posix shell urlencode and urldecode that handles multiline strings from both stdin and from arguments

urlencode.sh

urlencode() (
    string=${*:-$(
        cat -
        printf x
    )}
    [ -n "$*" ] || string=${string%x}
    # Zero index, + 1 to start from 1 since sed starts from 1
    lines=$(($(printf %s "$string" | wc -l) + 1))
    lineno=1
    while [ $lineno -le $lines ]; do
        currline=$(printf %s "$string" | sed "${lineno}q;d")
        pos=1
        chars=$(printf %s "$currline" | wc -c)
        while [ $pos -le "$chars" ]; do
            c=$(printf %s "$currline" | cut -b$pos)
            case $c in
            [-_.~a-zA-Z0-9]) printf %c "$c" ;;
            *) printf %%%02X "'${c:-\n}'" ;;
            esac
            pos=$((pos + 1))
        done
        [ $lineno -eq $lines ] || printf %%0A
        lineno=$((lineno + 1))
    done
)

urldecode() {
    string=${*:-$(
        cat -
        printf x
    )}
    [ -n "$*" ] || string=${string%x}
    # Zero index, + 1 to start from 1 since sed starts from 1
    lines=$(($(printf %s "$string" | wc -l) + 1))
    lineno=1
    while [ $lineno -le $lines ]; do
        currline=$(printf %s "$string" | sed "${lineno}q;d")
        pos=1
        chars=$(printf %s "$currline" | wc -c)
        while [ $pos -le "$chars" ]; do
            c=$(printf %s "$currline" | cut -b$pos)
            case $c in
            \+) printf ' ' ;;
            %)
                # shellcheck disable=SC2059
                printf "$(
                    printf '\\%03o' "$(
                        printf %s "$currline" | cut -b$pos-$((pos + 2)) | sed s/%/0x/
                    )"
                )"
                pos=$((pos + 2))
                ;;
            *) printf %c "$c" ;;
            esac
            pos=$((pos + 1))
        done
        [ $lineno -eq $lines ] || printf '\n'
        lineno=$((lineno + 1))
    done
}