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* [W-data format concept](wdata format concept)
* [Tags](Tags) * [Tags](Tags)
* [Data types](Data types) * [Data types](Data types)
* [C examples](C examples)
* [Python interface](https://hg.iscimath.org/forbes-group/wdata)
* [Tools](tools)
# Introduction
**W-data** was designed to satisfy the following requirements:
* Binary data is stored in a **conceptually easy format** that allows a variety of tools/languages to be used.
* Format provides storage for data with time stepping (frames / measurements / cycles).
* Data format is suitable for parallel processing (preferably with MPI I/O).
* Data is easy for processing via [VisIt](https://visit-dav.github.io/visit-website/index.html).
* It provides the extensible framework - new variables can be created and easily added to the existing dataset.
* Data is convenient for copy between computing systems.
* It allows for easy extraction / copy of selected variables
# W-data format concept
Data set will consist of set of files, example:
```bash
test.wtxt # metadata file, this one should be indicated when opening in VisIt
test_density_a.wdat # binary file with data
test_delta.wdat # binary file with data
test_current_a.wdat # binary file with data
```
Content of test.wtxt may look like:
```bash
# Comments with additional info about data set
# Comments are ignored when reading by parser
nx 24 # lattice
ny 28 # lattice
nz 32 # lattice
dx 1 # spacing
dy 1 # spacing
dz 1 # spacing
datadim 3 # dimension of block size: 1=nx, 2=nx*ny, 3=nx*ny*nz
prefix test # prefix for files belonging to this data set, binary files have names `prefix_variable-name.format`
cycles 10 # number of cycles (measurements)
t0 0 # time value for the first cycle
dt -1 # time interval between cycles. If `dt` negative then time step is varying and is read from binary file `prefix__t.wdata`
# variables
# tag name type unit format
var density_a real none wdat
var delta complex none wdat
var current_a vector none wdat
# links
# tag name link-to
link density_b density_a
link current_b current_a
# consts
# tag name value unit
const eF 0.5 MeV
const kF 1 1/fm
```
According to our experience, three types of variables (`real`, `complex`, `vector`) are sufficient and cover more less 90% of applications. However we have implemented a single precision (`float` in C notation) types denoted as `real4`, `complex8` and `vector4`, to be consistent with the notation one can use `real8`, `complex16` or `vector8` exchengable for `real`, `complex`, `vector`.
Binary files store data as row arrays, called datablocks:
![datablocks](uploads/41c52d993d9e6cda930a309e0e16d9ac/datablocks.png)
The size of datablock depends on variable type and data dimensionality.
* *real*/*real8*: `blocksize=blocklength*8 Bytes`
* *real4*: `blocksize=blocklength*4 Bytes`
* *complex*/*complex16*: `blocksize=blocklength*16 Bytes`
* *complex8*: `blocksize=blocklength*8 Bytes`
* *vector*/*vector8*: `blocksize=blocklength*3*8 Bytes`
* *vector4*: `blocksize=blocklength*3*4 Bytes`
where `blocklength` is
* for datadim=3: `blocklength=nx*ny*nz`
* for datadim=2: `blocklength=nx*ny`
* for datadim=1: `blocklength=nx`
Note that for vector variables we use following storage pattern:
![vecvar](uploads/64a47839f226cb58a60f81edabdf01eb/vecvar.png)
To compute time associated with given cycle we use the `wdata_set_time()` function. Which takes as parameters `wdata_metadata` struct, `icycle` current number of cycle and `current_time` which will be calculated.
!Note
If `dt` is positive (to be precise, non negative) then we assume evenly distributed times, according to formula: `current_time=t0 + icycle*dt`. Hovewer if `dt` is negative then `current_time` parameter has to be calculated by the user and parsed to the function in which binary file is created and the value is stored. For more information see [Example](Examples/Write#t_varying).
Let's get back to the `*wtxt` file.
W-data format allows for the representation of the following elements:
* *variable*:
Each variable is represented by the binary file of name `prefix_varname.format`. The variable description has following format:
`var name type unit format`
Following formats are allowed:
* `wdat`: default format for WSLDA codes. Binary files contain row data (no header).
* `dpca`: (*deprecated*) previous format of cold atomic codes. Binary file contains header of size 68B where additional info about file content is stored. For this format *wdata* lib provides only reading functionality.
* `npy`: binary files are *numpy* arrays. **Functionality under construction**
In order to switch WSLDA codes to writing in this format add to input file:
`dataformat npy`
* *link*:
It is an alternative name for a given variable. In the case of `WSLDA` codes in many cases, we do the computation for systems that exhibit some symmetries, like spin symmetry. Then densities for spin-a and spin-b particles are exactly the same. In order to save disk space we can save only one of them:
`var density_a real none wdat`
and for another one set link:
`link density_b density_a`
* *constant*:
Typically besides variables, we have some constants that are useful during the data analysis process. For example, when making plots it is convenient to express variables in dimensionless form, like delta/eF. To provide user info what are values of selected constants we use `const` field:
`const eF 0.5`
# W-data C library
Folder [lib/wdata](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/tree/public/lib/wdata) contains library that provides support for w-data processing.
List of examples demonstrating how to use this library are presented below. One can see more detailed descritption of each of it in [Examples](Examples/Examples)
* [example-write.c](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/tree/public/lib/wdata/c-examples/example-write.c): code creates artificial set of variables and writes them do wdat files.
* [example-write-many.c](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/tree/public/lib/wdata/c-examples/example-write-many.c): code creates artificial set of variables and writes them do wdat files. The differnence between the above is the non repeating parts of the code, however it introduces some limitations.
* [example-write-many-t_varying.c](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/tree/public/lib/wdata/c-examples/example-write-many-t_varying.c): code creates artificial set of variables and writes them do wdat files.
* [example-read.c](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/tree/public/lib/wdata/c-examples/example-read.c): code reads data from wdat files (created by [example-write.c](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/tree/public/lib/wdata/example-write.c)).
* [example-addvar.c](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/tree/public/lib/wdata/c-examples/example-addvar.c): code adds new variable to existing data set (created by [example-write.c](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/tree/public/lib/wdata/example-write.c)).
In order to compile examples (optionally you will need to modify Makefile):
```bash
[wtools@dell cold-atoms]$ cd lib/wdata/
[wtools@dell lib/wdata]$ make
```
# W-data python library
See [PyPi W-data Format](https://pypi.org/project/wdata/)
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