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:
The examples in this section use binary constants, starting with
‘0b’ (see Integer Constants). They stand for 32-bit integers
of type int.
unsigned int d = 12; /* 12 in binary is 0b1100. */ unsigned int m = 6; /* 6 in binary is 0b110. */ unsigned int y = 1983; /* 1983 in binary is 0b11110111111. */ unsigned int date = (((y << 4) + m) << 5) + d; /* Add 0b11110111111000000000 and 0b11000000 and 0b1100. Sum is 0b11110111111011001100. */
To extract the day, month, and year out of
date, use a combination of shift and remainder:
/* 32 in binary is 0b100000. */ /* Remainder dividing by 32 gives lowest 5 bits, 0b1100. */ d = date % 32; /* Shifting 5 bits right discards the day, leaving 0b111101111110110. Remainder dividing by 16 gives lowest remaining 4 bits, 0b110. */ m = (date >> 5) % 16; /* Shifting 9 bits right discards day and month, leaving 0b11110111111. */ 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, since negating a value
is equivalent to multiplying it by −1.