pthash

PTHash is a C++ library implementing fast and compact minimal perfect hash functions as described in the following research papers:

• PTHash: Revisiting FCH Minimal Perfect Hashing (SIGIR 2021),
• Parallel and External-Memory Construction of Minimal Perfect Hash Functions with PTHash (TKDE 2023),
• PHOBIC: Perfect Hashing with Optimized Bucket Sizes and Interleaved Coding (ESA 2024).

Please, cite these papers if you use PTHash.

Rust: if you use the Rust programming language, we recommend the PtrHash library.

Development note

The description of PTHash in the SIGIR and TKDE papers uses the c parameter to control the number of buckets for the search phase of the algorithm. You can get a version of the library using the c parameter here (Release v2). The current library uses instead a parameter called “lambda”, as described in the ESA paper.

Features

• Minimal and Non-Minimal Perfect Hash Functions
• Space/Time Efficiency: fast lookup within compressed space
• External-Memory Scaling
• Multi-Threaded Construction
• Configurable: can offer different trade-offs (between construction time, lookup time, and space effectiveness)

Introduction

Given a set S of n distinct keys, a function f that bijectively maps the keys of S into the first n natural numbers is called a minimal perfect hash function (MPHF) for S. Algorithms that find such functions when n is large and retain constant evaluation time are of practical interest. For instance, search engines and databases typically use minimal perfect hash functions to quickly assign identifiers to static sets of variable-length keys such as strings. The challenge is to design an algorithm which is efficient in three different aspects: time to find f (construction time), time to evaluate f on a key of S (lookup time), and space of representation for f.

PTHash is one such algorithm.

The following guide is meant to provide a brief overview of the library by illustrating its functionalities through some examples.

Table of contents

• Integration
• Compiling the benchmark and example code
• Quick start
• Build examples
• Reading keys from standard input
• Benchmarks

Integration

Integrating PTHash in your own project is very simple. If you use git, the easiest way to add PTHash is via git add submodule as follows.

git submodule add https://github.com/jermp/pthash.git
git submodule update --recursive --init

Then include the following in your CMakeLists.txt, which takes care of setting up the include paths and compiler flags of PTHash and its dependencies:

add_subdirectory(pthash)
target_link_libraries(MyTarget INTERFACE PTHASH)

To construct a perfect hash function, include pthash.hpp and create an instance of pthash::single_phf<...> (PTHash), pthash::partitioned_phf<...> (PTHash-HEM), or pthash::dense_partitioned_phf<...> (PHOBIC). For convenience, we also give pthash::phobic<...> which includes the configuration options for optimized bucket assignment function and interleaved coding. Refer to src/example.cpp for an example.

Compiling the Benchmark and Example Code

The code is tested on Linux with gcc and on Mac OS with clang (both Intel and ARM processors, like Apple M1). To build the code, CMake is required.

Clone the repository with

git clone --recursive https://github.com/jermp/pthash.git

If you have cloned the repository without --recursive, be sure you pull the dependencies with the following command before compiling:

git submodule update --init --recursive

To compile the code for a release environment (see file CMakeLists.txt for the used compilation flags), it is sufficient to do the following:

mkdir build
cd build
cmake ..
make -j

For a testing environment, use the following instead:

mkdir debug_build
cd debug_build
cmake .. -D CMAKE_BUILD_TYPE=Debug -D PTHASH_USE_SANITIZERS=On
make

NOTE: Beware that the software will result in a much slower execution when running in debug mode and using sanitizers. Use this only for debug purposes, not to run performance tests.)

Enable Large Bucket-Id Type

By default, PTHash assumes there are less than $2^{32}$ buckets, hence 32-bit integers are used for bucket ids. To overcome this, you can either lower the value of c or recompile with

cmake .. -D PTHASH_ENABLE_LARGE_BUCKET_ID_TYPE=On

to use 64-bit integers for bucket ids.

Quick Start

For a quick start, see the source file src/example.cpp. The example shows how to setup a simple build configuration for PTHash (parameters, base hasher, bucketer, and encoder types).

After compilation, run this example with

./example

Build Examples

After compilation (see the section Compiling the Code), the driver program called build can be used to run some examples.

Run

./build --help

shows the usage of the driver program. In the following, we illustrate some examples using this ./build program.

Example 1

The command

./build -n 10000000 -l 4.3 -a 0.99 -e D-D -b skew -q 3000000 -s 0 -p 2000000 -t 8 --verbose --minimal --check

builds a minimal (option --minimal) PHF over 10M random 64-bit integers, using

• avg. bucket size, “lambda”, of 4.3 (-l = 4.3);
• load factor of 0.99 (-a 0.99);
• the encoder D-D to compress the data structure;
• the skew bucketer type (-b skew);
• seed 0 (-s 0);
• avg. partition size 2M (-p 2000000);
• 8 parallel threads (-t 8).

Also, it performs 3M queries (-q 3000000) to benchmark the speed of lookup queries and check the correctness of the function (option --check).

Example 2

For the following example, we are going to use the strings from the UK-2005 URLs collection, which can be downloaded by clicking here or typing

wget http://data.law.di.unimi.it/webdata/uk-2005/uk-2005.urls.gz

The file is ~300 MB compressed using gzip (2.86 GB uncompressed).

After download, place the dataset in the build directory and run

gunzip uk-2005.urls.gz

to uncompress it. The file contains one string per line, for a total of 39,459,925 strings.

NOTE: Input files are read line by line (i.e., individual strings are assumed to be separated by the character \n). Be sure there are no blank lines.

The command

./build -n 39459925 -i uk-2005.urls -l 3.5 -e R-int -b skew -s 0 -q 3000000 -t 8 --dense --minimal --verbose --check

builds a MPHF using the strings of the file as input keys, where

• the function is of type dense partitioned" because option --dense is specified;
• the avg. number of buckets is set to 3.5 (-l 3.5);
• the data structure is compressed using interleaved Rice codes (-e R-int);
• the type of bucketer used is skew;
• the seed used for the construction is 0 (-s 0);
• the number of threads used for the construction are 8 (-t 8);
• 3M lookups are performed to benchmark query time (-q 3000000).

Example 3

The command

./build -n 39459925 -i uk-2005.urls -l 3.5 -a 0.94 -e PC -b skew -s 0 -q 3000000 -p 2500000 -t 8 -m 1 --minimal --verbose --check --external

builds a MPHF using most of the parameters used in Example 2 but the function is built in external memory (option --external) using 1 GB of RAM (option -m 1), with avg. partition size of 2.5M, and compressing the data structure with a partitioned compact encoding (-e PC).

Reading Keys from Standard Input

You can make the build tool read the keys from stardard input using bash pipelining (|) in combination with option -i -. This is very useful when building keys from compressed files.

Some examples below.

for i in $(seq 1 1000000) ; do echo $i ; done > foo.txt
cat foo.txt | ./build --minimal -l 3.4 -a 0.94 -e R-R -b skew -n 1000000 -q 0 -m 1 -i - -o foo.mph --verbose --external

gzip foo.txt
zcat foo.txt.gz | ./build --minimal -l 3.4 -a 0.94 -e R-R -b skew -n 1000000 -q 0 -m 1 -i - -o foo.mph --verbose --external

gunzip foo.txt.gz
zstd foo.txt
zstdcat foo.txt.zst | ./build --minimal -l 3.4 -a 0.94 -e R-R -b skew -n 1000000 -q 0 -m 1 -i - -o foo.mph --verbose --external

Note: you may need to write zcat < foo.txt.gz | (...) on Mac OSX.

One caveat of this approach is that it is not possible to use --check nor benchmark query time because these two options need to re-iterate over the keys from the stream.

Benchmarks

The directory benchmarks includes some performance benchmarks.