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+% \iffalse meta-comment
+%
+%% File: l3tl-build.dtx
+%
+% Copyright (C) 2018-2023 The LaTeX Project
+%
+% It may be distributed and/or modified under the conditions of the
+% LaTeX Project Public License (LPPL), either version 1.3c of this
+% license or (at your option) any later version. The latest version
+% of this license is in the file
+%
+% https://www.latex-project.org/lppl.txt
+%
+% This file is part of the "l3kernel bundle" (The Work in LPPL)
+% and all files in that bundle must be distributed together.
+%
+% -----------------------------------------------------------------------
+%
+% The development version of the bundle can be found at
+%
+% https://github.com/latex3/latex3
+%
+% for those people who are interested.
+%
+%<*driver>
+\documentclass[full,kernel]{l3doc}
+\begin{document}
+ \DocInput{\jobname.dtx}
+\end{document}
+%</driver>
+% \fi
+%
+% \title{^^A
+% The \textsf{l3tl-build} package\\ Piecewise \texttt{tl} constructions^^A
+% }
+%
+% \author{^^A
+% The \LaTeX{} Project\thanks
+% {^^A
+% E-mail:
+% \href{mailto:latex-team@latex-project.org}
+% {latex-team@latex-project.org}^^A
+% }^^A
+% }
+%
+% \date{Released 2023-11-01}
+%
+% \maketitle
+%
+% \begin{documentation}
+%
+% \section{Constructing \meta{tl~var} by accumulation}
+%
+% When creating a \meta{tl~var} by accumulation of many tokens, the performance
+% available using a combination of \cs{tl_set:Nn} and \cs{tl_put_right:Nn} or
+% similar begins to become an issue. To address this, a set of functions are
+% available to \enquote{build} a \meta{tl~var}. The performance of this approach
+% is much more efficient than the standard \cs{tl_put_right:Nn}, but the
+% constructed token list cannot be accessed during construction other than
+% by methods provided in this section.
+%
+% \begin{function}[added = 2018-04-01]{\tl_build_begin:N, \tl_build_gbegin:N}
+% \begin{syntax}
+% \cs{tl_build_begin:N} \meta{tl~var}
+% \end{syntax}
+% Clears the \meta{tl~var} and sets it up to support other
+% \cs[no-index]{tl_build_\ldots{}} functions. Until \cs{tl_build_end:N}
+% \meta{tl~var} is called, applying any function from \pkg{l3tl} other
+% than \cs[no-index]{tl_build_\ldots{}} will lead to incorrect
+% results. The |begin| and |gbegin| functions must be used for local
+% and global \meta{tl~var} respectively.
+% \end{function}
+%
+% \begin{function}[added = 2018-04-01]
+% {
+% \tl_build_put_left:Nn, \tl_build_put_left:Ne,
+% \tl_build_gput_left:Nn, \tl_build_gput_left:Ne,
+% \tl_build_put_right:Nn, \tl_build_put_right:Ne,
+% \tl_build_gput_right:Nn, \tl_build_gput_right:Ne
+% }
+% \begin{syntax}
+% \cs{tl_build_put_left:Nn} \meta{tl~var} \Arg{tokens}
+% \cs{tl_build_put_right:Nn} \meta{tl~var} \Arg{tokens}
+% \end{syntax}
+% Adds \meta{tokens} to the left or right side of the current contents
+% of \meta{tl~var}. The \meta{tl~var} must have been set up with
+% \cs{tl_build_begin:N} or \cs{tl_build_gbegin:N}. The |put| and
+% |gput| functions must be used for local and global \meta{tl~var}
+% respectively. The |right| functions are about twice faster than the
+% |left| functions.
+% \end{function}
+%
+% \begin{function}[added = 2018-04-01]{\tl_build_end:N, \tl_build_gend:N}
+% \begin{syntax}
+% \cs{tl_build_end:N} \meta{tl~var}
+% \end{syntax}
+% Gets the contents of \meta{tl~var} and stores that into the
+% \meta{tl~var} using \cs{tl_set:Nn} or \cs{tl_gset:Nn}.
+% The \meta{tl~var} must have
+% been set up with \cs{tl_build_begin:N} or \cs{tl_build_gbegin:N}.
+% The |end| and |gend| functions must be used for local and global
+% \meta{tl~var} respectively. These functions completely remove the
+% setup code that enabled \meta{tl~var} to be used for other
+% \cs[no-index]{tl_build_\ldots{}} functions. After the action of
+% |end|/|gend|, the \meta{tl~var} may be manipulated using standard
+% \texttt{tl} functions.
+% \end{function}
+%
+% \end{documentation}
+%
+% \begin{implementation}
+%
+% \section{\pkg{l3tl-build} implementation}
+%
+% \begin{macrocode}
+%<*package>
+% \end{macrocode}
+%
+% \begin{macrocode}
+%<@@=tl>
+% \end{macrocode}
+%
+% Between \cs{tl_build_begin:N} \meta{tl~var} and \cs{tl_build_end:N}
+% \meta{tl~var}, the \meta{tl~var} has the structure
+% \begin{quote}
+% \cs{exp_end:} \ldots{} \cs{exp_end:} \cs{@@_build_last:NNn}
+% \meta{assignment} \meta{next~tl} \Arg{left} \meta{right}
+% \end{quote}
+% where \meta{right} is not braced. The \enquote{data} it represents is
+% \meta{left} followed by the \enquote{data} of \meta{next~tl} followed
+% by \meta{right}. The \meta{next~tl} is a token list variable whose
+% name is that of \meta{tl~var} followed by~|'|. There are between $0$
+% and $4$ \cs{exp_end:} to keep track of when \meta{left} and
+% \meta{right} should be put into the \meta{next~tl}. The
+% \meta{assignment} is \cs{cs_set_nopar:Npe} if the variable is local,
+% and \cs{cs_gset_nopar:Npe} if it is global.
+%
+% \begin{macro}{\tl_build_begin:N, \tl_build_gbegin:N}
+% \begin{macro}{\@@_build_begin:NN, \@@_build_begin:NNN}
+% First construct the \meta{next~tl}: using a prime here conflicts
+% with the usual \pkg{expl3} convention but we need a name that can be
+% derived from |#1| without any external data such as a counter.
+% Empty that \meta{next~tl} and setup the structure. The local and
+% global versions only differ by a single function
+% \cs[no-index]{cs_(g)set_nopar:Npe} used for all assignments: this is
+% important because only that function is stored in the \meta{tl~var}
+% and \meta{next~tl} for subsequent assignments. In principle
+% \cs{@@_build_begin:NNN} could use \cs[no-index]{tl_(g)clear_new:N}
+% to empty |#1| and make sure it is defined, but logging the
+% definition does not seem useful so we just do |#3| |#1| |{}| to
+% clear it locally or globally as appropriate.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_build_begin:N #1
+ { \@@_build_begin:NN \cs_set_nopar:Npe #1 }
+\cs_new_protected:Npn \tl_build_gbegin:N #1
+ { \@@_build_begin:NN \cs_gset_nopar:Npe #1 }
+\cs_new_protected:Npn \@@_build_begin:NN #1#2
+ { \exp_args:Nc \@@_build_begin:NNN { \cs_to_str:N #2 ' } #2 #1 }
+\cs_new_protected:Npn \@@_build_begin:NNN #1#2#3
+ {
+ #3 #1 { }
+ #3 #2
+ {
+ \exp_not:n { \exp_end: \exp_end: \exp_end: \exp_end: }
+ \exp_not:n { \@@_build_last:NNn #3 #1 { } }
+ }
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}
+% {
+% \tl_build_put_right:Nn, \tl_build_put_right:Ne, \tl_build_put_right:Nx,
+% \tl_build_gput_right:Nn, \tl_build_gput_right:Ne, \tl_build_gput_right:Nx
+% }
+% \begin{macro}{\@@_build_last:NNn}
+% \begin{macro}{\@@_build_put:nn}
+% \begin{macro}{\@@_build_put:nw}
+% Similar to \cs{tl_put_right:Nn}, but apply \cs{exp:w} to |#1|. Most
+% of the time this just removes one \cs{exp_end:}. When there are
+% none left, \cs{@@_build_last:NNn} is expanded instead. It resets
+% the definition of the \meta{tl~var} by ending the \cs{exp_not:n} and
+% the definition early. Then it makes sure the \meta{next~tl} (its
+% argument |#1|) is set-up and starts a new definition. Then
+% \cs{@@_build_put:nn} and \cs{@@_build_put:nw} place the \meta{left}
+% part of the original \meta{tl~var} as appropriate for the definition
+% of the \meta{next~tl} (the \meta{right} part is left in the right
+% place without ever becoming a macro argument). We use
+% \cs{exp_after:wN} rather than some \cs{exp_args:No} to avoid reading
+% arguments that are likely very long token lists. We use
+% \cs[no-index]{cs_(g)set_nopar:Npe} rather than
+% \cs[no-index]{tl_(g)set:Ne} partly for the same reason and partly
+% because the assignments are interrupted by brace tricks, which
+% implies that the assignment does not simply set the token list to an
+% |e|-expansion of the second argument.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_build_put_right:Nn #1#2
+ {
+ \cs_set_nopar:Npe #1
+ { \__kernel_exp_not:w \exp_after:wN { \exp:w #1 #2 } }
+ }
+\cs_generate_variant:Nn \tl_build_put_right:Nn { Ne , Nx }
+\cs_new_protected:Npn \tl_build_gput_right:Nn #1#2
+ {
+ \cs_gset_nopar:Npe #1
+ { \__kernel_exp_not:w \exp_after:wN { \exp:w #1 #2 } }
+ }
+\cs_generate_variant:Nn \tl_build_gput_right:Nn { Ne , Nx }
+\cs_new_protected:Npn \@@_build_last:NNn #1#2
+ {
+ \if_false: { { \fi:
+ \exp_end: \exp_end: \exp_end: \exp_end: \exp_end:
+ \@@_build_last:NNn #1 #2 { }
+ }
+ }
+ \if_meaning:w \c_empty_tl #2
+ \@@_build_begin:NN #1 #2
+ \fi:
+ #1 #2
+ {
+ \__kernel_exp_not:w \exp_after:wN
+ {
+ \exp:w \if_false: } } \fi:
+ \exp_after:wN \@@_build_put:nn \exp_after:wN {#2}
+ }
+\cs_new_protected:Npn \@@_build_put:nn #1#2 { \@@_build_put:nw {#2} #1 }
+\cs_new_protected:Npn \@@_build_put:nw #1#2 \@@_build_last:NNn #3#4#5
+ { #2 \@@_build_last:NNn #3 #4 { #1 #5 } }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}
+% {
+% \tl_build_put_left:Nn, \tl_build_put_left:Ne, \tl_build_put_left:Nx,
+% \tl_build_gput_left:Nn, \tl_build_gput_left:Ne, \tl_build_gput_left:Nx
+% }
+% \begin{macro}{\@@_build_put_left:NNn}
+% See \cs{tl_build_put_right:Nn} for all the machinery. We could
+% easily provide \cs[no-index]{tl_build_put_left_right:Nnn}, by just
+% adding the \meta{right} material after the \Arg{left} in the
+% |e|-expanding assignment.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_build_put_left:Nn #1
+ { \@@_build_put_left:NNn \cs_set_nopar:Npe #1 }
+\cs_generate_variant:Nn \tl_build_put_left:Nn { Ne , Nx }
+\cs_new_protected:Npn \tl_build_gput_left:Nn #1
+ { \@@_build_put_left:NNn \cs_gset_nopar:Npe #1 }
+\cs_generate_variant:Nn \tl_build_gput_left:Nn { Ne , Nx }
+\cs_new_protected:Npn \@@_build_put_left:NNn #1#2#3
+ {
+ #1 #2
+ {
+ \__kernel_exp_not:w \exp_after:wN
+ {
+ \exp:w \exp_after:wN \@@_build_put:nn
+ \exp_after:wN {#2} {#3}
+ }
+ }
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_build_end:N, \tl_build_gend:N}
+% \begin{macro}{\@@_build_end_loop:NN}
+% Get the data then clear the \meta{next~tl} recursively until finding
+% an empty one. It is perhaps wasteful to repeatedly use
+% \cs{cs_to_str:N}. The local/global scope is checked by
+% \cs{tl_set:Ne} or \cs{tl_gset:Ne}.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_build_end:N #1
+ {
+ \@@_build_get:NNN \__kernel_tl_set:Ne #1 #1
+ \exp_args:Nc \@@_build_end_loop:NN { \cs_to_str:N #1 ' } \tl_clear:N
+ }
+\cs_new_protected:Npn \tl_build_gend:N #1
+ {
+ \@@_build_get:NNN \__kernel_tl_gset:Ne #1 #1
+ \exp_args:Nc \@@_build_end_loop:NN { \cs_to_str:N #1 ' } \tl_gclear:N
+ }
+\cs_new_protected:Npn \@@_build_end_loop:NN #1#2
+ {
+ \if_meaning:w \c_empty_tl #1
+ \exp_after:wN \use_none:nnnnnn
+ \fi:
+ #2 #1
+ \exp_args:Nc \@@_build_end_loop:NN { \cs_to_str:N #1 ' } #2
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\@@_build_get:NNN}
+% \begin{macro}{\@@_build_get:w, \@@_build_get_end:w}
+% The idea is to expand the \meta{tl~var} then the \meta{next~tl} and
+% so on, all within an |e|-expanding assignment, and wrap as
+% appropriate in \cs{exp_not:n}. The various \meta{left} parts are
+% left in the assignment as we go, which enables us to expand the
+% \meta{next~tl} at the right place. The various \meta{right} parts
+% are eventually picked up in one last \cs{exp_not:n}, with a brace
+% trick to wrap all the \meta{right} parts together.
+% \begin{macrocode}
+\cs_new_protected:Npn \@@_build_get:NNN #1#2#3
+ { #1 #3 { \if_false: { \fi: \exp_after:wN \@@_build_get:w #2 } } }
+\cs_new:Npn \@@_build_get:w #1 \@@_build_last:NNn #2#3#4
+ {
+ \exp_not:n {#4}
+ \if_meaning:w \c_empty_tl #3
+ \exp_after:wN \@@_build_get_end:w
+ \fi:
+ \exp_after:wN \@@_build_get:w #3
+ }
+\cs_new:Npn \@@_build_get_end:w #1#2#3
+ { \__kernel_exp_not:w \exp_after:wN { \if_false: } \fi: }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macrocode}
+%</package>
+% \end{macrocode}
+%
+% \end{implementation}
+%
+% \PrintIndex