From 879e5af4eb1a8972fc944853a515e1003f94bd7c Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Sat, 11 Apr 2026 12:27:23 -0700
Subject: Fix simloop divergence

---
 simloop/src/simloop.c | 32 +++++++++++++++++---------------
 1 file changed, 17 insertions(+), 15 deletions(-)

(limited to 'simloop/src/simloop.c')

diff --git a/simloop/src/simloop.c b/simloop/src/simloop.c
index 11f4d6d..aa2b6b7 100644
--- a/simloop/src/simloop.c
+++ b/simloop/src/simloop.c
@@ -12,12 +12,13 @@ Simloop simloop_make(const SimloopArgs* args) {
   assert(args->update_fps > 0);
 
   return (Simloop){
-      .frame  = 0,
-      .update = (SimloopTimeline){.ddt       = ddt_from_fps(args->update_fps),
-                                  .last_step = args->timer->start_time},
-      .render = (SimloopTimeline){  .ddt = ddt_from_fps(args->max_render_fps),
-                                  .last_step = args->timer->start_time},
-      .timer  = args->timer,
+      .frame      = 0,
+      .update     = (SimloopTimeline){.ddt       = ddt_from_fps(args->update_fps),
+                                      .last_step = args->timer->start_time},
+      .render     = (SimloopTimeline){  .ddt = ddt_from_fps(args->max_render_fps),
+                                      .last_step = args->timer->start_time},
+      .timer      = args->timer,
+      .first_iter = true,
   };
 }
 
@@ -26,15 +27,16 @@ static time_delta time_elapsed(const Simloop* sim, time_point t) {
   return time_diff(sim->timer->start_time, t);
 }
 
-static int step_update(const Simloop* sim, SimloopTimeline* timeline) {
+static bool step_update(const Simloop* sim, SimloopTimeline* timeline) {
   assert(sim);
   assert(timeline);
   assert(timeline->ddt > 0);
 
   const time_delta dt = time_diff(timeline->last_step, sim->timer->last_tick);
-  const time_delta steps = dt / timeline->ddt;
-  timeline->last_step    = time_add(timeline->last_step, dt);
-  return (int)steps;
+  const bool       should_step = dt >= timeline->ddt;
+  timeline->last_step =
+      should_step ? time_add(timeline->last_step, dt) : timeline->last_step;
+  return should_step;
 }
 
 static bool step_render(const Simloop* sim, SimloopTimeline* timeline) {
@@ -43,7 +45,7 @@ static bool step_render(const Simloop* sim, SimloopTimeline* timeline) {
 
   bool render = false;
   if (timeline->ddt > 0) {
-    render = step_update(sim, timeline) > 0;
+    render = step_update(sim, timeline);
   } else {
     timeline->last_step = sim->timer->last_tick;
     render              = true;
@@ -55,12 +57,12 @@ void simloop_update(Simloop* sim, SimloopOut* out) {
   assert(sim);
   assert(out);
 
-  const int new_frames = step_update(sim, &sim->update);
-  out->updates_pending = new_frames;
+  out->should_update = step_update(sim, &sim->update);
   out->should_render =
       step_render(sim, &sim->render) ||
-      (sim->frame == 0); // Trigger an initial render on the first frame.
-  sim->frame += new_frames;
+      (sim->first_iter); // Trigger an initial render on the first frame.
+  sim->frame += (out->should_update ? 1 : 0);
+  sim->first_iter     = false;
   out->frame          = sim->frame;
   out->render_elapsed = time_elapsed(sim, sim->render.last_step);
   out->update_elapsed = time_elapsed(sim, sim->update.last_step);
-- 
cgit v1.2.3