function moveSelection(direction) {
  switch (direction) {
    case DIRECTIONS.Y_PLUS:
      selectionCoords.y++;
      break;
    case DIRECTIONS.Y_MINUS:
      selectionCoords.y--;
      break;
    case DIRECTIONS.X_PLUS:
      selectionCoords.x++;
      break;
    case DIRECTIONS.X_MINUS:
      selectionCoords.x--;
      break;
  }

  // Prevent selection coords from going out-of-bounds
  selectionCoords.x = Math.max(selectionCoords.x, 0);
  selectionCoords.x = Math.min(selectionCoords.x, width() - 1);
  selectionCoords.y = Math.max(selectionCoords.y, 0);
  selectionCoords.y = Math.min(selectionCoords.y, height() / 2 - 1);

  // Update the selectionSprite
  updateSelectionSprite();
}
PIECE: select a piece to move
MOVE: make a valid move with the currently selected piece