% Khitan Small Script special builder.
%
% The current TeX/fontspec shaping path does not reliably expose the font's
% native cluster substitution behaviour, so IMPE LaTeX System composes Khitan Small Script
% clusters explicitly from the encoded character stream.
%
% Model used here:
% - clusters are separated by spaces
% - Type A starts with two adjacent characters
% - Type B inserts U+16FE4 KHITAN SMALL SCRIPT FILLER after the first character
% - Type A lays out rows of pairs, with a final centred character if odd
% - Type B lays out a first centred character, then applies the Type A rule to
%   the remaining characters

\ExplSyntaxOn

\tl_const:Nn \c__font_khs_filler_tl { 𖿤 }

\seq_new:N \l__font_khs_cluster_seq
\seq_new:N \l__font_khs_char_seq
\seq_new:N \l__font_khs_line_seq
\tl_new:N \l__font_khs_current_line_tl
\tl_new:N \l__font_khs_content_tl
\tl_new:N \l__font_khs_style_family_tl
\tl_new:N \l__font_khs_style_series_tl
\tl_new:N \l__font_khs_style_shape_tl
\tl_new:N \l__font_khs_style_setup_tl
\dim_new:N \l__font_khs_target_width_dim
\dim_new:N \l__font_khs_line_width_dim
\dim_new:N \l__font_khs_cluster_measure_dim
\dim_new:N \l__font_khs_cell_wd_dim
\dim_new:N \l__font_khs_cell_ht_dim
\dim_new:N \l__font_khs_cluster_wd_dim
\dim_new:N \l__font_khs_row_gap_dim
\dim_new:N \l__font_khs_col_gap_dim
\dim_new:N \l__font_khs_space_gap_dim
\dim_new:N \l__font_khs_target_ht_dim
\dim_new:N \l__font_khs_actual_ht_dim
\dim_new:N \l__font_khs_raise_dim
\bool_new:N \l__font_khs_type_b_bool
\bool_new:N \l__font_khs_first_row_bool
\bool_new:N \l__font_khs_first_cluster_bool
\int_new:N \l__font_khs_run_max_rows_int
\int_new:N \l__font_khs_cluster_rows_int
\box_new:N \l__font_khs_measure_box
\box_new:N \l__font_khs_cluster_box

\cs_new_protected:Npn \__font_khs_reset_style_parse:
  {
    \tl_clear:N \l__font_khs_style_family_tl
    \tl_clear:N \l__font_khs_style_series_tl
    \tl_clear:N \l__font_khs_style_shape_tl
    \tl_clear:N \l__font_khs_style_setup_tl
  }

\cs_new_protected:Npn \__font_khs_apply_style_setup:
  {
    \tl_clear:N \l__font_khs_style_setup_tl
    \tl_if_blank:VF \l__font_khs_style_family_tl
      { \tl_put_right:NV \l__font_khs_style_setup_tl \l__font_khs_style_family_tl }
    \tl_if_blank:VF \l__font_khs_style_series_tl
      { \tl_put_right:NV \l__font_khs_style_setup_tl \l__font_khs_style_series_tl }
    \tl_if_blank:VF \l__font_khs_style_shape_tl
      { \tl_put_right:NV \l__font_khs_style_setup_tl \l__font_khs_style_shape_tl }
  }

\cs_new_protected:Npn \__font_khs_prepare_content:n #1
  {
    \tl_set:Nn \l__font_khs_content_tl {#1}
    \__font_khs_reset_style_parse:
    \__font_khs_strip_wrappers:
    \__font_khs_apply_style_setup:
  }

\cs_new_protected:Npn \__font_khs_strip_wrappers:
  {
    \tl_if_head_eq_meaning:VNTF \l__font_khs_content_tl \textsf
      { \__font_khs_unwrap_textsf: }
      {
        \tl_if_head_eq_meaning:VNTF \l__font_khs_content_tl \texttt
          { \__font_khs_unwrap_texttt: }
          {
            \tl_if_head_eq_meaning:VNTF \l__font_khs_content_tl \textbf
              { \__font_khs_unwrap_textbf: }
              {
                \tl_if_head_eq_meaning:VNTF \l__font_khs_content_tl \textit
                  { \__font_khs_unwrap_textit: }
                  { }
              }
          }
      }
  }

\cs_new_protected:Npn \__font_khs_unwrap_textsf:
  { \exp_after:wN \__font_khs_unwrap_textsf_aux:w \l__font_khs_content_tl \q_stop }
\cs_new_protected:Npn \__font_khs_unwrap_textsf_aux:w \textsf #1#2 \q_stop
  {
    \tl_set:Nn \l__font_khs_style_family_tl { \sffamily }
    \tl_set:Nn \l__font_khs_content_tl {#1}
    \__font_khs_strip_wrappers:
  }

\cs_new_protected:Npn \__font_khs_unwrap_texttt:
  { \exp_after:wN \__font_khs_unwrap_texttt_aux:w \l__font_khs_content_tl \q_stop }
\cs_new_protected:Npn \__font_khs_unwrap_texttt_aux:w \texttt #1#2 \q_stop
  {
    \tl_set:Nn \l__font_khs_style_family_tl { \ttfamily }
    \tl_set:Nn \l__font_khs_content_tl {#1}
    \__font_khs_strip_wrappers:
  }

\cs_new_protected:Npn \__font_khs_unwrap_textbf:
  { \exp_after:wN \__font_khs_unwrap_textbf_aux:w \l__font_khs_content_tl \q_stop }
\cs_new_protected:Npn \__font_khs_unwrap_textbf_aux:w \textbf #1#2 \q_stop
  {
    \tl_set:Nn \l__font_khs_style_series_tl { \bfseries }
    \tl_set:Nn \l__font_khs_content_tl {#1}
    \__font_khs_strip_wrappers:
  }

\cs_new_protected:Npn \__font_khs_unwrap_textit:
  { \exp_after:wN \__font_khs_unwrap_textit_aux:w \l__font_khs_content_tl \q_stop }
\cs_new_protected:Npn \__font_khs_unwrap_textit_aux:w \textit #1#2 \q_stop
  {
    \tl_set:Nn \l__font_khs_style_shape_tl { \itshape }
    \tl_set:Nn \l__font_khs_content_tl {#1}
    \__font_khs_strip_wrappers:
  }
\cs_new:Npn \__font_khs_strip_fillers:n #1
  {
    \tl_clear:N \l_tmpa_tl
    \tl_map_inline:nn {#1}
      {
        \str_if_eq:eeF
          { \tl_to_str:n {##1} }
          { \tl_to_str:V \c__font_khs_filler_tl }
          { \tl_put_right:Nn \l_tmpa_tl {##1} }
      }
    \tl_use:N \l_tmpa_tl
  }

\cs_new_protected:Npn \__font_khs_render_linear:n #1
  {
    \__font_khs_prepare_content:n {#1}
    \tl_set_eq:NN \l_tmpa_tl \l__font_khs_content_tl
    \tl_replace_all:NVn \l_tmpa_tl \c__font_khs_filler_tl { }
    \cs_if_exist:NT \XeTeXinterchartokenstate
      { \XeTeXinterchartokenstate = 0 }
    \tl_use:N \l__font_khs_style_setup_tl
    \tl_use:N \l_tmpa_tl
  }
\cs_new_protected:Npn \__font_khs_measure_cells:
  {
    \dim_zero:N \l__font_khs_cell_wd_dim
    \dim_zero:N \l__font_khs_cell_ht_dim
    \seq_map_inline:Nn \l__font_khs_char_seq
      {
        \hbox_set:Nn \l__font_khs_measure_box { ##1 }
        \dim_set:Nn \l_tmpa_dim { \box_wd:N \l__font_khs_measure_box }
        \dim_set:Nn \l_tmpb_dim
          { \box_ht:N \l__font_khs_measure_box + \box_dp:N \l__font_khs_measure_box }
        \dim_compare:nNnT { \l_tmpa_dim } > { \l__font_khs_cell_wd_dim }
          { \dim_set_eq:NN \l__font_khs_cell_wd_dim \l_tmpa_dim }
        \dim_compare:nNnT { \l_tmpb_dim } > { \l__font_khs_cell_ht_dim }
          { \dim_set_eq:NN \l__font_khs_cell_ht_dim \l_tmpb_dim }
      }
    \dim_set:Nn \l__font_khs_col_gap_dim { 0.2em }
    \dim_set:Nn \l__font_khs_row_gap_dim { 0.05em }
    \dim_set:Nn \l__font_khs_space_gap_dim { 0.5em }
    \dim_set:Nn \l__font_khs_cluster_wd_dim
      { 2\l__font_khs_cell_wd_dim + \l__font_khs_col_gap_dim }
  }

\cs_new_protected:Npn \__font_khs_set_run_max_rows:n #1
  { \int_set:Nn \l__font_khs_run_max_rows_int {#1} }

\cs_new_protected:Npn \__font_khs_compute_cluster_rows:
  {
    \bool_if:NTF \l__font_khs_type_b_bool
      {
        \int_set:Nn \l__font_khs_cluster_rows_int
          { 1 + \int_div_truncate:nn { \seq_count:N \l__font_khs_char_seq } { 2 } }
      }
      {
        \int_set:Nn \l__font_khs_cluster_rows_int
          {
            \int_div_truncate:nn
              { \seq_count:N \l__font_khs_char_seq + 1 }
              { 2 }
          }
      }
  }

\cs_new_protected:Npn \__font_khs_center_row:n #1
  {
    \bool_if:NF \l__font_khs_first_row_bool
      { \kern \l__font_khs_row_gap_dim }
    \bool_set_false:N \l__font_khs_first_row_bool
    \hbox to \l__font_khs_cluster_wd_dim { \hfil #1 \hfil }
  }

\cs_new_protected:Npn \__font_khs_pair_row:nn #1#2
  {
    \bool_if:NF \l__font_khs_first_row_bool
      { \kern \l__font_khs_row_gap_dim }
    \bool_set_false:N \l__font_khs_first_row_bool
    \hbox to \l__font_khs_cluster_wd_dim
      {
        \hbox to \l__font_khs_cell_wd_dim { \hss #1 \hss }
        \kern \l__font_khs_col_gap_dim
        \hbox to \l__font_khs_cell_wd_dim { \hss #2 \hss }
      }
  }

\cs_new_protected:Npn \__font_khs_render_rows_from:n #1
  {
    \int_step_inline:nnnn { #1 } { 2 } { \seq_count:N \l__font_khs_char_seq }
      {
        \int_compare:nNnTF { ##1 } = { \seq_count:N \l__font_khs_char_seq }
          { \__font_khs_center_row:n { \seq_item:Nn \l__font_khs_char_seq { ##1 } } }
          {
            \__font_khs_pair_row:nn
              { \seq_item:Nn \l__font_khs_char_seq { ##1 } }
              { \seq_item:Nn \l__font_khs_char_seq { \int_eval:n { ##1 + 1 } } }
          }
      }
  }

\cs_new_protected:Npn \__font_khs_render_cluster_body:
  {
    \__font_khs_measure_cells:
    \__font_khs_compute_cluster_rows:
    \int_compare:nNnTF { \seq_count:N \l__font_khs_char_seq } = { 1 }
      { \hbox_set:Nn \l__font_khs_cluster_box { \seq_item:Nn \l__font_khs_char_seq { 1 } } }
      {
        \vbox_set:Nn \l__font_khs_cluster_box
          {
            \offinterlineskip
            \bool_set_true:N \l__font_khs_first_row_bool
            \bool_if:NTF \l__font_khs_type_b_bool
              {
                \__font_khs_center_row:n { \seq_item:Nn \l__font_khs_char_seq { 1 } }
                \int_compare:nNnT { \seq_count:N \l__font_khs_char_seq } > { 1 }
                  { \__font_khs_render_rows_from:n { 2 } }
              }
              { \__font_khs_render_rows_from:n { 1 } }
          }
      }
    \dim_set:Nn \l__font_khs_actual_ht_dim
      { \box_ht:N \l__font_khs_cluster_box + \box_dp:N \l__font_khs_cluster_box }
    \dim_set:Nn \l__font_khs_target_ht_dim
      {
        \int_max:nn { 1 } { \l__font_khs_run_max_rows_int }
        \l__font_khs_cell_ht_dim
        +
        \int_eval:n { \int_max:nn { 1 } { \l__font_khs_run_max_rows_int } - 1 }
        \l__font_khs_row_gap_dim
      }
    \dim_set:Nn \l__font_khs_raise_dim
      { \dim_eval:n { (\l__font_khs_target_ht_dim - \l__font_khs_actual_ht_dim) / 2 } }
    \raisebox{\dim_use:N \l__font_khs_raise_dim}{\box_use_drop:N \l__font_khs_cluster_box}
  }

\cs_new_protected:Npn \__font_khs_begin_cluster:
  {
    \seq_clear:N \l__font_khs_char_seq
    \bool_set_false:N \l__font_khs_type_b_bool
  }

\cs_new_protected:Npn \__font_khs_push_char:n #1
  {
    \seq_put_right:Nn \l__font_khs_char_seq { #1 }
  }

\cs_new_protected:Npn \__font_khs_mark_type_b:
  {
    \bool_set_true:N \l__font_khs_type_b_bool
  }

\cs_new_protected:Npn \__font_khs_space:
  { \hskip \l__font_khs_space_gap_dim }

\cs_new_protected:Npn \__font_khs_breakable_space:
  {
    \penalty0
    \hskip \l__font_khs_space_gap_dim plus 0.15em minus 0.08em\relax
  }

\cs_new_protected:Npn \__font_khs_parse_cluster:n #1
  {
    \__font_khs_begin_cluster:
    \tl_map_inline:nn {#1}
      {
        \str_if_eq:eeTF
          { \tl_to_str:n {##1} }
          { \tl_to_str:V \c__font_khs_filler_tl }
          {
            \int_compare:nNnT { \seq_count:N \l__font_khs_char_seq } = { 1 }
              { \__font_khs_mark_type_b: }
          }
          { \__font_khs_push_char:n {##1} }
      }
  }

\cs_new_protected:Npn \__font_khs_compute_run_max_rows:
  {
    \int_set:Nn \l__font_khs_run_max_rows_int { 1 }
    \seq_map_inline:Nn \l__font_khs_cluster_seq
      {
        \tl_if_blank:nF {##1}
          {
            \__font_khs_parse_cluster:n {##1}
            \__font_khs_compute_cluster_rows:
            \int_set:Nn \l__font_khs_run_max_rows_int
              {
                \int_max:nn
                  { \l__font_khs_run_max_rows_int }
                  { \l__font_khs_cluster_rows_int }
              }
          }
      }
  }

\cs_new_protected:Npn \__font_khs_render_text:n #1
  {
    \regex_split:nnN { \s+ } {#1} \l__font_khs_cluster_seq
    \__font_khs_compute_run_max_rows:
    \bool_set_true:N \l__font_khs_first_cluster_bool
    \seq_map_inline:Nn \l__font_khs_cluster_seq
      {
        \tl_if_blank:nF {##1}
          {
            \bool_if:NF \l__font_khs_first_cluster_bool
              { \__font_khs_space: }
            \bool_set_false:N \l__font_khs_first_cluster_bool
            \__font_khs_parse_cluster:n {##1}
            \__font_khs_render_cluster_body:
          }
      }
  }

\cs_new_protected:Npn \__font_khs_render_text_breakable:n #1
  {
    \regex_split:nnN { \s+ } {#1} \l__font_khs_cluster_seq
    \__font_khs_compute_run_max_rows:
    \bool_set_true:N \l__font_khs_first_cluster_bool
    \seq_map_inline:Nn \l__font_khs_cluster_seq
      {
        \tl_if_blank:nF {##1}
          {
            \bool_if:NF \l__font_khs_first_cluster_bool
              { \__font_khs_breakable_space: }
            \bool_set_false:N \l__font_khs_first_cluster_bool
            \__font_khs_parse_cluster:n {##1}
            \__font_khs_render_cluster_body:
          }
      }
  }

\cs_new_protected:Npn \__font_khs_measure_cluster:n #1
  {
    \hbox_set:Nn \l__font_khs_measure_box
      {
        \__font_khs_parse_cluster:n {#1}
        \__font_khs_render_cluster_body:
      }
    \dim_set:Nn \l__font_khs_cluster_measure_dim
      { \box_wd:N \l__font_khs_measure_box }
  }

\cs_new_protected:Npn \__font_khs_flush_current_line:
  {
    \tl_if_blank:VF \l__font_khs_current_line_tl
      {
        \seq_put_right:NV \l__font_khs_line_seq \l__font_khs_current_line_tl
        \tl_clear:N \l__font_khs_current_line_tl
        \dim_zero:N \l__font_khs_line_width_dim
      }
  }

\cs_new_protected:Npn \__font_khs_compute_lines:n #1
  {
    \seq_clear:N \l__font_khs_line_seq
    \tl_clear:N \l__font_khs_current_line_tl
    \dim_zero:N \l__font_khs_line_width_dim
    \dim_set:Nn \l__font_khs_target_width_dim { \linewidth }
    \seq_map_inline:Nn \l__font_khs_cluster_seq
      {
        \tl_if_blank:nF {##1}
          {
            \__font_khs_measure_cluster:n {##1}
            \dim_set:Nn \l_tmpa_dim { \l__font_khs_cluster_measure_dim }
            \tl_if_blank:VTF \l__font_khs_current_line_tl
              {
                \tl_set:Nn \l__font_khs_current_line_tl {##1}
                \dim_set:Nn \l__font_khs_line_width_dim { \l_tmpa_dim }
              }
              {
                \dim_set:Nn \l_tmpb_dim
                  {
                    \l__font_khs_line_width_dim
                    + \l__font_khs_space_gap_dim
                    + \l_tmpa_dim
                  }
                \dim_compare:nNnTF { \l_tmpb_dim } > { \l__font_khs_target_width_dim }
                  {
                    \__font_khs_flush_current_line:
                    \tl_set:Nn \l__font_khs_current_line_tl {##1}
                    \dim_set:Nn \l__font_khs_line_width_dim { \l_tmpa_dim }
                  }
                  {
                    \tl_put_right:Nn \l__font_khs_current_line_tl { ~ ##1 }
                    \dim_set:Nn \l__font_khs_line_width_dim { \l_tmpb_dim }
                  }
              }
          }
      }
    \__font_khs_flush_current_line:
  }

\cs_new_protected:Npn \__font_khs_render_block:n #1
  {
    \group_begin:
      \parindent=0pt
      \parskip=0pt
      \noindent
      \__font_khs_prepare_content:n {#1}
      \tl_use:N \l__font_khs_style_setup_tl
      \__font_khs_render_text_breakable:n { \l__font_khs_content_tl }
      \par
    \group_end:
  }

\cs_new_protected:Npn \__font_define_public_command_khitan:nn #1#2
  {
    \cs_set_protected:cpn { #1stack } ##1
      {
        {
          \use:c { __font_prepare_local_#2: }
          \cs_if_exist:NT \XeTeXinterchartokenstate
            { \XeTeXinterchartokenstate = 0 }
          \__font_khs_render_text:n {##1}
        }
      }

    \exp_args:Nc \DeclareDocumentCommand { #1stackblock } { +m }
      {
        {
          \use:c { __font_prepare_local_#2: }
          \cs_if_exist:NT \XeTeXinterchartokenstate
            { \XeTeXinterchartokenstate = 0 }
          \__font_khs_render_block:n { ##1 }
        }
      }
  }

\cs_new_protected:Npn \__font_define_public_command_khitan_linear:nn #1#2
  {
    \cs_set_protected:cpn { #1 } ##1
      {
        {
          \use:c { __font_prepare_local_#2: }
          \__font_khs_render_linear:n {##1}
        }
      }
  }

\cs_new_protected:Npn \__font_define_special_khitan_small:
  {
    \tl_clear:N \l__font_script_tl
    \tl_clear:N \l__font_language_tl
    \tl_set:Nn \l__font_backend_tl { generic }
    \__font_special_use_generic:
    \exp_args:NVV \__font_define_public_command_khitan:nn
      \l__font_command_tl
      \l__font_name_tl
    \exp_args:NVV \__font_define_public_command_khitan_linear:nn
      \l__font_command_tl
      \l__font_name_tl
  }

\ExplSyntaxOff