# Libigl Style Guidelines¶

Libigl is used and developed by many people. This document highlights some style guidelines for developers of the library, but also acts as best-practices for users.

## One function, one .h/.cpp pair¶

The structure of libigl is very flat and function-based. For every function/sub-routine, create a single .h and .cpp file. For example, if you have a function that determines connected components from a face list F you would create the header connected_components.h and connected_components.cpp and the only function defined should be void connected_components(const ... F, ... C). If the implementation of connected_components requires a subroutine to compute an adjacency matrix then create another pair adjacency_matrix.h and adjacency_matrix.cpp with a single function void adjacency_matrix(const ... F, ... A).

### Example¶

Here is an example function that would be defined in include/igl/example_fun.h and implemented in include/igl/example_fun.cpp.

#### example_fun.h¶

// This file is part of libigl, a simple c++ geometry processing library.
//
//
// This Source Code Form is subject to the terms of the Mozilla Public License
// v. 2.0. If a copy of the MPL was not distributed with this file, You can
// obtain one at http://mozilla.org/MPL/2.0/
#ifndef IGL_EXAMPLE_FUN_H
#define IGL_EXAMPLE_FUN_H

#include "igl_inline.h"

namespace igl
{
// This is an example of a function, it takes a templated parameter and
// shovels it into cout
//
// Input:
//   input  some input of a Printable type
// Returns true for the sake of returning something
template <typename Printable>
IGL_INLINE bool example_fun(const Printable & input);
}

#ifndef IGL_STATIC_LIBRARY
#  include "example_fun.cpp"
#endif

#endif


#### example_fun.cpp¶

// This file is part of libigl, a simple c++ geometry processing library.
//
//
// This Source Code Form is subject to the terms of the Mozilla Public License
// v. 2.0. If a copy of the MPL was not distributed with this file, You can
// obtain one at http://mozilla.org/MPL/2.0/
#include "igl/example_fun.h"
#include <iostream>

template <typename Printable>
IGL_INLINE bool igl::example_fun(const Printable & input)
{
using namespace std;
cout<<"example_fun: "<<input<<endl;
return true;
}

#ifdef IGL_STATIC_LIBRARY
template bool igl::example_fun<double>(const double& input);
template bool igl::example_fun<int>(const int& input);
#endif


### Avoid static “helper” functions¶

Strive to encapsulate sub-functions that could possibly be useful outside of the implementation of your current function. This might mean abstracting the interface a bit. If it doesn’t dramatically effect performance then create a new pair of .h/.cpp files with this sub-function.

#### Lambda functions¶

If encapsulation in a separate file is not possible or does not make sense, then avoid crowding the namespace by creating lambda functions within the function implementation.

These lambda functions must still be documented with clear input and output arguments. Avoid using full capturing of all automatic variables: do not use [&] or [=]. Rather specify each captured variable individually.

### Avoid “helper” classes¶

Libigl is built around the high-performance paradigm of “struct of arrays” rather than “array of structs”. The way we achieve this is to avoid classes and pass “basic types” directly. The price we pay is long function interfaces, but this increases code reuse dramatically. A “basic type” in our context is a Eigen type, stl type, or basic C type.

Each function prototype should be well documented in its corresponding .h header file. A typical documentation consists of four parts:

// [A human readable description of what the function does.]
//
// Inputs:
//   [variable name of first (const) input]   [dimensions and description of
//     this input variable]
//   [variable name of second (const) input]   [dimensions and description of
//     this input variable]
//   ...
// Outputs:
//   [variable name of first output ]   [dimensions and description of this
//     output variable]
//   [variable name of second output ]   [dimensions and description of this
//     output variable]
//   ...
// Returns [description of return value]


### Example¶

For example the header barycenter.h

// Computes the barycenter of every simplex
//
// Inputs:
//   V  #V by dim matrix of vertex coordinates
//   F  #F by simplex_size  matrix of indices of simplex corners into V
// Output:
//   BC  #F by dim matrix of 3d vertices
//


## Const inputs¶

All input parameters should be demarcated const. If an input is also an output than consider exposing two parameters (one const) or be sure to list the variable under both // Inputs: and // Outputs: in the header comments.

## Reference parameters¶

All but simple types should be passed by reference (e.g. Matrix & mat) rather than pointers (e.g. Matrix * mat) or value (e.g. Matrix mat).

## Returns vs output parameters¶

All functions should be implemented with at least one overload that has a void or simple return type (e.g. bool on success/failure). With this implementation its then possible to write an overload that returns a single output. Please see Templating with Eigen.

For example:

template <typename Atype>
void adjacency_matrix(const ... & F, Eigen::SparseMatrix<AType> & A);

template <typename Atype>


## Templating with Eigen¶

Functions taking Eigen dense matrices/arrays as inputs and outputs (but not return arguments), should template on top of Eigen::MatrixBase. Each parameter should be derived using its own template.

For example,

template <typename DerivedV, typename DerivedF, typename DerivedBC>
void barycenter(
const Eigen::MatrixBase<DerivedV> & V,
const Eigen::MatrixBase<DerivedF> & F,
const Eigen::MatrixBase<DerivedBC> & BC);


The Derived* template encodes the scalar type (e.g. double, int), the number of rows and cols at compile time, and the data storage (Row-major vs. column-major).

Returning Eigen types is discouraged. In cases where the size and scalar type are a fixed and matching function of an input Derived* template, then return that Derived* type. Do not return Eigen::PlainObjectBase<...> types. For example, this function scales fits a given set of points to the unit cube. The return is a new set of vertex positions so its type should match that of the input points:

template <typename DerivedV>
void DerivedV fit_to_unit_cube(const Eigen::PlainObjectBase<DerivedV> & V);


To implement this function, it is required to implement a more generic output-argument version and call that. So a full implementation looks like:

In igl/fit_in_unit_cube.h:

template <typename DerivedV, typename DerivedW>
void fit_to_unit_cube(
const Eigen::MatrixBase<DerivedV> & V,
Eigen::PlainObjectBase<DerivedW> & W);
template <typename DerivedV>
void DerivedV fit_to_unit_cube(const Eigen::PlainObjectBase<DerivedV> & V);


In igl/fit_in_unit_cube.cpp:

template <typename DerivedV, typename DerivedW>
void fit_to_unit_cube(
const Eigen::MatrixBase<DerivedV> & V,
Eigen::PlainObjectBase<DerivedW> & W)
{
W = (V.rowwise()-V.colwise().minCoeff()).array() /
(V.maxCoeff()-V.minCoeff());
}

template <typename DerivedV>
void DerivedV fit_to_unit_cube(const Eigen::MatrixBase<DerivedV> & V)
{
DerivedV W;
fit_to_unit_cube(V,W);
return W;
}


Notice that W is declared as a DerivedV type and not Eigen::PlainObjectBase<DerivedV> type.

Note: Not all functions are suitable for returning Eigen types. For example igl::barycenter above outputs a #F by dim list of barycenters. Returning a DerivedV type would be inappropriate since the number of rows in DerivedV will be #V and may not match the number of rows in DerivedF (#F).

## Function naming conventions¶

Functions (and thus also files) should have simple, descriptive names using lowercase letters and underscores between words. Avoid unnecessary prefaces. For example, instead of compute_adjacency_matrix, construct_adjacency_matrix, extract_adjacency_matrix, get_adjacency_matrix, or set_adjacency_matrix just call the function adjacency_matrix.

## Variable naming conventions¶

Libigl prefers short (even single character) variable names with heavy documentation in the comments in the header file or above the declaration of the function. When possible use V to mean a list of vertex positions and F to mean a list of faces/triangles.

## Class naming conventions¶

Classes should be avoided. When naming a class use CamelCase (e.g. SortableRow.h).

## Enum naming conversion¶

Enums types should be placed in the appropriate igl:: namespace and should be named in CamelCase (e.g. igl::SolverStatus) and instances should be named in ALL_CAPS with underscores between words and prefaced with the name of the enum. For example:

namespace igl
{
enum SolverStatus
{
// Good
SOLVER_STATUS_CONVERGED = 0,
// OK
SOLVER_STATUS_MAX_ITER = 1,
SOLVER_STATUS_ERROR = 2,
NUM_SOLVER_STATUSES = 3,
};
};


### Exception for file IO¶

For legacy reasons, file reading and writing functions use a different naming convention. A functions reading a .xyz file should be named readXYZ and a function writing .xyz files should be names writeXYZ.

## using namespace ... in global scope¶

Writing using namespace std;, using namespace Eigen; etc. outside of a global scope is strictly forbidden. Place these lines at the top of each function instead.

## Namespaces and external dependencies¶

Functions in the main library (directly in include/igl) should only depend on Eigen and stl. These functions should have the igl:: namespace.

Functions with other dependencies should be placed into appropriate sub-directories (e.g. if myfunction depends on tetgen then create igl/copyleft/tetgen/myfunction.h and igl/copyleft/tetgen/myfunction.cpp and give the function the namespace igl::copyleft::tetgen::myfunction.

### copyleft subdirectory/namespace¶

Dependencies that require users of libigl to release their projects open source (e.g. GPL) are considered aggressively “copyleft” and should be placed in the include/igl/copyleft/ sub-directory and igl::copyleft:: namespace.

## Assertions¶

Be generous with assertions and always identify the assertion with strings:

assert(m < n && "m must be less than n");


## ifndef include guard¶

Every header file should be wrapped in an #ifndef compiler directive. The name of the guard should be in direct correspondence with the path of the .h file. For example, include/igl/copyleft/tetgen/tetrahedralize.h should be

#ifndef IGL_COPYLEFT_TETGEN_TETRAHEDRALIZE_H
#define IGL_COPYLEFT_TETGEN_TETRAHEDRALIZE_H
...
#endif


## Spaces vs. tabs indentation¶

Do not use tabs. Use 2 spaces for each indentation level.

## Max line length¶

Limit lines to 80 characters. Break up long lines into many operations (this also helps performance).

## Include order¶

#include directives at the top of a .h or .cpp file should be sorted according to a simple principle: place headers of files most likely to be edited by you first. This means for include/igl/copyleft/tetgen/tetrahedralize.cpp you might see

// [Includes of headers in this directory]
#include "tetrahedralize.h"
#include "mesh_to_tetgenio.h"
#include "tetgenio_to_tetmesh.h"
// [Includes of headers in this project]
#include "../../matrix_to_list.h"
#include "../../list_to_matrix.h"
#include "../../boundary_facets.h"
// [Includes of headers of related projects]
#include <Eigen/Core>
// [Includes of headers of standard libraries]
#include <cassert>
#include <iostream>


## Placement of includes¶

Whenever possible #include directives should be placed in the .cpp implementation file rather than the .h header file.

## Warnings¶

Code should compile without firing any warnings.

### An Exception¶

The only exception is for the use of the deprecated Eigen::DynamicSparseMatrix in core sub-routines (e.g. igl::cat). This class is still supported and faster than the standard, non-deprecated Eigen implementation so we’re keeping it as long as possible and profitable.