Move all area logic to Piano privates, to simplify drawing logic.

Logic outside of Piano shouldn't have to care about the relationship
between key-index odd/even and big-vs-small-ness of the keys.
We're not completely there yet, due to the z-index draw ordering,
but it's definitely an improvement.

- Style: convert snake_case names to proper camelCase, where touched.

app/src/main/java/com/nicobrailo/pianoli/Piano.java

@@ -165,32 +165,43 @@ int pos_to_key_idx(float pos_x, float pos_y) {
         if (pos_y > keys_flats_height) return big_key_idx;
 
         // Check if press is inside rect of flat key
-        Key flat = get_area_for_flat_key(big_key_idx);
+        Key flat = getAreaForSmallKey(big_key_idx + 1);
         if (flat.contains(pos_x, pos_y)) return big_key_idx + 1;
 
         if (big_key_idx > 0) {
-            Key prev_flat = get_area_for_flat_key(big_key_idx - 2);
+            Key prev_flat = getAreaForSmallKey(big_key_idx - 1);
             if (prev_flat.contains(pos_x, pos_y)) return big_key_idx - 1;
         }
 
         // If not in the current or previous flat, it must be a hit in the big key
         return big_key_idx;
     }
 
-    Key get_area_for_key(int key_idx) {
-        int x_i = key_idx / 2 * keys_width;
+    @NonNull
+    Key getAreaForKey(int keyIdx) {
+        if ((keyIdx & 1) == 0) { // even positions are the full, big keys
+            return getAreaForBigKey(keyIdx);
+        } else {
+            return getAreaForSmallKey(keyIdx); // odd positions are the small/black/flat keys.
+        }
+    }
+
+    @NonNull
+    private Key getAreaForBigKey(int keyIdx) {
+        int x_i = keyIdx / 2 * keys_width;
         return new Key(x_i, x_i + keys_width, 0, keys_height);
     }
 
-    Key get_area_for_flat_key(int key_idx) {
-        final int octave_idx = (key_idx / 2) % 7;
-        if (octave_idx == 2 || octave_idx == 6) {
+    @NonNull
+    private Key getAreaForSmallKey(int keyIdx) {
+        final int octaveIdx = (keyIdx / 2) % 7;
+        if (octaveIdx == 2 || octaveIdx == 6) {
             // Keys without flat get a null-area
             return Key.CANT_TOUCH_THIS;
         }
 
         final int offset = keys_width - (keys_flat_width / 2);
-        int x_i = (key_idx / 2) * keys_width + offset;
+        int x_i = (keyIdx / 2) * keys_width + offset;
         return new Key(x_i, x_i + keys_flat_width, 0, keys_flats_height);
     }
 }

app/src/main/java/com/nicobrailo/pianoli/PianoCanvas.java

@@ -144,20 +144,6 @@ private static void resetCanvas(Canvas canvas) {
         canvas.drawPaint(p);
     }
 
-    private void drawBigKeys(Canvas canvas) {
-        for (int i = 0; i < piano.get_keys_count(); i += 2) {
-            draw_key(canvas, piano.get_area_for_key(i), i);
-        }
-    }
-
-    private void drawSmallKeys(Canvas canvas) {
-        for (int i = 1; i < piano.get_keys_count(); i += 2) {
-            if (piano.get_area_for_flat_key(i) != Key.CANT_TOUCH_THIS) {
-                draw_key(canvas, piano.get_area_for_flat_key(i), i);
-            }
-        }
-    }
-
     /**
      * Overlays gear icons onto the currently-held and next expected flat keys.
      */
@@ -173,7 +159,12 @@ void drawConfigGears(Canvas androidCanvas) {
         draw_icon_on_black_key(androidCanvas, gearIcon, appConfigTrigger.getNextExpectedKey(), normalSize, normalSize);
     }
 
-    void draw_key(final Canvas canvas, final Key rect, int i) {
+    void drawKey(final Canvas canvas, final int i) {
+        Key rect = piano.getAreaForKey(i);
+        if (rect == Key.CANT_TOUCH_THIS) {
+            return; // don't waste performance drawing the skipped black keys.
+        }
+
         Paint p = new Paint();
         p.setColor(theme.getColorForKey(i, piano.is_key_pressed(i)));
 
@@ -253,7 +244,7 @@ void draw_key(final Canvas canvas, final Key rect, int i) {
      */
     void draw_icon_on_black_key(final Canvas canvas, final Drawable icon, int key_idx,
                                 final int icon_width, final int icon_height) {
-        final Key key = piano.get_area_for_flat_key(key_idx);
+        final Key key = piano.getAreaForKey(key_idx);
         int icon_x = ((key.x_f - key.x_i) / 2) + key.x_i;
         int icon_y = icon_height;
 
@@ -285,9 +276,17 @@ public void redraw(SurfaceHolder surfaceHolder) {
         if (canvas == null) return;
 
         resetCanvas(canvas);
-        drawBigKeys(canvas);
-        // Small keys drawn after big keys to ensure z-index
-        drawSmallKeys(canvas);
+
+        // draw main, big keys (even key index)
+        for (int i = 0; i < piano.get_keys_count(); i += 2) {
+            drawKey(canvas, i);
+        }
+
+        // Small keys drawn after big keys to ensure z-index (odd key index)
+        for (int i = 1; i < piano.get_keys_count(); i += 2) {
+            drawKey(canvas, i);
+        }
+
         // Gear icons drawn after small keys, since they go on top of those.
         drawConfigGears(canvas);