Working with Bitmaps

1. Setting Bit Values

Command	Returns
`SETBIT key offset 0\|1`	Previous bit value; auto-extends string

Command	Returns
`GETBIT key offset`	`0` or `1`

Command	Options
`BITCOUNT key [start end [BYTE\|BIT]]`	Population count; BIT range 7.0+

Command	Returns
`BITPOS key 0\|1 [start [end [BYTE\|BIT]]]`	Offset of first matching bit, `-1` if none

Command	Operation
`BITOP AND\|OR\|XOR\|NOT dst k1 [k2 ...]`	Combine bitmaps; length = longest input

SETBIT dau:2026-05-19 42 1
SETBIT dau:2026-05-20 42 1
BITOP AND dau:both dau:2026-05-19 dau:2026-05-20
BITCOUNT dau:both

Command	Description
`BITFIELD key GET type offset`	Read typed integer (e.g. `u8`, `i16`)
Offset prefix `#`	`#3` means index 3 * type-width

BITFIELD counter SET u16 #0 1000 INCRBY u16 #0 5 GET u16 #0
BITFIELD stats OVERFLOW SAT INCRBY i8 #1 200

Command	Notes
`BITFIELD_RO key GET ...`	Replica-safe; only GET ops allowed

Population	Memory (1M users)
All bits set	~125 KB
Sparse	Allocated to max offset / 8 bytes
Equivalent SET	Tens of MB

Pattern	Implementation
Daily active users	`dau:YYYY-MM-DD` bitmap, `SETBIT` per user id
Retention	`BITOP AND` of two days
Funnel	Sequential `BITOP AND` across steps

# 16-bit counter per user, supports 1M users in ~2MB
BITFIELD events INCRBY u16 #42 1     # user 42 +1 event
BITFIELD events GET u16 #42