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#### 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;		/* 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;
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 0b111101111110.  */
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.