# GCE-Math
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GCE-Math (**G**eneralized **C**onstant **E**xpression Math) is a templated C++ library enabling compile-time computation of mathematical functions.

Features:

* The library is written in C++11 `constexpr` format, and is C++11/14/17 compatible.
* Continued fraction expansions and series expansions are implemented using recursive templates.
* The `gcem::` syntax is identical to the C++ standard library (`std::`).
* Tested and accurate to floating-point precision against the C++ standard library.
* Released under a permissive, non-GPL license.

**Author**: Keith O'Hara

[![License](https://img.shields.io/badge/Licence-Apache%202.0-blue.svg)](./LICENSE)

### Contents:
* [Status and Documentation](#status-and-documentation) 
* [Installation](#installation)
* [Test Suite](#test-suite)
* [Jupyter Notebook](#jupyter-notebook)
* [General Syntax](#general-syntax)
* [Examples](#examples)

## Status and Documentation

The library is actively maintained and is still being extended. A list of features includes:

* Basic library functions:
    - `abs`, `max`, `min`, `pow`, `sqrt`, 
    - `ceil`, `floor`, `round`, `trunc`, `fmod`,
    - `exp`, `expm1`, `log`, `log1p`, `log2`, and more
* Trigonometric functions:
    - basic: `cos`, `sin`, `tan`
    - inverse: `acos`, `asin`, `atan`, `atan2`
* Hyperbolic (area) functions: 
    - `cosh`, `sinh`, `tanh`, `acosh`, `asinh`, `atanh`
* Algorithms:
    - `gcd`, `lcm`
* Special functions:
    - factorials and the binomial coefficient: `factorial`, `binomial_coef`
    - beta, gamma, and multivariate gamma functions: `beta`, `lbeta`, `lgamma`, `tgamma`, `lmgamma`
    - the Gaussian error function and inverse error function: `erf`, `erf_inv`
    - (regularized) incomplete beta and incomplete gamma functions: `incomplete_beta`, `incomplete_gamma`
    - inverse incomplete beta and incomplete gamma functions: `incomplete_beta_inv`, `incomplete_gamma_inv`

Full documentation is available online:

[![Documentation Status](https://readthedocs.org/projects/gcem/badge/?version=latest)](https://gcem.readthedocs.io/en/latest/?badge=latest)

A PDF version of the documentation is available [here](https://buildmedia.readthedocs.org/media/pdf/gcem/latest/gcem.pdf).

## Installation

GCE-Math is a header-only library and does not require any additional libraries or utilities (beyond a C++11 compatible compiler). Simply add the header files to your project using:
```cpp
#include "gcem.hpp"
```

### Conda 

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You can install GCE-Math using the conda package manager.

```bash
conda install -c kthohr gcem
```

### CMake

You can also install the library from source using CMake.

```bash
# clone gcem from GitHub
git clone https://github.com/kthohr/gcem ./gcem

# make a build directory
cd ./gcem
mkdir build
cd build

# generate Makefiles and install
cmake .. -DCMAKE_INSTALL_PREFIX=/gcem/install/location
make install
```
For example, `/gcem/install/location` could be `/usr/local/`.

## Test Suite

There are two ways to build the test suite. On Unix-alike systems, a Makefile is available under `tests/`.

```bash
cd ./gcem/tests
make
./run_tests
```

With CMake, the option `GCEM_BUILD_TESTS=1` generates the necessary Makefiles to build the test suite.
```bash
cd ./gcem
mkdir build

cd build
cmake ../ -DGCEM_BUILD_TESTS=1 -DCMAKE_INSTALL_PREFIX=/gcem/install/location
make gcem_tests

cd tests
./exp.test
```

## Jupyter Notebook

You can test the library online using an interactive Jupyter notebook: 

[![Binder](https://mybinder.org/badge.svg)](https://mybinder.org/v2/gh/kthohr/gcem/master?filepath=notebooks%2Fgcem.ipynb)

## General Syntax

GCE-Math functions are written as C++ templates with `constexpr` specifiers, the format of which might appear confusing to users unfamiliar with template-based programming. 

For example, the [Gaussian error function](https://en.wikipedia.org/wiki/Error_function) (`erf`) is defined as:
```cpp
template<typename T>
constexpr
return_t<T>
erf(const T x) noexcept;
```
A set of internal templated `constexpr` functions will implement a continued fraction expansion and return a value of type `return_t<T>`. The output type ('`return_t<T>`') is generally determined by the input type, e.g., `int`, `float`, `double`, `long double`, etc.; when `T` is an intergral type, the output will be upgraded to `return_t<T> = double`, otherwise `return_t<T> = T`. For types not covered by `std::is_integral`, recasts should be used.

## Examples

To calculate 10!:

```cpp
#include "gcem.hpp"

int main()
{
    constexpr int x = 10;
    constexpr int res = gcem::factorial(x);

    return 0;
}
```
Inspecting the assembly code generated by Clang 7.0.0:
```assembly
        push    rbp
        mov     rbp, rsp
        xor     eax, eax
        mov     dword ptr [rbp - 4], 0
        mov     dword ptr [rbp - 8], 10
        mov     dword ptr [rbp - 12], 3628800
        pop     rbp
        ret
```
We see that a function call has been replaced by a numeric value (10! = 3628800).

Similarly, to compute the log Gamma function at a point:

```cpp
#include "gcem.hpp"

int main()
{
    constexpr long double x = 1.5;
    constexpr long double res = gcem::lgamma(x);

    return 0;
}
```
Assembly code:
```assembly
.LCPI0_0:
        .long   1069547520              # float 1.5
.LCPI0_1:
        .quad   -622431863250842976     # x86_fp80 -0.120782237635245222719
        .short  49147
        .zero   6
main:                                   # @main
        push    rbp
        mov     rbp, rsp
        xor     eax, eax
        mov     dword ptr [rbp - 4], 0
        fld     dword ptr [rip + .LCPI0_0]
        fstp    tbyte ptr [rbp - 32]
        fld     tbyte ptr [rip + .LCPI0_1]
        fstp    tbyte ptr [rbp - 48]
        pop     rbp
        ret
```

## Related libraries

* [StatsLib](https://github.com/kthohr/stats) is built on GCEM's compile-time functionality.
