% http://www.tondering.dk/claus/cal/week.php#calcweekno
% вычисляем номер недели
\int_new:N \l_week_number_year_int
\int_new:N \l_week_number_month_int
\int_new:N \l_week_number_day_int
\int_new:N \l_week_number_a_int
\int_new:N \l_week_number_b_int
\int_new:N \l_week_number_c_int
\int_new:N \l_week_number_s_int
\int_new:N \l_week_number_e_int
\int_new:N \l_week_number_f_int
\int_new:N \l_week_number_g_int
\int_new:N \l_week_number_d_int
\int_new:N \l_week_number_n_int
\int_new:N \l_week_number_W_int

\cs_new:Nn \week_number:nnn {

  \int_set:Nn \l_week_number_year_int { #1 }
  \int_set:Nn \l_week_number_month_int { #2 }
  \int_set:Nn \l_week_number_day_int { #3 }

  \int_compare:nNnTF { \l_week_number_month_int } < { 3 } % jan or feb
  { % true

    \int_set:Nn \l_week_number_a_int { \l_week_number_year_int - 1 }

    \int_set:Nn \l_week_number_b_int {
      \int_div_truncate:nn { \l_week_number_a_int } { 4 }
      - \int_div_truncate:nn { \l_week_number_a_int } { 100 }
      + \int_div_truncate:nn { \l_week_number_a_int } { 400 }
    }

    \int_set:Nn \l_week_number_c_int {
      \int_div_truncate:nn { \l_week_number_a_int - 1 } { 4 }
      - \int_div_truncate:nn { \l_week_number_a_int - 1 } { 100 }
      + \int_div_truncate:nn { \l_week_number_a_int - 1 } { 400 }
    }

    \int_set:Nn \l_week_number_s_int {
      \l_week_number_b_int - \l_week_number_c_int }

    \int_zero:N \l_week_number_e_int

    \int_set:Nn \l_week_number_f_int { \l_week_number_day_int - 1
      + 31 * ( \l_week_number_month_int - 1 ) }

  } % end true
  { % false

    \int_set_eq:NN \l_week_number_a_int \l_week_number_year_int

    \int_set:Nn \l_week_number_b_int {
      \int_div_truncate:nn { \l_week_number_a_int } { 4 }
      - \int_div_truncate:nn { \l_week_number_a_int } { 100 }
      + \int_div_truncate:nn { \l_week_number_a_int } { 400 }
    }

    \int_set:Nn \l_week_number_c_int {
      \int_div_truncate:nn { \l_week_number_a_int - 1 } { 4 }
      - \int_div_truncate:nn { \l_week_number_a_int - 1 } { 100 }
      + \int_div_truncate:nn { \l_week_number_a_int - 1 } { 400 }
    }

    \int_set:Nn \l_week_number_s_int {
      \l_week_number_b_int - \l_week_number_c_int }

    \int_set:Nn \l_week_number_e_int { \l_week_number_s_int + 1 }

    \int_set:Nn \l_week_number_f_int { \l_week_number_day_int
      + \int_div_truncate:nn {
        153 * ( \l_week_number_month_int - 3 ) + 2 } { 5 }
      + 58 + \l_week_number_s_int }

  } % end false

  \int_set:Nn \l_week_number_g_int {
    \int_mod:nn { \l_week_number_a_int + \l_week_number_b_int } { 7 }  }

  \int_set:Nn \l_week_number_d_int {
    \int_mod:nn { \l_week_number_f_int + \l_week_number_g_int
      - \l_week_number_e_int } { 7 }  }

  \int_set:Nn \l_week_number_n_int {
    \l_week_number_f_int + 3 - \l_week_number_d_int }

  \int_compare:nNnTF { \l_week_number_n_int } < { 0 }
  { %true

    \int_set:Nn \l_week_number_W_int { 53
      - \int_div_truncate:nn { \l_week_number_g_int
        - \l_week_number_s_int } { 5 } }

  } % end true
  { % false

    \int_compare:nNnTF { \l_week_number_n_int } > { 364
      + \l_week_number_s_int }
    { % true

      \int_set:Nn \l_week_number_W_int { 1 }

    } % end true
    { % false

      \int_set:Nn \l_week_number_W_int { \int_div_truncate:nn {
          \l_week_number_n_int } { 7 } + 1 }

    } % end false

  } % end false

}

