Since 1990 I've written code in assembly language (mainly on MicroChip PIC's) thus I'm not too familiar with the C constructs but hopefully the Fast LED library will be update to account for the newer type of inclusive-white 'addressable' led's.
The modified code is to enable use or RGBW type, instead of the older RGB ones, the white being much much better. Each led then requires RGB+W specific data sent to in in the data stream, but the standard library files will only be sending RGB data . . . . that hack' basically simulates there being more led's in one's led string to permit the additional data required by the white led's, but it's not working too well.