XQuery - wrong indexes in substring after reverse-string function use

Im trying to implement base64 coding in a very simple way. In my approach (lets for a second put away whether its appropriate or not) I need to reverse strings and then concate them. After that this concated string is used in substring function. Strings are joined properly but when I use substring basex seems to lose it.

Funny thing is substring works for well for all indexes starting at 8. So substring($string, 1, 8) and higher gives correct output. But everything below that is messed up. Starting with one disappeared number: substring($string, 1, 7 (and below) ) results in 6 length string. Moreover substring can start only with 1st or 0 index. Anything greater results in empty return.

declare variable $array := [];

declare function bs:encode
  ( $input as xs:string ) {
    bs:integer-to-binary(string-to-codepoints($input), "", $array)
} ;

declare function bs:integer-to-binary
  ( $input as xs:integer*, $string as xs:string, $array as array(xs:string) ) {
    let $strings :=
    for $i in $input
    return
    if ($i != 0)
    then if ($i mod 2 = 0)
         then  bs:integer-to-binary(xs:integer($i div 2), concat($string, 0), $array)
         else bs:integer-to-binary(xs:integer($i div 2), concat($string, 1), $array) 
    else if ($i <= 0) 
    then array:append($array, $string)
    return bs:check-if-eight($strings)
 } ;

 declare function bs:check-if-eight
  ( $strings as item()+ ) {
    let $fullBinary :=
    for $string in $strings
    return if (string-length($string) < 8)
    then bs:check-if-eight(concat($string, 0))
    else $string (: add as private below :)
    return bs:concat-strings($fullBinary)
 } ;

declare function bs:concat-strings
  ( $strings as item()+ ) {
    let $firstStringToConcat := functx:reverse-string($strings[position() = 1])
    let $secondStringToConcat := functx:reverse-string($strings[position() = 2])
    let $thirdStringToConcat := functx:reverse-string($strings[position() = 3])
    let $concat :=
                    concat
                          ($firstStringToConcat, 
                           $secondStringToConcat, 
                           $thirdStringToConcat)
(: this returns correct string of binary value for Cat word :)
                           return bs:divide-into-six($concat)
 } ;  

declare function bs:divide-into-six
   ( $binaryString as xs:string) {
     let $sixBitString := substring($binaryString, 1, 6)

(: this should return 010000 instead i get 000100 which is not even in $binaryString at all :)     

     return $sixBitString
 } ;

 bs:encode("Cat")

I expect first six letters from string (010000) instead I get some random sequence I guess (00100). The whole module is meant to encode strings into base64 format but for now (the part i uploaded) should just throw first six bits for 'C'

Solution

Alright so I figured it out I guess.

First of all in function concat-strings I changed concat to fn:string-join. It allowed me to pass as an argument symbol that separates joined strings.

declare function bs:concat-strings   ( $strings as item()+ )  {
let $firstStringToConcat := xs:string(functx:reverse-string($strings[position() = 1]))
let $secondStringToConcat := xs:string(functx:reverse-string($strings[position() = 2]))
let $thirdStringToConcat := xs:string(functx:reverse-string($strings[position() = 3]))
let $concat :=
                ****fn:string-join(****
                      ($firstStringToConcat, 
                       $secondStringToConcat, 
                       $thirdStringToConcat),****'X'****)
                       return bs:divide-into-six($concat)  } ;

I saw that my input looked like this: XXXXXXXX01000011XXXXXXXXXXXXXXXXX01100001XXXXXXXXXXXXXXXXX01110100XXXXXXXX

Obviously it had to looping somewhere without clear for loop and as I novice to Xquery i must have been missed that. And indeed. I found it in check-if-eight function:

>  declare function bs:check-if-eight   ( $strings as item()+ ) {
>     **let $fullBinary :=**
>     for $string in $strings
>     return if (string-length($string) < 8)
>            then bs:check-if-eight(concat($string, 0))
>            else $string (: add as private below :)
>     **return bs:concat-strings($fullBinary)**  } ;

Despite being above FOR keyword, $fullBinary variable was in a loop and produced empty spaces(?) and it was clearly shown when i used X as a separator. DISCLAIMER: I thought about this before and used functx:trim but for some reason it doesnt work like I expected. So it might not for you too if having similar issue.

At this point it was clear that let $fullBinary cannot be bided in FLWR statement at least can't trigger concat-strings function. I changed it and now it produces only string and now im trying to figure out new sequence of running whole module but I think the main problem here is solved.