BITPOS

Introduction and Use Case(s)

The BITPOS command in Redis is used to find the position of the first bit set to 1 or 0 in a string. It’s particularly useful in scenarios where you need to quickly locate a specific bit within a binary string, such as finding the first occurrence of an event in a bitmap.

Syntax

  1. BITPOS key bit [start] [end]

Parameter Explanations

  • key: The name of the key holding the string value.
  • bit: The bit value to search for (must be 0 or 1).
  • start (optional): The start index of the search range (inclusive). Defaults to 0 if not provided.
  • end (optional): The end index of the search range (inclusive). If not specified, the search goes until the end of the string.

Return Values

  • Returns the position of the first bit set to the specified value.
  • If no matching bit is found, it returns -1.

Examples

  1. If the bit exists:

    1. dragonfly> SET mykey "\x00\x00\x00\xff"
    2. OK
    3. dragonfly> BITPOS mykey 1
    4. (integer) 24

  2. If the bit does not exist:

    1. dragonfly> SET mykey "\xff\xf0\x00"
    2. OK
    3. dragonfly> BITPOS mykey 0 0 1
    4. (integer) 12

  3. When specifying a range:

    1. dragonfly> SET mykey "\x00\xff\x00"
    2. OK
    3. dragonfly> BITPOS mykey 1 1 2
    4. (integer) 8

Code Examples

  1. dragonfly> SET mybitmap "\x00\xff\xf0"
  2. OK
  3. dragonfly> BITPOS mybitmap 1
  4. (integer) 8
  5. dragonfly> BITPOS mybitmap 0 1
  6. (integer) 16
  7. dragonfly> BITPOS mybitmap 1 2
  8. (integer) 20
  9. dragonfly> BITPOS mybitmap 0 0 1
  10. (integer) 0

Common Mistakes

  • Invalid Bit Values: Using values other than 0 or 1 for the bit parameter will result in an error.

    1. dragonfly> BITPOS mybitmap 2
    2. (error) ERR bit must be 0 or 1

  • Out of Range Indexes: Specifying a start or end index outside the actual length of the string leads to incorrect results.

FAQs

What happens if the key does not exist?

If the key doesn’t exist, BITPOS returns -1 because there are no bits to search through.

Can BITPOS handle large strings efficiently?

Yes, BITPOS is optimized for efficiency even with large strings, making it suitable for scenarios involving large bitmaps or binary data.