README.md

# S2 Geometry Library

[![Build Status](https://travis-ci.org/google/s2geometry.svg?branch=master)](https://travis-ci.org/google/s2geometry)

## Overview

This is a package for manipulating geometric shapes. Unlike many geometry
libraries, S2 is primarily designed to work with _spherical geometry_, i.e.,
shapes drawn on a sphere rather than on a planar 2D map. This makes it
especially suitable for working with geographic data.

If you want to learn more about the library, start by reading the
[overview](http://s2geometry.io/about/overview) and [quick start
document](http://s2geometry.io/devguide/cpp/quickstart), then read the
introduction to the [basic types](http://s2geometry.io/devguide/basic_types).

S2 documentation can be found on [s2geometry.io](http://s2geometry.io).

## Requirements for End Users

* [CMake](http://www.cmake.org/)
* A C++ compiler with C++11 support, such as [g++ >= 4.7](https://gcc.gnu.org/)
* [Abseil](https://github.com/abseil/abseil-cpp) (standard library extensions)
* [OpenSSL](https://github.com/openssl/openssl) (for its bignum library)
* [gflags command line flags](https://github.com/gflags/gflags), optional
* [glog logging module](https://github.com/google/glog), optional
* [googletest testing framework](https://github.com/google/googletest)
  (to build tests and example programs, optional)

On Ubuntu, all of these other than abseil can be installed via apt-get:

```
sudo apt-get install cmake libgflags-dev libgoogle-glog-dev libgtest-dev libssl-dev
```

Otherwise, you may need to install some from source.

Currently, Abseil must always be installed from source.  See the use of
`-DCMAKE_PREFIX_PATH` in the [build instructions below](#building).
This is likely to change.

On macOS, use [MacPorts](http://www.macports.org/) or
[Homebrew](http://brew.sh/).  For MacPorts:

```
sudo port install cmake gflags google-glog openssl
```

Do not install `gtest` from MacPorts; instead download [release
1.8.0](https://github.com/google/googletest/releases/tag/release-1.8.0), unpack,
and substitute

```
cmake -DGTEST_ROOT=/...absolute path to.../googletest-release-1.8.0/googletest ..
```

in the build instructions below.

Thorough testing has only been done on Ubuntu 14.04.3 and macOS 10.12.

## Build and Install

You may either download the source as a ZIP archive, or [clone the git
repository](https://help.github.com/articles/cloning-a-repository/).

### Via ZIP archive

Download [ZIP file](https://github.com/google/s2geometry/archive/master.zip)

```
cd [parent of directory where you want to put S2]
unzip [path to ZIP file]/s2geometry-master.zip
cd s2geometry-master
```

### Via `git clone`

```
cd [parent of directory where you want to put S2]
git clone https://github.com/google/s2geometry.git
cd s2geometry
```

### Building

First, [install Abseil](https://github.com/abseil/abseil-cpp/blob/master/CMake/README.md#traditional-cmake-set-up).
It must be configured with `-DCMAKE_POSITION_INDEPENDENT_CODE=ON`.
s2geometry must be configured to use the came C++ version that
abseil uses.  The easiest way to achieve this is to pass
`-DCMAKE_CXX_STANDARD=11` (or `-DCMAKE_CXX_STANDARD=17`) to `cmake`
when compiling both abseil and s2geometry.

From the appropriate directory depending on how you got the source:

```
mkdir build
cd build
# You can omit -DGTEST_ROOT to skip tests; see above for macOS.
# Use the same CMAKE_CXX_STANDARD value that was used with absl.
cmake -DGTEST_ROOT=/usr/src/gtest -DCMAKE_PREFIX_PATH=/path/to/absl/install -DCMAKE_CXX_STANDARD=11 ..
make -j $(nproc)
make test ARGS="-j$(nproc)"  # If GTEST_ROOT specified above.
sudo make install
```

On macOS, `sysctl -n hw.logicalcpu` is the equivalent of `nproc`.

Enable gflags and glog with `cmake -DWITH_GFLAGS=ON -DWITH_GLOG=ON ...`.

Disable building of shared libraries with `-DBUILD_SHARED_LIBS=OFF`.

Enable the python interface with `-DWITH_PYTHON=ON`.

## Installing

From `build` subdirectory:

```
make install
```

Prefix it with `sudo` if needed:

```
sudo make install
```

_NOTE_: There is not `uninstall` target but `install_manifest.txt` may be helpfull.

All files will be installed at location specified in `CMAKE_INSTALL_PREFIX` variable.

Several suffix variables used for some file groups:

Variable | Default | Description
-------- | ------- | -----------
`CMAKE_INSTALL_INCLUDEDIR` | `include` | For header files
`CMAKE_INSTALL_BINDIR`     | `bin`     | For executables and `*.dll` files on `DLL`-based platforms
`CMAKE_INSTALL_LIBDIR`     | `lib`     | For library files (`*.so`, `*.a`, `*.lib` etc)

If needed set this variables on command line as `cmake` arguments with `-D` prefix or edit from `build` subdirectory:

```
make edit_cache
```

For more info read: [The CMake Cache](https://cmake.org/cmake/help/latest/guide/user-interaction/index.html#the-cmake-cache).

## Python

If you want the Python interface, you need to run cmake using
`-DWITH_PYTHON=ON`. You will also need to install the following dependencies:

* [SWIG 4](https://github.com/swig/swig) (for Python support, optional)
* python3-dev (for Python support, optional)

which can be installed via

```
sudo apt-get install swig python3-dev
```

or on macOS:

```
sudo port install swig
```
Version 4.0 is required, but it should be easy to make it work 3.0 or probably
even 2.0.

Python 3 is required.

## Other S2 implementations

* [Go](https://github.com/golang/geo) (Approximately 40% complete.)
* [Java](https://github.com/google/s2-geometry-library-java) (Some newer C++ features are missing;
  last updated in 2021.)
* [Kotlin](https://github.com/Enovea/s2-geometry-kotlin) (Complete except binary serialization)

## Disclaimer

This is not an official Google product.