Enemy {
+	int health, damage;
+};
+
+struct Sprite {
+	int id;
+};
+
+void transform_system(World& world) {
+	world.iterate_with<Transform>([&](Entity e) {
+		Transform& t = world.get_component<Transform>(e);
+		printf("Updating transform for %d\n", e);
+		printf("The position is: %d, %d\n", t.x, t.y);
+	});
+}
+
+void enemy_system(World& world) {
+	world.iterate_with<Transform, Enemy>([&](Entity e) {
+		Transform& t = world.get_component<Transform>(e);
+		Enemy& enemy = world.get_component<Enemy>(e);
+
+		printf("Updating enemy for %d\n", e);
+		printf("The health is: %d\n", enemy.health);
+	});
+}
+
+void sprite_system(World& world) {
+	world.iterate_with<Transform, Sprite>([&](Entity e) {
+		Transform& t = world.get_component<Transform>(e);
+		Sprite& sprite = world.get_component<Sprite>(e);
+
+		printf("Updating sprite for %d\n", e);
+		printf("The id is: %d\n", sprite.id);
+	});
+}
+
+int main() {
+	World world;
+	world.register_component_type<Transform>();
+	world.register_component_type<Enemy>();
+	world.register_component_type<Sprite>();
+
+	Entity a = world.create_entity();
+	world.add_component(a, Transform { 25, 25, 100, 100 });
+	world.add_component(a, Enemy { 10, 5 });
+	world.add_component(a, Sprite { 0 });
+
+	Entity b = world.create_entity();
+	world.add_component(b, Transform { 1, 3, 50, 4 });
+	world.add_component(b, Enemy { 5, 20 });
+
+	/* The game loop. */
+	while (true) {
+		enemy_system(world);
+		transform_system(world);
+		sprite_system(world);
+	}
+
+	world.deinit();
+}
-- 
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