Merge pull request #5 from gedankenexperimenter/bug/out-of-bounds

Fix for all out of bounds LED addressing

src/Kaleidoscope-LEDEffect-Chase.cpp

@@ -18,19 +18,45 @@
 
 namespace kaleidoscope {
 void LEDChaseEffect::update(void) {
+  // Check to see if it's time to change the positions of the red and blue lights
   if (current_chase_counter++ < chase_threshold) {
     return;
   }
   current_chase_counter = 0;
-  ::LEDControl.setCrgbAt(pos - (chase_sign * chase_pixels), {0, 0, 0});
+
+  // The red LED is at `pos`; the blue one follows behind. `chase_sign` is either +1 or
+  // -1; `chase_pixels` is the gap between them.
+  byte pos2 = pos - (chase_sign * chase_pixels);
+
+  // First, we turn off the LEDs that were turned on in the previous update. `pos` is
+  // always in the valid range (0 <= pos < LED_COUNT), but after it changes direction, for
+  // the first few updates, `pos2` will be out of bounds. Since it's an unsigned integer,
+  // even when it would have a value below zero, it underflows and so one test is good for
+  // both ends of the range.
   ::LEDControl.setCrgbAt(pos, {0, 0, 0});
+  if (pos2 < LED_COUNT)
+    ::LEDControl.setCrgbAt(pos2, {0, 0, 0});
 
+  // Next, we adjust the red light's position. If the direction hasn't changed (the red
+  // light isn't out of bounds), we also adjust the blue light's position to match the red
+  // one. If the new position puts it out of bounds, we reverse the direction, and bring
+  // it back in bounds. When this happens, the blue light "jumps" behind the red one, and
+  // will be out of bounds. The simplest way to do this is to assign it a value that is
+  // known to be invalid (LED_COUNT).
   pos += chase_sign;
-  if (pos >= (LED_COUNT - 1) || pos <= 0) {
+  if (pos < LED_COUNT) {
+    pos2 += chase_sign;
+  } else {
     chase_sign = -chase_sign;
+    pos += chase_sign;
+    pos2 = LED_COUNT;
   }
+
+  // Last, we turn on the LEDs at their new positions. As before, the blue light (pos2) is
+  // only set if it's in the valid LED range.
   ::LEDControl.setCrgbAt(pos, {0, 0, 255});
-  ::LEDControl.setCrgbAt(pos - (chase_sign * chase_pixels), {255, 0, 0});
+  if (pos2 < LED_COUNT)
+    ::LEDControl.setCrgbAt(pos2, {255, 0, 0});
 }
 }