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

Question

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'

Answer 1

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.