src:arduino-timer: fix timer() return value

Calculate the return value for tick() taking into account handler
duration and repeating task expirations.

The old calculation didn't consider the total elapsed time of all
handlers, so the ticks returned could be wrong if their total elapsed
time was greater than the expiration of the next task.

Author: contrem

src/arduino-timer.h

@@ -100,7 +100,8 @@ class Timer {
     unsigned long
     tick()
     {
-        unsigned long ticks = (unsigned long)-1;
+        unsigned long ticks = (unsigned long)-1, elapsed;
+        const unsigned long start = time_func();
 
         for (size_t i = 0; i < max_tasks; ++i) {
             struct task * const task = &tasks[i];
@@ -112,16 +113,23 @@ class Timer {
                 if (duration >= task->expires) {
                     task->repeat = task->handler(task->opaque) && task->repeat;
 
-                    if (task->repeat) task->start = t;
-                    else remove(task);
+                    if (task->repeat) {
+                        task->start = t;
+                        ticks = task->expires < ticks ? task->expires : ticks;
+                    } else remove(task);
                 } else {
                     const unsigned long remaining = task->expires - duration;
                     ticks = remaining < ticks ? remaining : ticks;
                 }
             }
         }
 
-        return ticks == (unsigned long)-1 ? 0 : ticks;
+        elapsed = time_func() - start;
+
+        if (elapsed >= ticks || ticks == (unsigned long)-1) ticks = 0;
+        else ticks -= elapsed;
+
+        return ticks;
     }
 
   private: