From e1f3765cae319e313fd86e83e05e1654fbb37dc4 Mon Sep 17 00:00:00 2001
From: Nathaniel Walizer <robots.are.love@gmail.com>
Date: Sat, 11 Jan 2025 15:23:17 -0800
Subject: [PATCH] Simplify timers and automagically correct for time jumps

---
 src/nese.c      | 17 +++++++++++------
 src/sdl_timer.c | 26 +++++++++++++-------------
 src/timer.h     | 23 ++++++-----------------
 3 files changed, 30 insertions(+), 36 deletions(-)

diff --git a/src/nese.c b/src/nese.c
index d221352..a19c749 100644
--- a/src/nese.c
+++ b/src/nese.c
@@ -74,8 +74,7 @@ int main(int argc, char* argv[]) {
 
         nes_render(rend, &sys.ppu);
 
-        struct timespec t_target = {0};
-        time_get(&t_target);
+        time_us t_target = time_now();
         uint64_t cycle_last_frame = 0;
 
         uint64_t total_cycles = 0;
@@ -92,14 +91,20 @@ int main(int argc, char* argv[]) {
                     // Sleep to catch up to master clock
                     uint64_t elapsed_cycles = total_cycles -
                                               cycle_last_frame;
-                    int elapsed_ns = (  elapsed_cycles *
+                    int elapsed_us = (  elapsed_cycles *
                                         nes_clock_master_den *
-                                        NS_PER_S ) /
+                                        US_PER_S ) /
                                      nes_clock_master_num;
 
-                    t_add_ns(&t_target, &t_target, elapsed_ns);
+                    t_target += elapsed_us;
 
-                    time_sleep_until(&t_target);
+                    time_us slept_us = time_sleep_until(t_target);
+                    (void)slept_us;
+
+                    if (slept_us <= -elapsed_us) {
+                        // We're way out of sync.
+                        t_target = time_now();
+                    }
 
                     cycle_last_frame = total_cycles;
 
diff --git a/src/sdl_timer.c b/src/sdl_timer.c
index b444882..eca8aa4 100644
--- a/src/sdl_timer.c
+++ b/src/sdl_timer.c
@@ -1,21 +1,21 @@
-#include <time.h>
-
-#include "time.h"
+#include "timer.h"
 
 #include <SDL2/SDL.h>
 
 
-void time_get(struct timespec* ts) {
-    uint64_t time = SDL_GetTicks64();
-    ts->tv_sec = time / 1000U;
-    ts->tv_nsec = (time % 1000U) * 1000U * 1000U;
+time_us time_now(void) {
+    return (SDL_GetTicks64() * 1000U);
+}
+
+void time_sleep(time_us sleep_us) {
+    SDL_Delay(sleep_us / 1000U);
 }
 
-void time_sleep_until(const struct timespec* ts) {
-    uint64_t t_target = (   (ts->tv_sec * 1000ULL) +
-                            (ts->tv_nsec / (1000U * 1000U)));
-    uint64_t t_now = SDL_GetTicks64();
-    if (t_target > t_now) {
-        SDL_Delay(t_target - t_now);
+time_us time_sleep_until(time_us t_target) {
+    time_us t_now = time_now();
+    time_us t_diff = t_target - t_now;
+    if (t_diff > 0) {
+        time_sleep(t_diff);
     }
+    return t_diff;
 }
diff --git a/src/timer.h b/src/timer.h
index 81669c5..ebf23b4 100644
--- a/src/timer.h
+++ b/src/timer.h
@@ -1,31 +1,20 @@
 #ifndef NESE_TIMER_H_
 #define NESE_TIMER_H_
 
-#include <time.h>
+#include <stdint.h>
 
 
-#define NS_PER_S    (1000U * 1000U * 1000U)
+#define US_PER_S    (1000U * 1000U)
 
 
-static inline int t_diff_ns(const struct timespec* b,
-                            const struct timespec* a) {
-    int sec = (b->tv_sec - a->tv_sec);
-    int nsec = (b->tv_nsec - a->tv_nsec);
-    return ((sec * NS_PER_S) + nsec);
-}
+typedef int64_t time_us;
 
-static inline void t_add_ns(struct timespec* s,
-                            const struct timespec* a,
-                     int b) {
-    int nsec = a->tv_nsec + b;
-    s->tv_sec = a->tv_sec + (nsec / NS_PER_S);
-    s->tv_nsec = nsec % NS_PER_S;
-}
 
+time_us time_now(void);
 
-void time_get(struct timespec*);
+void time_sleep(time_us);
 
-void time_sleep_until(const struct timespec*);
+time_us time_sleep_until(time_us);
 
 
 #endif // NESE_TIME_H_