From 5458f3db7b08bf06c1f5c99a5851f6b270126b92 Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Sun, 5 Apr 2026 14:48:01 -0700
Subject: Initial simloop module

---
 simloop/src/simloop.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 simloop/src/simloop.c

(limited to 'simloop/src')

diff --git a/simloop/src/simloop.c b/simloop/src/simloop.c
new file mode 100644
index 0000000..11f4d6d
--- /dev/null
+++ b/simloop/src/simloop.c
@@ -0,0 +1,68 @@
+#include <simloop.h>
+
+#include <assert.h>
+
+static uint64_t ddt_from_fps(int fps) {
+  static constexpr double NANOSECONDS = 1e9;
+  return (fps == 0) ? 0 : (uint64_t)(NANOSECONDS / (double)fps);
+}
+
+Simloop simloop_make(const SimloopArgs* args) {
+  assert(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,
+  };
+}
+
+static time_delta time_elapsed(const Simloop* sim, time_point t) {
+  assert(sim);
+  return time_diff(sim->timer->start_time, t);
+}
+
+static int 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;
+}
+
+static bool step_render(const Simloop* sim, SimloopTimeline* timeline) {
+  assert(sim);
+  assert(timeline);
+
+  bool render = false;
+  if (timeline->ddt > 0) {
+    render = step_update(sim, timeline) > 0;
+  } else {
+    timeline->last_step = sim->timer->last_tick;
+    render              = true;
+  }
+  return render;
+}
+
+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_render =
+      step_render(sim, &sim->render) ||
+      (sim->frame == 0); // Trigger an initial render on the first frame.
+  sim->frame += new_frames;
+  out->frame          = sim->frame;
+  out->render_elapsed = time_elapsed(sim, sim->render.last_step);
+  out->update_elapsed = time_elapsed(sim, sim->update.last_step);
+  out->update_dt      = sim->update.ddt;
+}
-- 
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