From c2b831a4d68f1fbc60a78f964a7e1a08839190c2 Mon Sep 17 00:00:00 2001
From: sergeypdev <sergeypoznyak@protonmail.com>
Date: Sun, 9 Feb 2025 19:54:44 +0400
Subject: [PATCH] Implement simulation rewind

---
 game/game.odin          |  8 ++++++++
 game/physics/debug.odin |  2 +-
 game/physics/scene.odin | 20 +++++++++++++++-----
 3 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/game/game.odin b/game/game.odin
index d326946..a226e92 100644
--- a/game/game.odin
+++ b/game/game.odin
@@ -58,6 +58,7 @@ Runtime_World :: struct {
 	camera:                 rl.Camera3D,
 	orbit_camera:           Orbit_Camera,
 	dt:                     f32,
+	rewind_simulation: bool,
 	step_simulation:        bool,
 	single_step_simulation: bool,
 }
@@ -420,6 +421,7 @@ update_runtime_world :: proc(runtime_world: ^Runtime_World, dt: f32) {
 
 		runtime_world.dt = dt
 		should_single_step := rl.IsKeyPressed(.PERIOD)
+		runtime_world.rewind_simulation = rl.IsKeyPressed(.COMMA)
 		runtime_world.step_simulation = !g_mem.physics_pause || should_single_step
 		runtime_world.single_step_simulation = should_single_step
 	}
@@ -629,6 +631,9 @@ draw :: proc() {
 			car_model.transform = car_matrix
 
 			if !runtime_world.pause {
+				if runtime_world.rewind_simulation {
+					world.physics_scene.simulation_state_index = physics.get_prev_sim_state_index(&world.physics_scene)
+				} else {
 				physics.simulate(
 					&world.physics_scene,
 					&runtime_world.solver_state,
@@ -637,6 +642,7 @@ draw :: proc() {
 					commit = runtime_world.step_simulation,
 					step_mode = g_mem.physics_pause ? physics.Step_Mode.Single : physics.Step_Mode.Accumulated_Time,
 				)
+				}
 			}
 
 			// rl.DrawModel(car_model, physics.body_get_shape_pos(car_body), 1, rl.WHITE)
@@ -944,6 +950,8 @@ game_init :: proc() {
 
 	g_mem^ = Game_Memory{}
 
+	physics.init_physics_scene(&g_mem.runtime_world.world.physics_scene, 100)
+
 	game_hot_reloaded(g_mem)
 }
 
diff --git a/game/physics/debug.odin b/game/physics/debug.odin
index ac16f22..21c3178 100644
--- a/game/physics/debug.odin
+++ b/game/physics/debug.odin
@@ -40,7 +40,7 @@ draw_debug_shape :: proc(
 draw_debug_scene :: proc(scene: ^Scene) {
 	tracy.Zone()
 
-	sim_state := get_next_sim_state(scene)
+	sim_state := get_sim_state(scene)
 
 	for _, i in sim_state.bodies {
 		body := &sim_state.bodies_slice[i]
diff --git a/game/physics/scene.odin b/game/physics/scene.odin
index 5fdebaf..f81cdb8 100644
--- a/game/physics/scene.odin
+++ b/game/physics/scene.odin
@@ -25,10 +25,14 @@ Sim_State :: struct {
 }
 
 Scene :: struct {
-	simulation_states:      [2]Sim_State,
+	simulation_states:      []Sim_State,
 	simulation_state_index: i32,
 }
 
+init_physics_scene :: proc(scene: ^Scene, max_history := int(2)) {
+	scene.simulation_states = make([]Sim_State, max(max_history, 2))
+}
+
 Body :: struct {
 	// Is this body alive (if not it doesn't exist)
 	alive:              bool,
@@ -132,19 +136,24 @@ _invalid_body_slice := _invalid_body[:]
 _invalid_suspension_constraint: #soa[1]Suspension_Constraint
 _invalid_suspension_constraint_slice := _invalid_suspension_constraint[:]
 
+get_prev_sim_state_index :: proc(scene: ^Scene) -> i32 {
+	return (scene.simulation_state_index - 1) %% i32(len(scene.simulation_states))
+}
+
 get_sim_state :: proc(scene: ^Scene) -> ^Sim_State {
 	return &scene.simulation_states[scene.simulation_state_index]
 }
 
 get_prev_sim_state :: proc(scene: ^Scene) -> ^Sim_State {
-	return &scene.simulation_states[(scene.simulation_state_index + 1) % 2]
+	return &scene.simulation_states[get_prev_sim_state_index(scene)]
 }
 
-// lol
-get_next_sim_state :: get_prev_sim_state
+get_next_sim_state :: proc(scene: ^Scene) -> ^Sim_State {
+	return &scene.simulation_states[(scene.simulation_state_index + 1) %% i32(len(scene.simulation_states))]
+}
 
 flip_sim_state :: proc(scene: ^Scene) {
-	scene.simulation_state_index = (scene.simulation_state_index + 1) % 2
+	scene.simulation_state_index = (scene.simulation_state_index + 1) %% i32(len(scene.simulation_states))
 }
 
 /// Returns pointer to soa slice. NEVER STORE IT
@@ -361,4 +370,5 @@ destroy_physics_scene :: proc(scene: ^Scene) {
 	for &sim_state in scene.simulation_states {
 		destry_sim_state(&sim_state)
 	}
+	delete(scene.simulation_states)
 }