code: private int getNumberOfPlaceholderViews() {
          int numColumns = getNumColumnsCompatible();
          int adapterCount = adapter.getEncapsulatedAdapter().getCount();
          int lastLineCount = adapterCount % numColumns;
          return lastLineCount > 0 ? numColumns - lastLineCount : 0;
      }
docs: Returns the number of placeholder views, which are necessary to complement the items of the
  encapsulated adapter in order to fill up all columns..
  
  @return The number of placeholder views, which are necessary to complement the items of the
  encapsulated adapter in order to fill up all columns, as an {@link Integer} value