feat: dynamic constraints (Fixes #8)

app/src/main/java/sudoku/structure/Difficulty.java

@@ -8,7 +8,7 @@ public enum Difficulty {
 	double factor;
 	String displayName;
 
-	Difficulty(String displayName, double factor) {
+	private Difficulty(String displayName, double factor) {
 		this.factor = factor;
 		this.displayName = displayName;
 	}
@@ -21,4 +21,18 @@ public enum Difficulty {
 		return factor;
 	}
 
+	private static final String[] difficultyNames;
+
+	static {
+		Difficulty[] diffs = Difficulty.values();
+		difficultyNames = new String[diffs.length];
+		for (int i = 0; i < diffs.length; i++) {
+			difficultyNames[i] = diffs[i].getDisplayName();
+		}
+	}
+
+	public static String[] getDifficultyNames() {
+		return difficultyNames;
+	}
+
 }