SETBIT

SETBIT key offset value

Available since

2.2.0

Time complexity

O(1)

ACL categories

@write, @bitmap, @slow

Sets or clears the bit at offset 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 2^32 (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 2^32 -1) and the string value stored at key does not yet hold a string value, or holds a small string value, Redict needs to allocate all intermediate memory which can block the server for some time. On a 2010 MacBook Pro, setting bit number 2^32 -1 (512MB allocation) takes ~300ms, setting bit number 2^30 -1 (128MB allocation) takes ~80ms, setting bit number 2^28 -1 (32MB allocation) takes ~30ms and setting bit number 2^26 -1 (8MB allocation) takes ~8ms. Note that once this first allocation is done, subsequent calls to SETBIT for the same key will not have the allocation overhead.

Examples #

SETBIT mykey 7 1
SETBIT mykey 7 0
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.

Because Redict’ strings are binary-safe, 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:

> SETBIT bitmapsarestrings 2 1
> SETBIT bitmapsarestrings 3 1
> SETBIT bitmapsarestrings 5 1
> SETBIT bitmapsarestrings 10 1
> SETBIT bitmapsarestrings 11 1
> SETBIT bitmapsarestrings 14 1
> 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

Advanced Pattern: accessing bitmap ranges #

It is also possible to use the GETRANGE and SETRANGE string commands to efficiently access a range of bit offsets in a bitmap. Below is a sample implementation in idiomatic Redict Lua scripting that can be run with the EVAL command:

--[[
Sets a bitmap range

Bitmaps are stored as Strings in Redict. A range spans one or more bytes,
so we can call `SETRANGE` when entire bytes need to be set instead of flipping
individual bits. Also, to avoid multiple internal memory allocations in
Redict, we traverse in reverse.
Expected input:
  KEYS[1] - bitfield key
  ARGV[1] - start offset (0-based, inclusive)
  ARGV[2] - end offset (same, should be bigger than start, no error checking)
  ARGV[3] - value (should be 0 or 1, no error checking)
]]--

-- A helper function to stringify a binary string to semi-binary format
local function tobits(str)
  local r = ''
  for i = 1, string.len(str) do
    local c = string.byte(str, i)
    local b = ' '
    for j = 0, 7 do
      b = tostring(bit.band(c, 1)) .. b
      c = bit.rshift(c, 1)
    end
    r = r .. b
  end
  return r
end

-- Main
local k = KEYS[1]
local s, e, v = tonumber(ARGV[1]), tonumber(ARGV[2]), tonumber(ARGV[3])

-- First treat the dangling bits in the last byte
local ms, me = s % 8, (e + 1) % 8
if me > 0 then
  local t = math.max(e - me + 1, s)
  for i = e, t, -1 do
    redict.call('SETBIT', k, i, v)
  end
  e = t
end

-- Then the danglings in the first byte
if ms > 0 then
  local t = math.min(s - ms + 7, e)
  for i = s, t, 1 do
    redict.call('SETBIT', k, i, v)
  end
  s = t + 1
end

-- Set a range accordingly, if at all
local rs, re = s / 8, (e + 1) / 8
local rl = re - rs
if rl > 0 then
  local b = '\255'
  if 0 == v then
    b = '\0'
  end
  redict.call('SETRANGE', k, rs, string.rep(b, rl))
end

Note: the implementation for getting a range of bit offsets from a bitmap is left as an exercise to the reader.