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+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_.
+
+Like for 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.
+
+@return
+
+@integer-reply
+
+The number of bits set to 1.
+
+@examples
+
+```cli
+SET mykey "foobar"
+BITCOUNT mykey
+BITCOUNT mykey 0 0
+BITCOUNT mykey 1 1
+```
+
+## Pattern: real-time metrics using bitmaps
+
+Bitmaps 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][hbgc212fermurb]".
+
+[hbgc212fermurb]:
+  http://blog.getspool.com/2011/11/29/fast-easy-realtime-metrics-using-redis-bitmaps
+
+## Performance considerations
+
+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)
+Redis command like `GET` or `INCR`.
+
+When the bitmap is big, there are two alternatives:
+
+- Taking a separated key that is incremented every time the bitmap is modified.
+  This can be very efficient and atomic using a small Redis Lua script.
+- Running the bitmap incrementally using the `BITCOUNT` _start_ and _end_
+  optional parameters, accumulating the results client-side, and optionally
+  caching the result into a key.