6.7.3 Shift Hacks

You can use the shift operators for various useful hacks. For example, given a date specified by day of the month d, month m, and year y, you can store the entire date in a single integer date:

unsigned int d = 12;
unsigned int m = 6;
unsigned int y = 1983;
unsigned int date = ((y << 4) + m) << 5) + d;

To extract the original day, month, and year out of date, use a combination of shift and remainder.

d = date % 32;
m = (date >> 5) % 16;
y = date >> 9;

-1 << LOWBITS is a clever way to make an integer whose LOWBITS lowest bits are all 0 and the rest are all 1. -(1 << LOWBITS) is equivalent to that, due to associativity of multiplication, since negating a value is equivalent to multiplying it by -1.