Improve framerate counting

The FPS counter was called only on new frames, so it could not print
values regularly, especially when there are very few FPS (when the
device surface does not change).

To the extreme, it was never able to display 0 fps.

Add a separate thread to print framerate every second.

app/src/fps_counter.c

@@ -1,60 +1,169 @@
 #include "fps_counter.h"
 
+#include <SDL2/SDL_assert.h>
 #include <SDL2/SDL_timer.h>
 
+#include "lock_util.h"
 #include "log.h"
 
-void
+#define FPS_COUNTER_INTERVAL_MS 1000
+
+bool
 fps_counter_init(struct fps_counter *counter) {
-    counter->started = false;
-    // no need to initialize the other fields, they are meaningful only when
-    // started is true
+    counter->mutex = SDL_CreateMutex();
+    if (!counter->mutex) {
+        return false;
+    }
+
+    counter->state_cond = SDL_CreateCond();
+    if (!counter->state_cond) {
+        SDL_DestroyMutex(counter->mutex);
+        return false;
+    }
+
+    counter->thread = NULL;
+    SDL_AtomicSet(&counter->started, 0);
+    // no need to initialize the other fields, they are unused until started
+
+    return true;
 }
 
 void
-fps_counter_start(struct fps_counter *counter) {
-    counter->started = true;
-    counter->slice_start = SDL_GetTicks();
+fps_counter_destroy(struct fps_counter *counter) {
+    SDL_DestroyCond(counter->state_cond);
+    SDL_DestroyMutex(counter->mutex);
+}
+
+// must be called with mutex locked
+static void
+display_fps(struct fps_counter *counter) {
+    unsigned rendered_per_second =
+        counter->nr_rendered * 1000 / FPS_COUNTER_INTERVAL_MS;
+    if (counter->nr_skipped) {
+        LOGI("%u fps (+%u frames skipped)", rendered_per_second,
+                                            counter->nr_skipped);
+    } else {
+        LOGI("%u fps", rendered_per_second);
+    }
+}
+
+// must be called with mutex locked
+static void
+check_interval_expired(struct fps_counter *counter, uint32_t now) {
+    if (now < counter->next_timestamp) {
+        return;
+    }
+
+    display_fps(counter);
     counter->nr_rendered = 0;
     counter->nr_skipped = 0;
+    // add a multiple of the interval
+    uint32_t elapsed_slices =
+        (now - counter->next_timestamp) / FPS_COUNTER_INTERVAL_MS + 1;
+    counter->next_timestamp += FPS_COUNTER_INTERVAL_MS * elapsed_slices;
+}
+
+static int
+run_fps_counter(void *data) {
+    struct fps_counter *counter = data;
+
+    mutex_lock(counter->mutex);
+    while (!counter->interrupted) {
+        while (!counter->interrupted && !SDL_AtomicGet(&counter->started)) {
+            cond_wait(counter->state_cond, counter->mutex);
+        }
+        while (!counter->interrupted && SDL_AtomicGet(&counter->started)) {
+            uint32_t now = SDL_GetTicks();
+            check_interval_expired(counter, now);
+
+            SDL_assert(counter->next_timestamp > now);
+            uint32_t remaining = counter->next_timestamp - now;
+
+            // ignore the reason (timeout or signaled), we just loop anyway
+            cond_wait_timeout(counter->state_cond, counter->mutex, remaining);
+        }
+    }
+    mutex_unlock(counter->mutex);
+    return 0;
+}
+
+bool
+fps_counter_start(struct fps_counter *counter) {
+    mutex_lock(counter->mutex);
+    counter->next_timestamp = SDL_GetTicks() + FPS_COUNTER_INTERVAL_MS;
+    counter->nr_rendered = 0;
+    counter->nr_skipped = 0;
+    mutex_unlock(counter->mutex);
+
+    SDL_AtomicSet(&counter->started, 1);
+    cond_signal(counter->state_cond);
+
+    // counter->thread is always accessed from the same thread, no need to lock
+    if (!counter->thread) {
+        counter->thread =
+            SDL_CreateThread(run_fps_counter, "fps counter", counter);
+        if (!counter->thread) {
+            LOGE("Could not start FPS counter thread");
+            return false;
+        }
+    }
+
+    return true;
 }
 
 void
 fps_counter_stop(struct fps_counter *counter) {
-    counter->started = false;
+    SDL_AtomicSet(&counter->started, 0);
+    cond_signal(counter->state_cond);
 }
 
-static void
-display_fps(struct fps_counter *counter) {
-    if (counter->nr_skipped) {
-        LOGI("%d fps (+%d frames skipped)", counter->nr_rendered,
-                                            counter->nr_skipped);
-    } else {
-        LOGI("%d fps", counter->nr_rendered);
+bool
+fps_counter_is_started(struct fps_counter *counter) {
+    return SDL_AtomicGet(&counter->started);
+}
+
+void
+fps_counter_interrupt(struct fps_counter *counter) {
+    if (!counter->thread) {
+        return;
     }
+
+    mutex_lock(counter->mutex);
+    counter->interrupted = true;
+    mutex_unlock(counter->mutex);
+    // wake up blocking wait
+    cond_signal(counter->state_cond);
 }
 
-static void
-check_expired(struct fps_counter *counter) {
-    uint32_t now = SDL_GetTicks();
-    if (now - counter->slice_start >= 1000) {
-        display_fps(counter);
-        // add a multiple of one second
-        uint32_t elapsed_slices = (now - counter->slice_start) / 1000;
-        counter->slice_start += 1000 * elapsed_slices;
-        counter->nr_rendered = 0;
-        counter->nr_skipped = 0;
+void
+fps_counter_join(struct fps_counter *counter) {
+    if (counter->thread) {
+        SDL_WaitThread(counter->thread, NULL);
     }
 }
 
 void
 fps_counter_add_rendered_frame(struct fps_counter *counter) {
-    check_expired(counter);
+    if (!SDL_AtomicGet(&counter->started)) {
+        return;
+    }
+
+    mutex_lock(counter->mutex);
+    uint32_t now = SDL_GetTicks();
+    check_interval_expired(counter, now);
     ++counter->nr_rendered;
+    mutex_unlock(counter->mutex);
 }
 
 void
 fps_counter_add_skipped_frame(struct fps_counter *counter) {
-    check_expired(counter);
+    if (!SDL_AtomicGet(&counter->started)) {
+        return;
+    }
+
+    mutex_lock(counter->mutex);
+    uint32_t now = SDL_GetTicks();
+    check_interval_expired(counter, now);
     ++counter->nr_skipped;
+    mutex_unlock(counter->mutex);
 }