math in the <dim> element

[prjemian]

Some NXDL files use mathematics involving one or more symbols to describe a <dim /> element.

definitions/applications/NXtomophase.nxdl.xml

Line 162 in ed064a1

<dim index="1" value="nDarkFrames + nBrightFrames + nSampleFrame" />

definitions/contributed_definitions/NXsnshisto.nxdl.xml

Line 196 in ed064a1

<dim index="1" value="numtof + 1"/>

definitions/contributed_definitions/NXsnshisto.nxdl.xml

Line 314 in ed064a1

<dim index="1" value="numtimechannels + 1"/>

[prjemian]

It was suggested at the 2022 Code Camp that such math be expressed in a single symbol (in the symbol table) and that symbol be used in the <dim /> element. Such as:

	<symbols>
		<doc>symbolic array lengths to be coordinated between various fields</doc>
		<symbol name="nDarkFrames "><doc>number of dark frames </doc></symbol>
		<symbol name="nBrightFrames "><doc>number of bright frames </doc></symbol>
		<symbol name="nSampleFrame"><doc>number of sample frame </doc></symbol>
		<symbol name="nTotalFrames">
		  <doc>total number of frames</doc>
		  <math>nDarkFrames + nBrightFrames + nSampleFrame</math>
		</symbol>
	</symbols>

then later

    <dim index="1" value="nTotalFrames" />

[prjemian]

We would add a math element to the symbols element in the nxdl.xsd file. How should the math element be described (both in documentation and how it is prescribed in the XML Schema)?

[prjemian]

For example, that the math expression can be interpreted as javascript and the variables must be defined in the symbol table.

[prjemian]

@mkoennecke asked:

cnxvalidate is written in C, is there a library it can use?

@yayahjb suggests:

https://github.com/samthor/gumnut

[RussBerg]

A C library for processing math strings in EPICS is one used in the calc record
https://epics.anl.gov/EpicsDocumentation/AppDevManuals/RecordRef/Recordref-13.html

scrolling down a bit it discusses supported operators and some examples

[prjemian]

The EPICS library likely needs additional work to use in this context.

[RussBerg]

next code camp I could look at how much work this would be