Fix key press colour change

Old alpha-based implementation is dumb
because it depends on the activity background colour.
If the keyboard is being used in a dark mode activity
where the background is black like the keys,
changing the key fill alpha will have no effect.
---
Re-implementation lightens dark-coloured keys
and darkens light-coloured keys.

Author: yawnoc

CHANGELOG.md

@@ -4,6 +4,7 @@
 ## [Unreleased]
 
 - Fixed NullPointerException for `onSinglePointerTouchEvent` of null key
+- Fixed key press colour change dependent on activity background
 - Reduced space bar height
 - Reduced digit row key height
 - Added fillColour attribute for the keyboard as a whole

app/src/main/java/io/github/yawnoc/strokeinput/InputContainer.java

@@ -48,8 +48,7 @@ public class InputContainer
   private static final int KEY_REPEAT_START_MILLISECONDS = 500;
   private static final int KEY_LONG_PRESS_MILLISECONDS = 750;
 
-  private static final int DEFAULT_KEY_ALPHA = 0xFF;
-  private static final int PRESSED_KEY_ALPHA = 0x7F;
+  private static final float COLOUR_LIGHTNESS_CUTOFF = 0.7f;
 
   // Container meta-properties
   private OnInputListener inputListener;
@@ -177,11 +176,11 @@ public void onDraw(Canvas canvas) {
 
       keyRectangle.set(0, 0, key.width, key.height);
 
-      int key_fill_colour =
-        ColorUtils.setAlphaComponent(
-          key.keyFillColour,
-          key == currentlyPressedKey ? PRESSED_KEY_ALPHA : DEFAULT_KEY_ALPHA
-        );
+      int key_fill_colour = key.keyFillColour;
+      if (key == currentlyPressedKey) {
+        key_fill_colour = getContrastingColour(key_fill_colour);
+      }
+      
       keyFillPaint.setColor(key_fill_colour);
       keyBorderPaint.setColor(key.keyBorderColour);
       keyBorderPaint.setStrokeWidth(key.keyBorderThickness);
@@ -214,6 +213,28 @@ public void onDraw(Canvas canvas) {
 
   }
 
+  /*
+    Lighten a dark colour and darken a light colour.
+    Used for key press colour changes.
+  */
+  private static int getContrastingColour(int colour) {
+    
+    float[] colourHSL = new float[3];
+    ColorUtils.colorToHSL(colour, colourHSL);
+    
+    float colourLightness = colourHSL[2];
+    if (colourLightness < COLOUR_LIGHTNESS_CUTOFF) {
+      colourLightness = (2 * colourLightness + 1) / 3;
+    }
+    else {
+      colourLightness = (2 * colourLightness) / 3;
+    }
+    
+    colourHSL[2] = colourLightness;
+    
+    return ColorUtils.HSLToColor(colourHSL);
+  }
+  
   /*
     Handle logic for multiple pointers (e.g. two-thumb typing).
     // TODO: fix unexpected behaviour when two pointers merge