(xcor module)=
# **xcor module**

This optional module allows to compute cross-correlation from a 2D ndarray (or a A1Section) containing a DAS section.
The core function is written in fortran and parallelized using openMP.

Instead of processing cross-correlation using individual records, the __xcor__  module uses directly the 2D float array containing DAS data.
A pre-processing step to select the couple of traces to be cross-correlated is achieved by calling the [register_couple()](#register_couple) function. A number of pre-processing are built-in and available (one-bit, whitenning, bandpass filtering, ...) by calling the [register_par()](#register_par) function.

## Requirements
This module requires:

- C and fortran compilers (e.g gfortran) and make (gmake is ok)
- numpy 
- one of the **MKL** or **OpenBlas**+**FFTW3** libraries. 

For linux, these are standard and free libraries that can be easily download using apt-get.
Try;
apt-get install libfftw3
apt-get install libfftw3-dev
apt-get install libopenblas-dev

For mac with __arm__ cpu (M1, M2, ...) using OpenBlas and FFTW is mandatory, they can be get from __brew__. Getting a fortran compiler usually requires to install Apple xcode.


## Download and installation
0) Install (tar.gz archive) the xcor module in the same directory as a1das from the gitlab repository (e.g. [in2p3 gitlab](https://a1das-v2-olivier-coutant-fa984751404466eb2f1c34128bcd5750d522ff.pages.in2p3.fr/A1-XCORPY.tar.gz)), you should see a directory named A1-XCORPY.
1) Check that the required libraries are installed and find their locations (/opt/..., /usr/local/lib, /usr/lib/x86_64-linux-gnu, /opt/homebrew, ....).
You should look for **libopenblas.a**, **libfftw3.a** or **libmkl**
2) change to A1-XCORPY directory and under a python environment, run the shell **configure_var.sh**, this will create the __Makefile.python__ file
3) select one of the proposed Makefile.inc.???, edit and change according to your needs, then copy it as **Makefile.inc**
4) run the compilation step by typing "**make python**"

This should have installed the binary module under the **a1das** directory. If you have already installed the a1das package you can either 

a) uninstall and reinstall it:
python -m pip uninstall a1das
pip install . 

b) or copy the _a1xcorPy.cpython.?? file in the directory where a1das has been installed (e.g. 

## Usage

```{eval-rst}
 .. autofunction:: xcor.compute_xcorr
```
(register_couple)=
```{eval-rst}
 .. autofunction:: xcor.register_couple
```
```{eval-rst}
 .. autofunction:: xcor.reset_couple
```
(register_par)=
```{eval-rst}
 .. autofunction:: xcor.register_par
```
```{eval-rst}
 .. autofunction:: xcor.list_processing
```
```{eval-rst}
 .. autofunction:: xcor.trace_index_list
```
```{eval-rst}
 .. autofunction:: xcor.sort_xcorr
```
```{eval-rst}
 .. autofunction:: xcor.get_nxcorr
```