Try to run at a more consistent frame rate

Before we were taking however long the simulation took, and then waiting
an additional 0.1s between each frame. Now we only wait the difference
between the simulation time and 0.1s, so we should get a consistent
10fps on machines that can keep up.

main.cxx

@@ -6,6 +6,7 @@
 #include <errno.h>
 #include <unistd.h>
 #include <math.h>
+#include <time.h>
 
 #include <algorithm>
 #include <random>
@@ -1148,6 +1149,35 @@ void handle_window_size_changed(int signal) {
   u_clear_screen();
 }
 
+timespec subtract(const timespec& lhs, const timespec& rhs) {
+  timespec diff;
+  diff.tv_sec = lhs.tv_sec - rhs.tv_sec;
+  diff.tv_nsec = lhs.tv_nsec - rhs.tv_nsec;
+  if (lhs.tv_nsec < rhs.tv_nsec) {
+    diff.tv_sec -= 1;
+    diff.tv_nsec += 1e9;
+  }
+  return diff;
+}
+
+// wait for up to one second from the given start time
+// returns the current time when it exits
+timespec wait(long desired_interval_nsec, timespec start) {
+  timespec now;
+  clock_gettime(CLOCK_MONOTONIC, &now);
+
+  timespec diff = subtract(now, start);
+  if (diff.tv_sec == 0) {
+    long wait_for = (desired_interval_nsec - diff.tv_nsec) / 1000;
+    if (wait_for > 0) {
+      usleep(wait_for);
+      clock_gettime(CLOCK_MONOTONIC, &now);
+    }
+  }
+
+  return now;
+}
+
 int main(int argc, char** argv) {
   args_t in = parse_args(argc, argv);
   g_rainbow_enabled = in.rainbow;
@@ -1162,8 +1192,10 @@ int main(int argc, char** argv) {
   sim_init(in);
   draw(&buf);
 
+  timespec interval_start;
   while (process_keypress()) {
     sim_step();
+    interval_start = wait(1e8, interval_start);
     draw(&buf);
   }
 

misc/terminal.cxx

@@ -66,7 +66,7 @@ void enable_raw_mode() {
   raw.c_cflag |= (CS8);
   raw.c_lflag &= ~(ECHO | ICANON | IEXTEN | ISIG);
   raw.c_cc[VMIN] = 0;
-  raw.c_cc[VTIME] = 1; // tenths of a second
+  raw.c_cc[VTIME] = 0; // tenths of a second
   if (tcsetattr(STDIN_FILENO, TCSAFLUSH, &raw) == -1) {
     die("failed to enable raw mode");
   }