HyperLogLog is an algorithm that can solve Count Distinct problem. A Count Distinct problem is something like getting number 5 for a data set like [1, 3, 2, 1, 5, 2, 4], for it has [1, 2, 3, 4, 5] 5 elements.

HyperLogLog can provide estimated count on a very large data stream.

- Advantage
- Fast (O(1)).
- Memory efficient.
- Can be distributed and paralleled.

- Disadvantage
- Only provides estimated count.

- Hash each item.
- Place each item into 32 buckets based on first 5-bits of the hash.
- Update value for each item in 32 buckets based on last 27-bits of the hash.
- Apply to some fancy mathematical formulas to get the number based on the 32 buckets data.

References:

- Damn Cool Algorithms: Cardinality Estimation
- Using Linear Counting, LogLog, and HyperLogLog to Estimate Cardinality
- HyperLogLog Playground

HyperLogLogPlusPlus is an enhancement of HyperLogLog.

- Uses 64-bit integers rather than 32-bit
- Sparse data structure rather than one huge array
- Better bias correction algorithm at lower cardinalities.

Commands `PFADD`

, `PFCOUNT`

, and `PFMERGE`

are the main interfaces for manipulating HyperLogLog data structure in Redis.

```
redis> PFADD hll a b c d e f g
(integer) 1
redis> PFCOUNT hll
(integer) 7
redis> PFADD hll1 foo bar zap a
(integer) 1
redis> PFADD hll2 a b c foo
(integer) 1
redis> PFMERGE hll3 hll1 hll2
"OK"
redis> PFCOUNT hll3
(integer) 6
```

References:

- Redis new data structure: the HyperLogLog: If you're developing a web service, Redis might be a good choice.
- HyperLogLogs in Redis
- PFADD: Add all the element arguments to the HyperLogLog data structure referenced as
`key`

. - PFCOUNT: Get the approximated cardinality computed by the HyperLogLog data structure
- PFMERGE: Merge multiple HyperLogLog values into one.

Source code of the script `test.py`

:

```
from datasketch import HyperLogLog
data = [1, 2, 3, 5, 2, 4, 1, 6, 7]
hll = HyperLogLog()
for element in data:
hll.update(str(element).encode('utf8'))
print(f'estimate: {hll.count()}')
print(f'real: {len(set(data))}')
```

Run the script:

```
$ mkdir testdatasketch
$ python3 -mvenv venv
$ source venv/bin/activate
(venv) $ pip install datasketch
(venv) $ python test.py
estimate: 7.097484291802821
real: 7
```

References:

- datasketch - big data looks small: If you're trying to write a simple Python script to solve problem, datasketch might be a good choice.

Logswan can find unique IP addresses in very large log files but only consumes 4MB memory.

```
$ logswan access.log
```

References:

- logswan - Fast Web log analyzer using probabilistic data structures: Good choice for unique visitors counting on common logging file.

- Keep HyperLogLog data for every 10 minutes for the past 24 hours.
- Merge HyperLogLog data into hourly HLL after 24 hours.
- Merge HyperLogLog data into daily HLL after 60 days.
- ...

Example redis usage: `PFADD users:<date>:<hour>:<MM> <UID>`

, MM=[00, 10, 20, 30, 40, 50]

References: