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# Introduction
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**W-data** was designed to satisfy the following requirements:
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* Binary data is stored in a **conceptually easy format** that allows a variety of tools/languages to be used.
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* Format provides storage for data with time stepping (frames/measurements/cycles).
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* Data format is suitable for parallel processing (preferably with MPI I/O).
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* Data is easy for processing via [VisIt](https://visit-dav.github.io/visit-website/index.html).
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* It provides an extensible framework - new variables can be created and easily added to the existing dataset.
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* Data is convenient for copy between computing systems.
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* It allows for easy extraction/copy of selected variables
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# W-data format concept
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Data set will consist of a set of files, for example:
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```bash
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test.wtxt # metadata file, this one should be indicated when opening in VisIt
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test_density_a.wdat # binary file with data
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test_delta.wdat # binary file with data
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test_current_a.wdat # binary file with data
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```
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Content of test.wtxt may look like:
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```bash
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# Comments with additional info about the data set
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# Comments are ignored when reading by the parser
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nx 24 # lattice
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ny 28 # lattice
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nz 32 # lattice
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dx 1 # spacing
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dy 1 # spacing
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dz 1 # spacing
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datadim 3 # dimension of block size: 1=nx, 2=nx*ny, 3=nx*ny*nz
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prefix test # prefix for files belonging to this data set, binary files have names `prefix_variable-name.format`
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cycles 10 # number of cycles (measurements)
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t0 0 # time value for the first cycle
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dt -1 # time interval between cycles. If `dt` negative then time step is varying and is read from binary file `prefix__t.wdata`
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# variables
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# tag name type unit format
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var density_a real none wdat
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var delta complex none wdat
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var current_a vector none wdat
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# links
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# tag name link-to
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link density_b density_a
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link current_b current_a
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# consts
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# tag name value unit
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const eF 0.5 MeV
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const kF 1 1/fm
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```
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According to our experience, three [types of variables](https://gitlab.fizyka.pw.edu.pl/wtools/wdata/-/wikis/Data%20types) (`real`, `complex`, `vector`) are sufficient and cover more than 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` exchangeable for `real`, `complex`, `vector`.
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Binary files store data as row arrays, called datablocks:
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The size of `datablock` depends on variable type and data dimensionality.
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* *real*/*real8*: `blocksize=blocklength*8 Bytes`
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* *real4*: `blocksize=blocklength*4 Bytes`
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* *complex*/*complex16*: `blocksize=blocklength*16 Bytes`
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* *complex8*: `blocksize=blocklength*8 Bytes`
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* *vector*/*vector8*: `blocksize=blocklength*3*8 Bytes`
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* *vector4*: `blocksize=blocklength*3*4 Bytes`
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where `blocklength` is
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* for datadim=3: `blocklength=nx*ny*nz`
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* for datadim=2: `blocklength=nx*ny`
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* for datadim=1: `blocklength=nx`
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Note that for vector variables we use following storage pattern:
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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.
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!Note
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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).
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Let's get back to the `*wtxt` file.
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W-data format allows for the representation of the following elements:
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* *variable*:
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Each variable is represented by the binary file of name `prefix_varname.format`. The variable description has the following format:
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`var name type unit format`
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Following formats are allowed:
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* `wdat`: default format for WSLDA codes. Binary files contain row data (no header).
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* `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.
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* `npy`: binary files are *numpy* arrays. **Functionality under construction**
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In order to switch WSLDA codes to writing in this format add to input file:
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`dataformat npy`
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* *link*:
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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:
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`var density_a real none wdat`
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and for another one set link:
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`link density_b density_a`
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* *constant*:
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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:
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`const eF 0.5`
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See [here](https://gitlab.fizyka.pw.edu.pl/wtools/wdata/-/wikis/Tags) for complete list of tags. |
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\ No newline at end of file |
