% datemultical-engine-gr.def
%
% datemultical: Named dates with multi-calendar formatting for LaTeX.
%
% Copyright (C) 2026 Amer Amikhteh
%
% This file is part of the datemultical-engine package.
%
% This work is free software: you may redistribute it and/or modify
% it under the terms of the LaTeX Project Public License as published
% by the LaTeX Project, either version 1.3c of the License, or
% (at your option) any later version.
%
% This work is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty
% of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
%
% The full license text is available at:
% https://www.latex-project.org/lppl/
%
% This work is "maintained" by Amer Amikhteh.
%
% The current maintainer of this work is Amer Amikhteh.


\ProvidesFile{datemultical-engine-gr.def}
[2026/09/17 v0.1.0 Gregorian calendar rules for the TeX backend]

% =================================================
% Gregorian -> JDN
% =================================================

\cs_new:Npn \dmc_gr_to_jdn:nnn #1#2#3
{
  \int_eval:n
  {
    \int_div_truncate:nn
    {
      1461 *
      (
      #1 + 4800
      + \int_div_truncate:nn { #2 - 14 } {12}
      )
    }
    {4}
    +
    \int_div_truncate:nn
    {
      367 *
      (
      #2 - 2
      - 12 * \int_div_truncate:nn { #2 - 14 } {12}
      )
    }
    {12}
    -
    \int_div_truncate:nn
    {
      3 *
      \int_div_truncate:nn
      {
        #1 + 4900
        + \int_div_truncate:nn { #2 - 14 } {12}
      }
      {100}
    }
    {4}
    + #3 - 32075
  }
}

% =================================================
% Gregorian JDN conversion helpers
% =================================================

\cs_new:Npn \dmc_gr_a:n #1
{
  \int_eval:n
  {
    #1 + 68569
  }
}

\cs_new:Npn \dmc_gr_b:n #1
{
  \int_div_truncate:nn
  {
    4 * \dmc_gr_a:n {#1}
  }
  {146097}
}

\cs_new:Npn \dmc_gr_c:n #1
{
  \int_eval:n
  {
    \dmc_gr_a:n {#1}
    -
    \int_div_truncate:nn
    {
      146097 * \dmc_gr_b:n {#1} + 3
    }
    {4}
  }
}

\cs_new:Npn \dmc_gr_d:n #1
{
  \int_div_truncate:nn
  {
    4000 * ( \dmc_gr_c:n {#1} + 1 )
  }
  {1461001}
}

\cs_new:Npn \dmc_gr_e:n #1
{
  \int_div_truncate:nn
  {
    80 *
    (
    \dmc_gr_c:n {#1}
    -
    \int_div_truncate:nn
    {
      1461 * \dmc_gr_d:n {#1}
    }
    {4}
    + 31
    )
  }
  {2447}
}

\cs_new:Npn \dmc_gr_tmp:n #1
{
  \int_div_truncate:nn
  {
    \dmc_gr_e:n {#1}
  }
  {11}
}

% =================================================
% JDN -> Gregorian year
% Expandable
% =================================================

\cs_new:Npn \dmc_jdn_to_gr_year:n #1
{
  \int_eval:n
  {
    100 * ( \dmc_gr_b:n {#1} - 49 )
    + \dmc_gr_d:n {#1}
    + \dmc_gr_tmp:n {#1}
  }
}

% =================================================
% JDN -> Gregorian month
% Expandable
% =================================================

\cs_new:Npn \dmc_jdn_to_gr_month:n #1
{
  \int_eval:n
  {
    \dmc_gr_e:n {#1}
    + 2
    - 12 * \dmc_gr_tmp:n {#1}
  }
}

% =================================================
% JDN -> Gregorian day
% Expandable
% =================================================

\cs_new:Npn \dmc_jdn_to_gr_day:n #1
{
  \int_eval:n
  {
    \dmc_gr_c:n {#1}
    -
    \int_div_truncate:nn
    {
      1461 * \dmc_gr_d:n {#1}
    }
    {4}
    + 31
    -
    \int_div_truncate:nn
    {
      2447 * \dmc_gr_e:n {#1}
    }
    {80}
  }
}

% =================================================
% native procedural JDN -> Gregorian interface
% =================================================

\cs_new_protected:Npn \dmc_jdn_to_gr:n #1
{
  \group_begin:

  \int_set:Nn \l_dmc_a_int
  { \dmc_gr_a:n {#1} }

  \int_set:Nn \l_dmc_b_int
  { \dmc_gr_b:n {#1} }

  \int_set:Nn \l_dmc_c_int
  { \dmc_gr_c:n {#1} }

  \int_set:Nn \l_dmc_e_int
  { \dmc_gr_e:n {#1} }

  \int_gset:Nn \g_dmc_gr_d_int
  { \dmc_jdn_to_gr_day:n {#1} }

  \int_gset:Nn \g_dmc_gr_m_int
  { \dmc_jdn_to_gr_month:n {#1} }

  \int_gset:Nn \g_dmc_gr_y_int
  { \dmc_jdn_to_gr_year:n {#1} }

  \group_end:
}

\endinput