SETBIT

Syntax

SETBIT key offset value

Time complexity: O(1)

ACL categories: @write, @bitmap, @slow

Sets or clears the bit at offset (zero-indexed) in the string value stored at key.

The bit is either set or cleared depending on value, which can be either 0 or 1.

When key does not exist, a new string value is created. The string is grown to make sure it can hold a bit at offset. The offset argument is required to be greater than or equal to 0, and smaller than 2³² (this limits bitmaps to 512MB). When the string at key is grown, added bits are set to 0.

Warning: When setting the last possible bit (offset equal to 2³²-1), and the string value stored at key does not holds a value (or holds a small string), the operation will take some time, as Dragonfly is required to allocate all memory leading to that bit. Subsequent calls will not have the performance penalty.

Return

Integer reply: the original bit value stored at offset.

Examples

dragonfly> SETBIT mykey 7 1
(integer) 0
dragonfly> SETBIT mykey 7 0
(integer) 1
dragonfly> GET mykey
""

Pattern: accessing the entire bitmap

There are cases when you need to set all the bits of single bitmap at once, for example when initializing it to a default non-zero value. It is possible to do this with multiple calls to the SETBIT command, one for each bit that needs to be set. However, so as an optimization you can use a single SET command to set the entire bitmap.

Bitmaps are not an actual data type, but a set of bit-oriented operations defined on the String type (for more information refer to the Bitmaps section of the Data Types Introduction page). This means that bitmaps can be used with string commands, and most importantly with SET and GET.

A bitmap is trivially encoded as a bytes stream. The first byte of the string corresponds to offsets 0..7 of the bitmap, the second byte to the 8..15 range, and so forth.

For example, after setting a few bits, getting the string value of the bitmap would look like this:

dragonfly> SETBIT bitmapsarestrings 2 1
dragonfly> SETBIT bitmapsarestrings 3 1
dragonfly> SETBIT bitmapsarestrings 5 1
dragonfly> SETBIT bitmapsarestrings 10 1
dragonfly> SETBIT bitmapsarestrings 11 1
dragonfly> SETBIT bitmapsarestrings 14 1
dragonfly> GET bitmapsarestrings
"42"

By getting the string representation of a bitmap, the client can then parse the response's bytes by extracting the bit values using native bit operations in its native programming language. Symmetrically, it is also possible to set an entire bitmap by performing the bits-to-bytes encoding in the client and calling SET with the resultant string.

Pattern: setting multiple bits

SETBIT excels at setting single bits, and can be called several times when multiple bits need to be set. To optimize this operation you can replace multiple SETBIT calls with a single call to the variadic BITFIELD command and the use of fields of type u1.

For example, the example above could be replaced by:

BITFIELD bitsinabitmap SET u1 2 1 SET u1 3 1 SET u1 5 1 SET u1 10 1 SET u1 11 1 SET u1 14 1