README.md

# Apache DataSketches Core C++ Library Component

This is the core C++ component of the Apache DataSketches library.  It contains all the key sketching algorithms from the Java implementation and can be accessed directly by user applications. 

This component is also a dependency of other library components that create adaptors for target systems, such as [PostgreSQL](https://github.com/apache/datasketches-postgresql).

Note that we have parallel core library components for Java, Python, GO, and Rust implementations of many of the same sketch algorithms:

- [datasketches-java](https://github.com/apache/datasketches-java)
- [datasketches-python](https://github.com/apache/datasketches-python)
- [datasketches-go](https://github.com/apache/datasketches-go)
- [datasketches-rust](https://github.com/apache/datasketches-rust)

Please visit the main [Apache DataSketches website](https://datasketches.apache.org) for more information. 

If you are interested in making contributions to this site, please see our [Community](https://datasketches.apache.org/docs/Community/) page for how to contact us.

---

This code requires C++11.

This library is header-only. The provided build process is only for unit tests.

Building the unit tests requires CMake 3.12.0 or higher.

Installing the latest CMake on OSX: `brew install cmake`.

Building and running unit tests using CMake for OSX and Linux:

```shell
cmake -S . -B build/Release -DCMAKE_BUILD_TYPE=Release
cmake --build build/Release -t all test
```

Building and running unit tests using CMake for Windows from the command line:

```shell
cd build
cmake ..
cd ..
cmake --build build --config Release
cmake --build build --config Release --target RUN_TESTS
```

To install a local distribution (OSX and Linux), use the following command. The `CMAKE_INSTALL_PREFIX` variable controls the destination. If not specified, it defaults to installing in /usr (/usr/include, /usr/lib, etc). In the command below, the installation will be in /tmp/install/DataSketches (/tmp/install/DataSketches/include, /tmp/install/DataSketches/lib, etc).

```shell
cmake -S . -B build/Release -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/tmp/install/DataSketches
cmake --build build/Release -t install
```

To generate an installable package using CMake's built-in cpack packaging tool, use the following command. The type of packaging is controlled by the `CPACK_GENERATOR` variable (semi-colon separated list). CMake usually supports packaging formats such as RPM, DEB, STGZ, TGZ, TZ, and ZIP.

```shell
cmake -S . -B build/Release -DCMAKE_BUILD_TYPE=Release -DCPACK_GENERATOR="RPM;STGZ;TGZ" 
cmake --build build/Release -t package
```

The DataSketches project can be included in other projects' CMakeLists.txt files in one of two ways.

If DataSketches has been installed on the host (using an RPM, DEB, "make install" into /usr/local, or some way, then CMake's `find_package` command can be used like this:

```cmake
find_package(DataSketches 3.2 REQUIRED)
target_link_library(my_dependent_target PUBLIC ${DATASKETCHES_LIB})
```

When used with find_package, DataSketches exports several variables, including

- `DATASKETCHES_VERSION`: The version number of the datasketches package that was imported.
- `DATASKETCHES_INCLUDE_DIR`: The directory that should be added to access DataSketches include files. Because CMake automatically includes the interface directories for included target libraries when using `target_link_library`, under normal circumstances, there will be no need to include this directly
- `DATASKETCHES_LIB`: The name of the DataSketches target to include as a dependency. Projects pulling in DataSketches should reference this with `target_link_library` in order to set up all the correct dependencies and include paths.

If you don't have DataSketches installed locally, dependent projects can pull it directly from GitHub using CMake's `ExternalProject` module. The code would look something like this:

```cmake
cmake_policy(SET CMP0097 NEW)
include(ExternalProject)
ExternalProject_Add(datasketches
    GIT_REPOSITORY https://github.com/apache/datasketches-cpp.git
    GIT_TAG 3.2.0
    GIT_SHALLOW true
    GIT_SUBMODULES ""
    INSTALL_DIR /tmp/datasketches-prefix
    CMAKE_ARGS -DBUILD_TESTS=OFF -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} -DCMAKE_INSTALL_PREFIX=/tmp/datasketches-prefix

    # Override the install command to add DESTDIR
    # This is necessary to work around an oddity in the RPM (but not other) package
    # generation, as CMake otherwise picks up the Datasketch files when building
    # an RPM for a dependent package. (RPM scans the directory for files in addition to installing
    # those files referenced in an "install" rule in the cmake file)
    INSTALL_COMMAND env DESTDIR= ${CMAKE_COMMAND} --build . --target install
)
ExternalProject_Get_property(datasketches INSTALL_DIR)
set(datasketches_INSTALL_DIR ${INSTALL_DIR})
message("Source dir of datasketches = ${datasketches_INSTALL_DIR}")
target_include_directories(my_dependent_target 
                            PRIVATE ${datasketches_INSTALL_DIR}/include/DataSketches)
add_dependencies(my_dependent_target datasketches)
```