expl3 - Counting the number of leading spaces of an argument

Question

How can I count the number of leading spaces and not all spaces as with the use of \str_count_spaces ?

Concretly I would like to read a content line by line and for each line of input I would use the indentation to build the corresponding output.

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn
    \seq_new:N \l__pmbc_words_seq
    \tl_new:N \l__pmbc_one_temp_tl

    \NewExpandableDocumentCommand{\nbleadingsp}{m} {
        \projetmbc_nbleadingsp:n { #1 }
    }

    \cs_new:Nn \projetmbc_nbleadingsp:n {
        % ...
    }
\ExplSyntaxOff


\begin{document}

\nbleadingsp{ 1space}

\nbleadingsp{  two spaces}

\nbleadingsp{  2 * 2    spaces}

\end{document}

Answer 1

As Phelype Oleinik and egreg has said, it's possible only by reading the argument verbatim, which forbides the use in another command.

In the following code, the argument is taken verbatim by use of the v specifier of xparse.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand { \nbleadingsp } { v } 
  { \__pmbc_nbleadingsp:n { #1 } }

\cs_new_protected:Npn \__pmbc_nbleadingsp:n #1
  {
    \int_zero:N \l_tmpa_int
    \str_map_inline:nn { #1 }
      {
        \str_if_eq:nnTF { ##1 } { ~ } 
          { \int_incr:N \l_tmpa_int }
          { \str_map_break: }
      }
    \int_use:N \l_tmpa_int
  }
\ExplSyntaxOff


\begin{document}

\nbleadingsp{ 1space}

\nbleadingsp{  two spaces}

\nbleadingsp{  2 * 2    spaces}

\nbleadingsp{    four    spaces }

\end{document}