Perform a bitwise operation between multiple keys (containing string values) and
store the result in the destination key.

The `BITOP` command supports four bitwise operations: **AND**, **OR**, **XOR**
and **NOT**, thus the valid forms to call the command are:

- `BITOP AND destkey srckey1 srckey2 srckey3 ... srckeyN`
- `BITOP OR destkey srckey1 srckey2 srckey3 ... srckeyN`
- `BITOP XOR destkey srckey1 srckey2 srckey3 ... srckeyN`
- `BITOP NOT destkey srckey`

As you can see **NOT** is special as it only takes an input key, because it
performs inversion of bits so it only makes sense as an unary operator.

The result of the operation is always stored at `destkey`.

## Handling of strings with different lengths

When an operation is performed between strings having different lengths, all the
strings shorter than the longest string in the set are treated as if they were
zero-padded up to the length of the longest string.

The same holds true for non-existent keys, that are considered as a stream of
zero bytes up to the length of the longest string.

@return

@integer-reply

The size of the string stored in the destination key, that is equal to the size
of the longest input string.

@examples

```cli
SET key1 "foobar"
SET key2 "abcdef"
BITOP AND dest key1 key2
GET dest
```

## Pattern: real time metrics using bitmaps

`BITOP` is a good complement to the pattern documented in the `BITCOUNT` command
documentation. Different bitmaps can be combined in order to obtain a target
bitmap where the population counting operation is performed.

See the article called "[Fast easy realtime metrics using Redis
bitmaps][hbgc212fermurb]" for a interesting use cases.

[hbgc212fermurb]:
  http://blog.getspool.com/2011/11/29/fast-easy-realtime-metrics-using-redis-bitmaps

## Performance considerations

`BITOP` is a potentially slow command as it runs in O(N) time. Care should be
taken when running it against long input strings.

For real-time metrics and statistics involving large inputs a good approach is
to use a replica (with read-only option disabled) where the bit-wise operations
are performed to avoid blocking the master instance.