# kdcount

kdcount is a simple API for brute force pair counting, there is a C interface and a Python interface. It uses a KDTree to prune the K-D spatial data; for each pair within a given distance D, a callback function is called; the user-defined callback function does the actual counting.

To cite kdcount, use the DOI via Zenodo

Periodic boundary is supported, and it is non-trivial.

The python interface is more complicated, and powerful:

- paircounting via :py:mod:`kdcount.correlate`
- clustering via Friend-of-Friend algorithm; :py:mod:`kdcount.cluster`

The calculation can be made parallel, if :py:mod:`sharedmem` is installed.

`easy_install --user sharedmem`

Refer to the to the API Reference at http://rainwoodman.github.io/kdcount

The time complexity is `O[(D / n) ** d]`

,
where n is number density. Each pair is opened.

Note that smarter algorithms exist (more or less, O(D / n log Dn), I may remembered it wrongly See Gary and Moore 2001. The smarter algorithm is internally implemented, but not very much tested, and undocumented; do not use it.

Unfortunately in cosmology we usually want to project the pair separation along parallel + perpendicular direction relative to a given observer -- in this case, the smarter algorithm become very difficult to implement.

The spatial complexity is a constant, as we make extensive use of callback functions