How to manipulate file-paths - xquery

Question

I know this seems like a duplicate, and I am sure it more or less is ...
However, it really bugs me, and I cannot make anything of the posts before:
I am building a digital edition, utlizing TEI, XML, XSLT, (and probably existDB, maybe I switch to node/javascript).
I built a php-function that should transforme each file in a specified directory to html. (My xsl-file works well)
declare function app:XMLtoHTML-forAll ($node as node(), $model as map(*), $query as xs:string?){
let $ref := xs:string(request:get-parameter("document", ""))
let $xml := doc(concat("/db/apps/BookOfOrders/data/edition/",$ref))
let $xsl := doc("/db/apps/BookOfOrders/resources/xslt/xmlToHtml.xsl")
let $params :=
<parameters>
{for $p in request:get-parameter-names()
let $val := request:get-parameter($p,())
where not($p = ("document","directory","stylesheet"))
return
<param name="{$p}" value="{$val}"/>
}
</parameters>
return
transform:transform($xml, $xsl, $params)
};
There is a list of files in the apps/BookofOrders/data/edition/ named FolioX.html, where x is the page-number. (I'll probably change names to [FolioNumber].xml, but that's not the issue)
I am trying to make a text slider (so that when I open the page, a page is presented and further buttons are created, and I can slide to the right and read the rest of the pages).
I have a table of content, that is linked to the transformed files:
declare function app:toc($node as node(), $model as map(*)) {
for $doc in collection("/db/apps/BookOfOrders/data/edition")/tei:TEI
return
<li>{document-uri(root($doc))}</li>
};
I guess I am wondering on how to change the link inside to for example Folio29 to Folio30.
Can I take a part of the provided link and make the destination of a link flexible, similar but not identical to what I did in the toc-function above?
I'd be really happy if anyone could point me in the right direction.

Answer 1

Given an expression like document-uri(root($doc)) (perhaps more simply util:document-name($doc), since you're using eXist) that returns the path to (or filename of) the document ending in "FolioX", you just need to isolate X, then cast it as an integer so you can perform addition/subtraction on the value:
document-uri(root($doc)) => substring-after("Folio") => xs:integer()
util:document-name($doc) => substring-after("Folio") => xs:integer()
Then add 1, and you've got your next document. Subtract one, and you've got the previous
However, this could lead to broken links: Folio0 or Folio98 (assuming there are only 97). To avoid this, you might want to retrieve determine the complete list of Folios, find the current position, and then never hit 0 or 98:
let $this-folio := $doc => util:document-name()
let $collection := $doc => util:collection-name()
let $all-folios := xmldb:get-child-resources($collection)
(: sort the filenames using UCA Numeric collation to ensure Folio2 < Folio10.
: see https://www.w3.org/TR/xpath-functions-31/#uca-collations :)
let $sorted-folios := $all-folios => sort("?numeric=yes")
let $this-folio-n := index-of($all-folios, $this-folio)
let $prev-folio := if ($this-folio-n gt 1) then "Folio" || $this-folio-n - 1 else ()
let $next-folio := if ($this-folio-n lt count($all-folios)) then "Folio" || $this-folio-n + 1 else ()
return
<nav>
<prev>{$prev-folio}</prev>
<this>{"Folio" || $this-folio-n}</this>
<next>{$next-folio}</next>
</nav>