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/test/simloop_test.c | 72 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 52 insertions(+), 20 deletions(-)

(limited to 'simloop/test')

diff --git a/simloop/test/simloop_test.c b/simloop/test/simloop_test.c
index 3f2aa46..c79ee32 100644
--- a/simloop/test/simloop_test.c
+++ b/simloop/test/simloop_test.c
@@ -3,57 +3,89 @@
 #include <test.h>
 #include <timer.h>
 
-/// An initial render should always trigger on frame 0.
+/// At time/frame 0:
+///   1. An initial render is always triggered.
+///   2. No update is triggered (not enough time passed).
 TEST_CASE(simloop_initial_render) {
-  constexpr int UPDATE_FPS = 10;
+  Timer   timer   = {};
+  Simloop simloop = simloop_make(
+      &(SimloopArgs){.update_fps = 10, .max_render_fps = 0, .timer = &timer});
+  SimloopOut simout;
 
-  Timer      timer   = {};
-  Simloop    simloop = simloop_make(&(SimloopArgs){
-         .update_fps = UPDATE_FPS, .max_render_fps = 0, .timer = &timer});
+  simloop_update(&simloop, &simout);
+
+  TEST_TRUE(simout.should_render);
+  TEST_TRUE(!simout.should_update);
+  TEST_EQUAL(simout.frame, 0);
+}
+
+/// The initial render is not re-triggered if there is a render frame rate cap
+/// and time does not advance.
+TEST_CASE(simloop_initial_render_not_retriggered) {
+  Timer   timer   = {};
+  Simloop simloop = simloop_make(
+      &(SimloopArgs){.update_fps = 10, .max_render_fps = 10, .timer = &timer});
   SimloopOut simout;
 
   simloop_update(&simloop, &simout);
+
   TEST_TRUE(simout.should_render);
+  TEST_TRUE(!simout.should_update);
+  TEST_EQUAL(simout.frame, 0);
+
+  for (int i = 0; i < 10; i++) {
+    // Note that time does not advance.
+    simloop_update(&simloop, &simout);
+    TEST_TRUE(!simout.should_render);
+    TEST_TRUE(!simout.should_update);
+    TEST_EQUAL(simout.frame, 0);
+  }
 }
 
-/// A simulation loop with no render frame cap.
-TEST_CASE(simloop_test_no_render_frame_cap) {
-  constexpr int    UPDATE_FPS   = 10;
-  const time_delta STEP         = sec_to_time_delta(1);
-  const time_delta SIM_TIME_SEC = sec_to_time_delta(30);
+/// A simulation loop with no render frame cap:
+///   1. Updates based on the desired update frame rate.
+///   2. Renders at every loop.
+TEST_CASE(simloop_no_render_frame_cap) {
+  constexpr int    UPDATE_FPS    = 10;
+  const time_delta EXPECT_UPDATE = sec_to_time_delta(1.0 / (double)UPDATE_FPS);
+  const time_delta STEP          = sec_to_time_delta(1);
+  const time_delta SIM_TIME_SEC  = sec_to_time_delta(30);
 
   Timer      timer   = {};
   Simloop    simloop = simloop_make(&(SimloopArgs){
          .update_fps = UPDATE_FPS, .max_render_fps = 0, .timer = &timer});
   SimloopOut simout;
 
-  for (time_delta t = STEP; t < SIM_TIME_SEC; t += STEP) {
+  for (time_delta t = 0; t < SIM_TIME_SEC; t += STEP) {
     timer_advance(&timer, t);
     simloop_update(&simloop, &simout);
     TEST_TRUE(simout.should_render);
-    TEST_EQUAL(simout.updates_pending, UPDATE_FPS);
+    TEST_EQUAL((t > 0) && ((t % EXPECT_UPDATE) == 0), simout.should_update);
   }
 }
 
-/// A simulation loop with a render frame cap.
-TEST_CASE(simloop_test_with_render_frame_cap) {
+/// A simulation loop with a render frame cap:
+///   1. Updates based on the desired update frame rate.
+///   2. Renders based on the desired render frame rate.
+TEST_CASE(simloop_with_render_frame_cap) {
   constexpr int    UPDATE_FPS    = 10;
   constexpr int    RENDER_FPS    = 5;
-  const time_delta STEP          = sec_to_time_delta(0.1);
-  const time_delta SIM_TIME_SEC  = sec_to_time_delta(30);
   const time_delta EXPECT_UPDATE = sec_to_time_delta(1.0 / (double)UPDATE_FPS);
   const time_delta EXPECT_RENDER = sec_to_time_delta(1.0 / (double)RENDER_FPS);
+  const time_delta STEP          = sec_to_time_delta(0.1);
+  const time_delta SIM_TIME_SEC  = sec_to_time_delta(30);
 
   Timer      timer   = {};
   Simloop    simloop = simloop_make(&(SimloopArgs){
-         .update_fps = UPDATE_FPS, .max_render_fps = 0, .timer = &timer});
+         .update_fps = UPDATE_FPS, .max_render_fps = RENDER_FPS, .timer = &timer});
   SimloopOut simout;
 
-  for (time_delta t = STEP; t < SIM_TIME_SEC; t += STEP) {
+  for (time_delta t = 0; t < SIM_TIME_SEC; t += STEP) {
     timer_advance(&timer, t);
     simloop_update(&simloop, &simout);
-    TEST_TRUE(((STEP % EXPECT_RENDER) == 0) ? simout.should_render : true);
-    TEST_TRUE(((STEP % EXPECT_UPDATE) == 0) ? simout.updates_pending : true);
+    // Also expecting initial render at t=0.
+    TEST_EQUAL((t % EXPECT_RENDER) == 0, simout.should_render);
+    TEST_EQUAL((t > 0) && ((t % EXPECT_UPDATE) == 0), simout.should_update);
   }
 }
 
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