square_color method
Returns the color of the square ('light' or 'dark'), or null if square
is invalid
Implementation
String? square_color(square) {
if (SQUARES.containsKey(square)) {
final sq_0x88 = SQUARES[square];
return ((rank(sq_0x88) + file(sq_0x88)) % 2 == 0) ? 'light' : 'dark';
}
return null;
}