Counts the number of set bits (population counting) in a string.
BITCOUNT key [start
[end [BYTE |
BIT]]]
Count the number of set bits (population counting) in a string.
By default all the bytes contained in the string are examined. It is possible to specify the counting operation only in an interval passing the additional arguments start and end.
Similar to the GETRANGE
command, start and end can contain negative
values in order to index bytes starting from the end of the string,
where -1 is the last byte, -2 is the penultimate, and so forth.
Non-existent keys are treated as empty strings, so the command will return zero.
By default, the additional arguments start and
end specify a byte index. We can use an additional argument
BIT to specify a bit index. So 0 is the first bit, 1 is the
second bit, and so forth. For negative values, -1 is the last bit, -2 is
the penultimate, and so forth.
Integer reply: the number of bits set to 1.
O(N)
@bitmap @read @slow
127.0.0.1:6379> SET mykey "foobar"
OK
127.0.0.1:6379> BITCOUNT mykey
(integer) 26
127.0.0.1:6379> BITCOUNT mykey 0 0
(integer) 4
127.0.0.1:6379> BITCOUNT mykey 1 1
(integer) 6
127.0.0.1:6379> BITCOUNT mykey 1 1 BYTE
(integer) 6
127.0.0.1:6379> BITCOUNT mykey 5 30 BIT
(integer) 17Bitmaps are a very space-efficient representation of certain kinds of information. One example is a Web application that needs the history of user visits, so that for instance it is possible to determine what users are good targets of beta features.
Using the SETBIT command, this
is trivial to accomplish, identifying every day with a small progressive
integer. For instance day 0 is the first day the application was put
online, day 1 the next day, and so forth.
Every time a user performs a page view, the application can register
that in the current day the user visited the web site using the
SETBIT command setting the bit corresponding to the current
day.
Later it will be trivial to know the number of single days the user
visited the web site simply calling the BITCOUNT command
against the bitmap.
A similar pattern where user IDs are used instead of days is described in the article called “Fast easy realtime metrics using Redis bitmaps”.
In the above example of counting days, even after 10 years the
application is online we still have just 365*10 bits of
data per user, that is just 456 bytes per user. With this amount of data
BITCOUNT is still as fast as any other O(1) Valkey command
like GET or INCR.
When the bitmap is big, there are two alternatives:
BITCOUNT
start and end optional parameters,
accumulating the results client-side, and optionally caching the result
into a key.BYTE|BIT option.end made optional; when called without argument
the command reports the last BYTE.