Enforce realtime rendering

- Master clock cycles are truncated to nanoseconds, but this should
   be within tolerance of actual hardware implementations

src/nese.c

@@ -1,5 +1,6 @@
 #include <limits.h>
 #include <stdlib.h>
+#include <time.h>
 
 #include "ines.h"
 #include "render.h"
@@ -33,6 +34,23 @@ void e6502_dump_stack(e6502_Core* core, FILE* file) {
 }
 
 
+#define NS_PER_S    (1000U * 1000U * 1000U)
+/*
+static 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);
+}
+*/
+static 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;
+}
+
 extern nes_Renderer sdl_renderer;
 
 static nes sys = {0};
@@ -54,6 +72,10 @@ int main(int argc, char* argv[]) {
         nes_init(&sys);
         nes_reset(&sys);
 
+        struct timespec t_target = {0};
+        clock_gettime(CLOCK_MONOTONIC, &t_target);
+        uint64_t cycle_last_frame = 0;
+
         uint64_t total_cycles = 0;
         int last_frame_rendered = -1;
         for (int i = 0; i < n_loops && status == 0; ++i) {
@@ -68,11 +90,26 @@ int main(int argc, char* argv[]) {
                             us_run, run,
                             status == 0 ? "OK" : "Halted");
 */
-            // TODO: Check VBlank or scanline?
             if (    status == 0 &&
                     sys.ppu.frame != last_frame_rendered) {
+                // TODO: Check VBlank or scanline?
                 status = nes_render(rend, &sys.ppu);
                 last_frame_rendered = sys.ppu.frame;
+
+                // Sleep to catch up to master clock
+                uint64_t elapsed_cycles = total_cycles -
+                                          cycle_last_frame;
+                int elapsed_ns = (  elapsed_cycles *
+                                    nes_clock_master_den *
+                                    NS_PER_S ) /
+                                 nes_clock_master_num;
+
+                t_add_ns(&t_target, &t_target, elapsed_ns);
+
+                clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME,
+                                &t_target, NULL);
+
+                cycle_last_frame = total_cycles;
             }
         }
 

src/sdl_render.c

@@ -366,18 +366,7 @@ static int sdl_render(nes_Renderer* rend, nes_ppu* ppu) {
         SDL_DestroyTexture(texture);
     }
 
-/*
-    // HACK: THIS IS TERRIBLE
-    static uint32_t last_ms = 0;
-    uint32_t now_ms = SDL_GetTicks();
-    if (last_ms != 0) {
-        int delay = (last_ms + (1000 / 35)) - now_ms;
-        if (delay > 0) SDL_Delay(delay);
-//        else fprintf(stderr, "Delta %d\n", now_ms - last_ms);
-    }
-    last_ms = now_ms;
-*/
-
+    // TODO: Handle this in the input loop, or anywhere else
     SDL_Event event = {0};
     return (1 == SDL_PollEvent(&event) && event.type == SDL_QUIT) ?
            -1 : 0;