Re: Lua filter to automatically tag keywords for TeX indexing

[John MacFarlane <fiddlosopher-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

> On Nov 2, 2022, at 6:20 PM, bapt a <auguieba-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> 
> Hi all,
> 
> I've started writing a technical book using Quarto markdown, which uses pandoc with Lua filters under the hood to produce a website as well as the publisher's pdf format (via LaTeX). 
> I quite like to keep the source document as plain as possible, and I'm wondering if I could avoid the use of [concept]{.index}, which gets turned into \index{concept}, and instead write a Lua filter with my custom list of keywords, and have pandoc automatically match them as they appear in the text. 
> As a proof of principle I wrote the following code (see below), which matches specific keywords, and reformats them as small-caps. I quickly realised that trailing punctuation, such as "concept, ..." will fail to match, so I'm using gsub to strip such punctuation before matching. It works, but I'm a bit worried:
> 
> - what's the overhead of such a filter, in practice? From what I understand, every single string element in the AST will be processed by gsub then tested for a match. Are Lua filters walking down the AST fast enough that I shouldn't worry about it? (as far as I can tell on small examples, it seems fine)

The AST walking is very fast.  See the benchmarks at the beginning of https://pandoc.org/lua-filters.html for one example.

> - assuming this idea is reasonable, I might want to do a few similar operations, e.g. reformatting program languages (as in this example code), wrapping keywords in \index{}, etc., and the exact format will often depend on the output target (html vs TeX etc.). Is there a better construct for this than successive if/else statements to look for matches? (I don't know much Lua)

In lua you can do

string.gusb(val, [“(%l*)”], function (word)
    if indexable[word] then
      .. whatever ..
    end
  end)

This will run the function on every group of letters in the matched string.
Here I’m assuming you have a lua table indexable that maps words to true, e.g.

{ cow: true, horse: true }

That will be much faster than iterating through an array as you’re doing here.

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