# Contributing to Corrfunc¶

Corrfunc is written in a very modular fashion with minimal interaction between the various calculations. The algorithm presented in Corrfunc is applicable to a broad-range of astrophysical problems, viz., any situation that requires looking at all objects around a target and performing some analysis with this group of objects.

Here are the basic steps to get your statistic into the Corrfunc package:

• Add exhaustive tests. The output of your statistic should exactly agree with a brute-force implementation (under double-precision). Look at test_periodic.c and test_nonperiodic.c under theory/tests/ for tests on simulation volumes. For mock catalogs, look at mocks/tests/tests_mocks.c.
• Add a python extension for the new statistic. This extension should reside in file theory/python_bindings/_countpairs.c or mocks/python_bindings/_countpairs_mocks.c for statistics relevant for simulations and mocks respectively. It is preferred to have the extension documented but not necessary.
• Add a call to this new extension in the python_bindings/call_correlation_functions*.py script.

Note

Different from corresponding script in Corrfunc/ directory.

• Add a python wrapper for the previous python extension. This wrapper should exist in Corrfunc/theory/ or Corrfunc/mocks/. Wrapper must have inline API docs.
• Add the new wrapper to __all__ in __init__.py within the relevant directory.
• Add an example call to this wrapper in Corrfunc/call_correlation_functions.py or Corrfunc/call_correlation_functions_mocks.py for simulations and mocks respectively.

Note

Different from corresponding script in python_bindings directory.

• Add the new wrapper to the API docs within ROOT_DIR/docs/source/theory_functions.rst or ROOT_DIR/docs/source/mocks_functions.rst.
• Add to the contributors list under ROOT_DIR/docs/source/development/contributors.rst.
• Submit pull request

Note

Please feel free to email the author or the Corrfunc Google Groups if you need help at any stage.

## Corrfunc Design¶

All of the algorithms in Corrfunc have the following components:

• Reading in data. Relevant routines are in the io/ directory with a mapping within io.c to handle the file format
• Creating the 3-D lattice structure. Relevant routines are in the utils/gridlink_impl.c.src and utils/gridlink_mocks.c.src. This lattice grids up the particle distribution on cell-sizes of rmax (the maximum search radius).

Note

The current lattice code duplicates the particle memory. If you need a lattice that does not duplicate the particle memory, then please email the author. Relevant code existed in Corrfunc but has been removed in the current incarnation.

• Setting up the OpenMP sections such that threads have local copies of histogram arrays. If OpenMP is not enabled, then this section should not produce any compilable code.
• Looping over all cells in the 3-D lattice and then looping over all neighbouring cells for each cell.
• For a pair of cells, hand over the two sets of arrays into a specialized kernel (count*kernel.c.src) for computing pairs.
• Aggregate the results, if OpenMP was enabled.

## Directory and file layout¶

• Codes that compute statistics on simulation volumes (Cartesian XYZ as input) go into a separate directory within theory
• Codes that compute statistics on mock catalogs (RA, DEC [CZ]) go into a separate directory within mocks
• Public API in a count*.h file. Corresponding C file simply dispatches to appropriate floating point implementation.
• Floating point implmentation in file count*_impl.c.src. This file is processed via sed to generate both single and double precision implementations.
• A kernel named count*kernels.c.src containing implementations for counting pairs on two sets of arrays. This kernel file is also preprocessed to produce both the single and double precision kernels.
• Tests go within tests directory under theory or mocks, as appropriate. For simulation routines, tests with and without periodic boundaries go into test_periodic.c and test_nonperiodic.c
• C code to generate the python extensions goes under python_bindings directory into the file _countpairs*.c
• Each python extension has a python wrapper within Corrfunc directory

## Coding Guidelines¶

### C guidelines¶

#### Code contents¶

• Always check for error conditions when calling a function
• If an error condition occurs when making an kernel/external library call, first call perror and then return the error status. If calling a wrapper from within Corrfunc, assume that perror has already been called and simply return the status. Clean up memory before returning status.
• Declare variables in the smallest possible scope.
• Add const qualifiers liberally
• There must not be any compiler warnings (with gcc6.0) under the given set of Warnings already enabled within common.mk. If the warning can not be avoided because of logic issues, then suppress the warning but note why that suppression is required. Warnings are treated as errors on the continuous integration platform (TRAVIS)
• Valgrind should not report any fixable memory or file leaks (memory leaks in OpenMP library, e.g., libgomp, are fine)

#### Style¶

The coding style is loosely based on Linux Kernel Guideline. These are recommended but not strictly enforced. However, note that if you do contribute code to Corrfunc, the style may get converted.

• Braces - Opening braces start at the same line, except for functions - Closing braces on new line - Even single line conditionals must have opening and closing braces
• Comments - Explanatory comments on top of code segment enclosed with /**/ - Inline comments must be single-line on the right
• Indentation is tab:=4 spaces
• Avoid typedef for structs and unions

### Python guidelines¶

• Follow the astropy python code guide
• Docs are in numpydocs format. Follow any of the wrapper routines in Corrfunc (which are, in turn, taken from halotools)