The HyperLogLog algorithm [1] is a space efficient method to estimate the cardinality of extraordinarily large datasets. This module is written in C for Python >= 3.6 and Python 2.7.x. It implements a 64 bit version of HyperLogLog [2] using a Murmur64A hash.
Install Python development libraries. This step will depend on your OS. On Ubuntu:
sudo apt install python-dev
Install HLL:
pip install HLL
Example usage:
from HLL import HyperLogLog
hll = HyperLogLog(10) # use 2^10 registers
hll.add('some data')
estimate = hll.cardinality()
print(estimate)
- Remove support for Python 2.7.
Deprecation notice: this is the last supported version for Python 2.7.x.
- Fixed bug where HyperLogLogs of unequal sizes could be merged.
- Fixed bug causing cardinality estimates to be off when repeatedly merging sparse HyperLogLogs loaded from a pickle dump.
- Algorithm has been updated to a 64 bit version [2]. This fixes the spike in relative error when switching from linear counting in the original HyperLogLog algorithm.
- Hash function has been updated to the 64 bit Murmur64A function.
- More efficiently store registers using a combination of sparse and dense representations.
- Improved method for counting the number of leading zeroes.
- Changed the return type of
cardinality()
from float to integer. - Changed the return logic of
add()
. This method no longer always indicates if a register was updated using its return value. This behavior is only preserved in dense representation. In sparse representation,add()
always returnsFalse
. HyperLogLog
objects pickled in 1.x and 2.x are not compatible.- Added
get_register()
- Added
hash()
- Added
_get_meta()
- Deprecated
murmur2_hash()
- Deprecated
registers()
- Deprecated
set_register()
HyperLogLog
objects implement a 64 bit HyperLogLog algorithm [2]. They can
be used to estimate the cardinality of very large datasets. The estimation
accuracy is proportional to the number of registers. Using more registers
increases the accuracy and using less registers decreases the accuracy. The
number of registers is set in powers of 2 using the parameter p
and defaults
to p=12
or 2^12
registers.
>>> from hll import HyperLogLog
>>> hll = HyperLogLog() # Default to 2^12 registers
>>> hll.size()
4096
>>> hll = HyperLogLog(3) # Use 2^3 registers
>>> hll.size()
8
>>> for data in ['one', 'two', 'three', 'four',]:
... hll.add(data)
>>> hll.cardinality()
4
HyperLogLogs use a Murmur64A hash. This function is fast and has a good
uniform distribution of bits which is necessary for accurate estimations. The
seed to this hash function can be set in the HyperLogLog
constructor:
>>> hll = HyperLogLog(p=2, seed=123456789)
>>> hll.seed()
123456789
The hash function can also be called directly:
>>> hll.hash('something')
393810339
Individual registers can be printed:
>>> for i in range(2**4):
... print(hll.get_register(i))
0
0
3
0
4
HyperLogLog
objects can be merged. This is done by taking the maximum value
of their respective registers:
>>> A = HyperLogLog(p=4)
>>> A.add('hello')
>>> B = HyperLogLog(p=4)
>>> B.add('world')
>>> A.merge(B)
>>> A.cardinality()
2
Registers are stored using both sparse and dense representation. Originally
all registers are initialized to zero. However storing all these zeroes
individually is wasteful. Instead a sorted linked list [3] is used to store
only registers that have been set (e.g. have a non-zero value). When this list
reaches sufficient size the HyperLogLog
object will switch to using dense
representation where registers are stored invidiaully using 6 bits.
Sparse representation can be disabled using the sparse
flag:
>>> HyperLogLog(p=2, sparse=False)
The maximum list size for the sparse register list determines when the
HyperLogLog
object switches to dense representation. This can be set
using max_list_size
:
>>> HyperLogLog(p=15, max_list_size=10**6)
Traversing the sparse register list every time an item is added to the
HyperLogLog
to update a register is expensive. A temporary buffer is instead
used to defer this operation. Items added to the HyperLogLog
are first added
to the temporary buffer. When the buffer is full the items are sorted and then
any register updates occur. These updates can be done in one pass since both
the temproary buffer and sparse register list are sorted.
The buffer size can be set using max_buffer_size
:
>>> HyperLogLog(p=15, max_buffer_size=10**5)
This software is released under the MIT License.
[1] P. Flajolet, E. Fusy, O. Gandouet, F. Meunier. "HyperLogLog: the analysis of a near-optimal cardinality estimation algorithm," Conference on the Analysis of Algorithms 2007.
[2] O. Ertl, "New Cardinality Estimation Methods for HyperLogLog Sketches," arXiv:1706.07290 [cs], June 2017.
[3] S. Heule, M. Nunkesser, A. Hall. "HyperLogLog in Practice: Algorithimic Engineering of a State of the Art Cardinality Estimation Algorithm," Proceedings of the EDBT 2013 Conference, ACM, Genoa March 2013.