% \iffalse meta-comment
% ======================================================================
% setspaceenhanced.dtx
% Copyright © 2008–2024 Markus Kohm
%
% This work is a KOMA-Script spin-off.  For the original sources of
% KOMA-Script's `float.hak' see file `scrhack.dtx' in the KOMA-Script
% sources at <https://sourceforge.net/p/koma-script/code/>.
%
% Development is taking place as part of `scrhack' at
% <https://github.com/komascript/third-party-enhancements>.  New issues
% should be reported there as well as known issues can be found.
%
% This work may be distributed and/or modified under the conditions of
% the LaTeX Project Public License, version 1.3c of the license.
% The latest version of this license is in
%   http://www.latex-project.org/lppl.txt
% and version 1.<3c or later is part of all distributions of LaTeX
% version 2005/12/01 or later and of this work.
%
% The Current Maintainer and author of this work is Markus Kohm.
%
% This work consists of the file `setspaceenhanced.dtx' and `README.md'.
%
% The recommended way to install `setspaceenhanced' is to use
% the package manager of your TeX distribution.
% ======================================================================
%
%<*dtx>
\ifx\ProvidesExplFile\undefined\def\ProvidesExplFile#1#2#3#4{}\fi
\ProvidesExplFile{setspaceenhanced.dtx}
%</dtx>
%<*dtx|package>
%<package>\ProvidesExplPackage{setspaceenhanced}
  {2024-11-05}{1.03}
  {%
%<*dtx>
    sources and unpack driver of
%</dtx>
    improved setspace package%
  }
%</dtx|package>
%<*dtx>
\ifx\documentclass\undefined
  \input docstrip.tex
  \generate{%
    \file{setspaceenhanced.sty}{%
      \from{setspaceenhanced.dtx}{package}%
    }%
  }%
\else
  \let\endbatchfile\ExplSyntaxOff
\fi
\endbatchfile
\documentclass[ngerman,USenglish]{koma-script-source-doc}
\NewDocElement[%
  macrolike = true,
  toplevel  = true,
  idxtype   = \textit{const.},
  idxgroup  = Constants,
  printtype = \textit{const.},
]{Constant}{Constant}
\NewDocElement[%
  macrolike = true,
  toplevel  = true,
  idxtype   = \textit{var.},
  idxgroup  = Variable,
  printtype = \textit{var.},
]{Variable}{Variable}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=alphabetic]{biblatex}
\begin{filecontents}[force]{\jobname.bib}
@online{pkg:setspace,
  author={David Carlisle and Erica Harris and Geoffrey Tobin and
          Robin Fairbairns},
  title={\pkg*{setspace} --- Set space between lines},
  version={6.7b},
  date={2022-12-04},
  url={https://ctan.org/pkg/setspace},
  urldate={2023-07-25},
  note={Provides support for setting the spacing between lines in a
        document. Package options include \opt{singlespacing},
        \opt{onehalfspacing}, and \opt{doublespacing}. Alternatively
        the spacing can be changed as required with the
        \cs{singlespacing}, \cs{onehalfspacing}, and
        \cs{doublespacing} commands. Other size spacings also
        available.},
}
@manual{pkg:setspace:manual,
  author={Geoffrey Tobin and Robin Fairbairns},
  date={2022-12-04},
  title={The \pkg*{setspace} Package},
  url={https://ctan.org/tex-archive/macros/latex/contrib/setspace},
  urldate={2023-07-25},
  abstract={},
}
@online{pkg:scrlfile,
  author   = {Markus Kohm},
  version  = {3.41},
  date     = {2023-07-07},
  title    = {\pkg*{scrlfile} – Installation control (not only) for
              \KOMAScript{} packages},
  url      = {https://ctan.org/pkg/scrlfile},
  urldate  = {2023-07-19},
  note     = {The package provides hooks for the execution of commands before
              or after loading files, classes or packages independent from the
              \LaTeX{} kernel version.},
}
@online{pkg:koma-script,
  author={Markus Kohm},
  version={3.41},
  date={2023-07-07},
  title={{\KOMAScript} --- A bundle of versatile classes and packages},
  url={https://ctan.org/pkg/koma-script},
  urldate={2023-07-14},
  note={The \KOMAScript{} bundle provides replacements for the \pkg*{article},
        \pkg*{report}, and \pkg*{book} classes with emphasis on typography and
        versatility. There is also a letter class.},
}
@manual{pkg:koma-script:manual:de,
  author={Markus Kohm},
  date={2023-06-16},
  title={{\KOMAScript}},
  subtitle={Die Anleitung},
  url={http://mirrors.ctan.org/macros/latex/contrib/koma-script/scrguide-de.pdf},
  urldate={2023-07-04},
}
@manual{pkg:koma-script:manual:en,
  author={Markus Kohm},
  date={2023-06-16},
  title={{\KOMAScript}},
  subtitle={The Guide},
  url={http://mirrors.ctan.org/macros/latex/contrib/koma-script/scrguide-en.pdf},
  urldate={2023-07-14},
}
@article{latexnews35,
  author={{The {\LaTeX} Project Team}},
  shortauthor={TLT},
  year={2022},
  month={6},
  title={Issue 35},
  journal={{\LaTeX} News},
  url={http://mirrors.ctan.org/macros/latex/base/ltnews35.pdf},
  urldate={2023-07-14},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\setcounter{StandardModuleDepth}{2}
\begin{document}
  \nocite{pkg:setspace,pkg:koma-script}
  \DocInput{setspaceenhanced.dtx}
\end{document}
%</dtx>
%\fi
%
% \changes{v0.1}{2023/06/01}{new \KOMAScript{} spin-off}
% \changes{v1.0}{2023/08/04}{release}
%
% \GetFileInfo{setspaceenhanced.dtx}
% \title{Improving ``\pkg{setspace}''}
% \author{\href{mailto:komascript@gmx.info}{Markus Kohm}\thanks{Repository and
%   bug reports:
%   \url{https://github.com/komascript/third-party-enhancements}}}
% \date{Version \filedate{} \fileversion}
% \maketitle
% \begin{abstract}
%   Package \pkg*{setspaceenhanced} has started as hack module of the
%   \KOMAScript{} package \pkg*{scrhack} years ago to fix an issue when using
%   package \pkg{setspace} with other document font sizes than 10\,pt, 11\,pt
%   or 12\,pt. This became necessary because package \pkg{setspace} originally
%   only supported these three font sizes and loading the package with a
%   floating point definition of \cs{@ptsize} even resulted in errors. These
%   two issues has been fixed some years ago. Now, \pkg{setspace} uses a
%   static factor for all font sizes but 10\,pt, 11\,pt, or
%   12\,pt.
%
%   Additionally, if you change font size inside the document
%   \pkg{setspace} still uses the stretch factor of the document font size
%   instead of using a proper stretch factor for the new font size.
%
%   Package \pkg*{setspaceenhanced} provides improvements for all these
%   limitations and also some additional enhancements.
% \end{abstract}
%
% \tableofcontents
%
%\iffalse
%<*doc>
%\fi
\section{Why should I use this package instead of
  \texorpdfstring{\pkg{setpace}}{setspace} at least if I use a \KOMAScript{}
  class or \KOMAScript{} package \texorpdfstring{\pkg{scrextend}}{scrextend}
  or a similar package?}
\label{sec:whywithkomascript}

From 2006 \href{https://www.ctan.org/pkg/koma-script}{\KOMAScript} classes and
\KOMAScript{} package \pkg{scrextend} provide option \opt{fontsize} for
setting document font sizes not limited to 10\,pt, 11\,pt, or 12\,pt. Using
this option is also not limited to integer font sizes but also supports
floating point sizes like 11.5\,pt. To support such font sizes, the
specification of macro \cs{@ptsize} has been changed to be no longer either 0,
1 or 2 but to be the font size minus 10 in pt.

By the way, e.g., Ivan Valbusa's package \pkg{fontsize} adopted this existing
definition of \cs{@ptsize} from \KOMAScript{} by using most of the font size
code of \KOMAScript.

Unfortunately package \pkg{setspace} provides stretch factors for individual
font sizes only for 10\,pt, 11\,pt or 12\,pt. For all sizes between these
size, it uses the factor of the down rounded integer. For all sizes below
10\,pt or 11\,pt it uses a static value, 1.25 for \cs{onehalfspacing} and
1.667 for \cs{doublespacing}. Package \pkg*{setspaceenhanced} uses for
\cs{onehalfspacing} and \cs{doublespacing} a calculation of the stretch factor
depending on the selected baseline skip and font size. With this, every font
size is supported.

But that's just the tip of the iceberg and even comparatively
unimportant. Much more important are the reasons in the following section,
which also apply when using a \KOMAScript{} class or the \KOMAScript{} package
\pkg{scrextend} or a similar package.


\section{Why should I use this package instead of
  \texorpdfstring{\pkg{setpace}}{setspace} independent from using a
  \KOMAScript{} class or \KOMAScript{} package
  \texorpdfstring{\pkg{scrextend}}{scrextend} or similar packages?}
\label{sec:whywithoutkomascript}

\pkg{setspace} does not care for the \cs{baselineskip} selected by
\cs{fontsize}. Instead it sets the stretch factor always depending on the
document font size. And using \cs{singlespacing}, \cs{onehalfspacing} or
\cs{doublespacing} or even \cs{setstretch}\SpecialCommandIndex{\setstretch}
after switching the font size using \cs{fontsize} instead of \cs{Huge},
\cs{huge}, \cs{LARGE}, \cs{Large}, \cs{large}, \cs{normalsize}, \cs{small},
\cs{footnotesize}, \cs{scriptsize}, \cs{tiny}, or another command defined
using \cs{@setsize}, reactivates the last used such font size commd. So using
something like
\begin{verbatim}
  \normalsize\fontsize{5pt}{7pt}\selectfont\onehalfspacing
\end{verbatim}
result in \cs{normalsize} with onehalfspacing, not 5\,pt with onehalfspacing!

Package \pkg*{setspaceenhanced} uses for
\cs{onehalfspacing} and \cs{doublespacing} a calculation of the stretch factor
depending on the selected baseline skip and font size. So using something like
\begin{verbatim}
  \normalsize\fontsize{5pt}{7pt}\onehalfspacing
\end{verbatim}
will not set the stretch factor based on \cs{normalsize} but based on font
size 5\,pt with baseline skip 7\,pt. So this results in real onehalfspacing
of the 5\,pt font.

In other words: Package \pkg*{setspaceenhanced} uses a completely different
definition of \cs{onehalfspacing} and \cs{doublespacing}. It always uses
\cs{f@size} and \cs{f@baselineskip} to calculate a factor resulting in real
onehalfspacing and doublespacing. This also means, if you use one of these
commands after changing the font size, a new stretch factor is calculated
depending on the current font size without changing the font size.

\section{How to use
  \texorpdfstring{\pkg*{setspaceenhanced}}{setspaceenhanced}?}
\label{sec:howtouse}

In the document preamble of your document you just can replace
\begin{verbatim}
  \usepackage{setspace}
\end{verbatim}
by
\begin{verbatim}
  \usepackage{setspaceenhanced}
\end{verbatim}
to load package \pkg*{setspaceenhanced}. This does still also load package
\pkg{setspace} but additionally replaces several commands of \pkg{setspace} to
avoid the issues shown in \autoref{sec:whywithkomascript} and
\autoref{sec:whywithoutkomascript}. \pkg*{setspaceenhanced} also does support
the same options as \pkg{setspace}. So you can also replace, e.g.
\begin{verbatim}
  \usepackage[onehalfspaceing]{setspace}
\end{verbatim}
by
\begin{verbatim}
  \usepackage[onehalfspacing]{setspaceenhanced}
\end{verbatim}

If you want you can alternatively also load both packages explicitly, either
\pkg{setspace} before \pkg{setspaceenhanced} or---if you
want---\pkg{setspaceenhanced} before \pkg{setspace}. In this case, you should
use the same options for both package.

This is also useful, if you use a package, that uses \pkg{setspace} itself. In
this case, you always should load \pkg*{setspaceenhanced} \emph{before} the
package, that uses \pkg{setspace}. Otherwise it is very likely that the
initial line spacing is still done with the unchanged commands and settings of
\pkg{setspace} and therefore the full functionality of \pkg*{setspaceenhanced}
cannot be reached.  Only if \pkg*{setspaceenhanced} is loaded before the first
use of \cs{singlespacing}, \cs{onehalfspacing}, \cs{doublespacing}, or
\cs{setstretch} can it be ensured that the enhancements of
\pkg*{setspaceenhanced} are initialized and used correctly.

When using a class that uses \pkg{setspace}, the correct operation can be
ensured with
\begin{verbatim}
  \AddToHook{package/setspace/after}{\RequirePackage{setspaceenhanced}}
\end{verbatim}
even before \cs{documentclass}. This requires at least \LaTeX{}
2020/10/01. For older versions of \LaTeX{} you can use
\begin{verbatim}
  \RequirePackage{scrlfile}
  \AfterPackage{setspace}{\RequirePackage{setspaceenhanced}}
\end{verbatim}
also before \cs{documentclass}. This would require the \KOMAScript{} package
\pkg{scrlfile}\nocite{pkg:scrlfile}. In both cases you should also use the
same optional argument for \cs{RequirePackage}, that is used for loading
\pkg{setspace}.

Package \pkg*{setspaceenhanced} provides all options and commands of the user
interface of \pkg{setspace}, see \autocite{pkg:setspace:manual}. Following we
document only the differences and enhancements.

There are some options influencing the behavior and result of the examples
shown in the section before. All these options are
\opt{\meta{key}=\meta{value}} options using the new \LaTeX{} kernel
interface. Therefore the package needs at least \LaTeX{} 2022-06-01.

\DescribeCommand{\spacesetup}
Options can be set as global option via \cs{documentclass}, as package option
via \cs{usepackage} or using:
\begin{quote}
  \cs{spacesetup}\marg{options}
\end{quote}
The command can be used in the document preamble and also in the document
body. In the document body the changes are local to the current
group.

\subsubsection*{Available options:}

\begin{description}\setkomafont{descriptionlabel}{\ttfamily}
\item[byselectfont=\meta{boolean}]%
  \DescribeOption{byselectfont}%
  \hfill initial=\optvalue{false}, default=\optvalue{true}\\
  In the \pkg*{setspaceenhanced} examples in the previous section, the correct
  factor has only been used, because of using \cs{onehalfspacing} after
  changing the font size, e.g., to \cs{small}. If you use \cs{onehalfspacing}
  before changing the font size, the factor is calculated with the previous
  valid font size, which is the document font size 12\,pt in all these
  examples. This\SpecialCommandIndex{\selectfont}\marginline{\cs{selectfont}}
  behavior can be changed to a more dynamic automatism using option
  \opt{byselectfont} or \opt{byselectfont=true}. This will use the generic
  \LaTeX{} hook \texttt{selectfont} to reactivate \cs{onehalfspacing} or
  \cs{doublespacing} after every \cs{selectfont} if the font size has been
  changed. However note, this is only done if baselineskip is not less than
  the font size. So for a font selection like |\fontsize{6}{0}\selectfont| the
  factor recalculation is ignored, because it does not make sense.\footnote{In
    my opinion using a baselineskip less than the font size never makes
    sense. But \LaTeX{} itself uses baselineskip 0 inside the definition of
    \cs{LaTeX}.}
\item[doublespacing=\meta{real}]%
  \DescribeOption{onehalfspacing}%
  \hfill initial=\texttt{nan}, default=\emph{empty}\\
  If this option is used without value, it is the same as \pkg{setspace}'s
  package option \opt{doublespacing} or using command \cs{doublespacing}. But
  if you assign a real number\footnote{\label{ftn:fp}Here are all values
    allowed, that would be allowed as second argument of \LaTeX3 function
    \cs{fp\string_set:Nn}} this would be used as the new stretch factor used
  for doublespacing. This also means, that the default calculation of the
  factor is deactivated. But a factor of \texttt{nan} would reactivate the
  calculation of the factor depending on the font size and the baseline skip
  set for the font size. It is recommended to use the option always without
  value!
\item[keepfontsize=\meta{boolean}]%
  \DescribeOption{keepfontsize}%
  \marginpar{\raggedleft\footnotesize (changed 2023-09-19)}%
  \DescribeCommand{\setstretch}%
  \hfill initial=\optvalue{false}, default=\optvalue{true}\\
  As explained in \autoref{sec:whywithoutkomascript}, \pkg{setspace}'s
  \cs{setstretch} behaves different after a font size command like \cs{Huge},
  \cs{huge}, \cs{LARGE}, \cs{Large}, \cs{large}, \cs{normalsize}, \cs{small},
  \cs{footnotesize}, \cs{scriptsize}, \cs{tiny}, or another command defined
  using \cs{@setsize} than after using |\fontsize{…}{…}\selectfont|. With the
  last the font size will be reset to the previous usage of one of the other
  or the document font size. For a lot of users this is somehow
  unexpected. With option \opt{keepfontsize} or \opt{keepfontsize=true} this
  is changed and using \cs{setstretch} does not reactivate the last used
  \cs{Huge}, \cs{huge}, \cs{LARGE}, \cs{Large}, \cs{large}, \cs{normalsize},
  \cs{small}, \cs{footnotesize}, \cs{scriptsize}, \cs{tiny}.
\item[onehalfspacing=\meta{real}]%
  \DescribeOption{onehalfspacing}%
  \hfill initial=\texttt{nan}, default=\emph{empty}\\
  If this option is used without value, it is the same as \pkg{setspace}'s
  package option \opt{onehalfspacing} or using command
  \cs{onehalfspacing}. But if you assign a real number\footref{ftn:fp} this
  would be used as the new stretch factor used for onehalfspacing. This also
  means, that the default calculation of the factor is deactivated. But a
  factor of \texttt{nan} would reactivate the calculation of the factor
  depending on the font size and the baseline skip set for the font size. It
  is recommended to use the option always without value!
\item[singlespacing=\meta{real}]%
  \DescribeOption{singlespacing}%
  \hfill initial=1, default=\emph{empty}\\
  If this option is used without value, it is the same as \pkg{setspace}'s
  package option \opt{singlespacing} or using command \cs{singlespacing}. But
  if you assign a real number\footref{ftn:fp} this would be used as the new
  stretch factor used for singlespacing. So this is similar to using
  \cs{SetSinglespace}\marg{real}\cs{singlespacing}.  A factor of \texttt{nan}
  would activate the calculation of the factor depending on the font size and
  the baseline skip set for the font size. It is recommended to use the option
  always without value!
\end{description}

\subsubsection*{Compatibility Notes:}

\begin{description}
\item[\pkg*{scrhack}] If you want to use this package together with package
  \pkg*{scrhack} from \href{https://www.ctan.org/pkg/koma-script}{\KOMAScript}
  before version 3.42, you should deactivate the \file{setspace} hack using
  \pkg*{scrhack}' option \opt{setspace=false}. From version 3.42
  \pkg*{scrhack} does not use the old hacks any longer but
  \pkg*{setspaceenhanced} and is therefore compatible again.
\end{description}

\section{How does the result of
  \texorpdfstring{\pkg*{setspaceenhanced}}{setspaceenhanced} differ from
  \texorpdfstring{\pkg{setspace}}{setspace} even for using the standard font
  sizes of the standard classes?}

For example if you have a document:
\begin{verbatim}
  \documentclass[12pt]{article}
  \usepackage{setspace}
  \begin{document}
  \small\onehalfspacing This is font size
  \csname f@size\endcsname pt with
  normal baseline skip \csname f@baselineskip\endcsname.
  The current stretch factor is \baselinestretch. This
  results in a baseline skip of \the\baselineskip.
  \end{document}
\end{verbatim}
this will result in:
\begin{quote}
  This is font size 10.95pt with normal baseline skip 13.6pt. The current
  stretch factor is 1.241. This results in a baseline skip of 16.87756pt.
\end{quote}
But one moment: $10.95\,\textrm{pt}\cdot 1.5 = 16.425\,\textrm{pt}$. So the
factor seems to be wrong. It is not real onehalfspacing depending on the used
font size. It is also not onehalfspacing depending on the document font size,
because this would need a baseline skip of 18\,pt. So what is it? It is using
the stretch factor of 12\,pt for the 10.95\,pt of \cs{small}.

With \pkg{setspaceenhanced}:
\begin{verbatim}
  \documentclass[12pt]{article}
  \usepackage{setspaceenhanced}
  \begin{document}
  \small\onehalfspacing This is font size
  \csname f@size\endcsname pt with
  normal baseline skip \csname f@baselineskip\endcsname.
  The current stretch factor is \baselinestretch. This
  results in a baseline skip of \the\baselineskip.
  \end{document}
\end{verbatim}
the result changes:
\begin{quote}
  This is font size 10.95pt with normal baseline skip 13.6pt. The current
  stretch factor is 1.207720046225135. This results in a baseline skip of
  16.42496pt.
\end{quote}
Here the difference from the correct value 16.425\,pt is very, very small:
0.00004\,pt. So you can say, this is really onehalfspacing depending on the
used font size.

Moreover if you have a document:
\begin{verbatim}
  \documentclass[12pt]{article}
  \usepackage{setspace}
  \begin{document}
  \fontsize{5pt}{7pt}\selectfont\onehalfspacing This is font size
  \csname f@size\endcsname pt with
  normal baseline skip \csname f@baselineskip\endcsname.
  The current stretch factor is \baselinestretch. This
  results in a baseline skip of \the\baselineskip.
  \end{document}
\end{verbatim}
this result in:
\begin{quote}
  This is font size 12pt with normal baseline skip 14.5pt. The current stretch
  factor is 1.241. This results in a baseline skip of 17.99446pt.
\end{quote}
But
\begin{verbatim}
  \documentclass[12pt]{article}
  \usepackage[keepfontsize]{setspaceenhanced}
  \begin{document}
  \fontsize{5pt}{7pt}\selectfont\onehalfspacing This is font size
  \csname f@size\endcsname pt with
  normal baseline skip \csname f@baselineskip\endcsname.
  The current stretch factor is \baselinestretch. This
  results in a baseline skip of \the\baselineskip.
  \end{document}
\end{verbatim}
results in:
\begin{quote}
  This is font size 5pt with normal baseline skip 7.0pt. The current stretch
  factor is 1.071428571428571. This results in a baseline skip of 7.49998pt.
\end{quote}
In my opinion this is more the expected result. See the previous
\autoref{sec:howtouse} for more information about options like
\opt{keepfontsize}.

% \iffalse
%</doc>
% \fi
%
% \MaybeStop{\printbibliography[heading=bibintoc]\PrintIndex}
%
% \iffalse
%<*package>
%<@@=setspaceenhanced>
% \fi
% \section{Implementation}
%
% We use the new \LaTeX{} kernel feature of \opt{\meta{key}=\meta{value}}
% options introduced in \autocite{latexnews35}. So we need at least \LaTeX{}
% 2022-06-01:
%    \begin{macrocode}
\ifnum 0=\ifcsname IfFormatAtLeastTF\endcsname
    \IfFormatAtLeastTF{2022-06-01}{1}{0}
  \else
    0
  \fi\relax
  \PackageError{setspaceenhanced}{LaTeX~kernel~too~old}{
    The~package~needs~at~least~LaTeX~2022-06-01.\MessageBreak
    This~error~is~fatal.~Loading~will~be~aborted
  }
  \endinput
\fi
%    \end{macrocode}
%
% We do not pass any options to \pkg{setspace}, because we handle them
% different. So we can just load the package here:
%    \begin{macrocode}
\RequirePackage{setspace}
%    \end{macrocode}
%
% \ExplSyntaxOn
% \begin{Constant}{\c_@@_single_fp,\c_@@_onehalf_fp,\c_@@_double_fp}
% \ExplSyntaxOff
% The internal constants store the absolute factor for singlespacing,
% onehalfspacing and doublespacing used to calculate the stretch factors.
%    \begin{macrocode}
\fp_const:Nn \c_@@_single_fp  { 1.2 }
\fp_const:Nn \c_@@_onehalf_fp { 1.5 }
\fp_const:Nn \c_@@_double_fp  { 2.0 }
%    \end{macrocode}
% \end{Constant}
%
% \begin{Variable}{\g_@@_single_factor_fp,
%   \g_@@_onehalf_factor_fp,
%   \g_@@_double_factor_fp}
% \ExplSyntaxOff
% \changes{v1.02}{2023-10-09}{missing prefix \texttt{g} added to variable
%   name}
% The internal variables used to store the configured stretch factors for
% singlespacing, onehalfspacing and doublespacing. If \texttt{nan}
% \cs{@@\_linespread} is calculated.
%    \begin{macrocode}
\fp_new:N \g_@@_single_factor_fp
\fp_set:Nn \g_@@_single_factor_fp { 1.0 }
\fp_new:N \g_@@_onehalf_factor_fp
\fp_set_eq:NN \g_@@_onehalf_factor_fp \c_nan_fp
\fp_new:N \g_@@_double_factor_fp
\fp_set_eq:NN \g_@@_double_factor_fp \c_nan_fp
%    \end{macrocode}
% \end{Variable}
%
% \begin{Variable}{\g_@@_linespread_fp,\g_@@_fp}
% \ExplSyntaxOff
% \changes{v1.02}{2023-10-09}{missing prefix \texttt{g} added to variable
%   name}
% Storage of the current calculated stretch factor and the used constant.
%    \begin{macrocode}
\fp_new:N \g_@@_linespread_fp
\fp_set:Nn \g_@@_linespread_fp { 1.0 }
\fp_new:N \g_@@_fp
%    \end{macrocode}
% \end{Variable}
%
% \begin{macro}{\@@_set_spacing:nn}
% \ExplSyntaxOff
% This function is used to set the stretch factor for one of the
% spacings.
%    \begin{macrocode}
\cs_new:Nn \@@_set_spacing:nn
  {
    \tl_if_blank:nF { #1 }
      {
        \fp_set:cn { g_@@_#2_factor_fp } { #1 }
      }
    \fp_if_nan:nTF { \tl_use:c { g_@@_#2_factor_fp } }
      {
        \fp_set_eq:Nc \g_@@_fp { c_@@_#2_fp }
        \@@_calc_stretch:
      }
      {
        \fp_set_eq:Nc \g_@@_linespread_fp { g_@@_#2_factor_fp }
        \fp_set_eq:NN \g_@@_fp \c_nan_fp
      }
      \setstretch{ \fp_to_decimal:N \g_@@_linespread_fp }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_calc_stretch:}
% \ExplSyntaxOff
% \changes{v1.03}{2024/11/05}{special case \cs{f@baselineskip} < \cs{f@size}}
% This macro is used to (re-)calculate the stretch factor
% \cs{@@\_calcstretch} if the currently used constant is not
% \texttt{nan}.
%    \begin{macrocode}
\cs_new:Nn \@@_calc_stretch:
  {
    \fp_if_nan:nF { \g_@@_fp }
      {
        \fp_compare:nNnTF { \dim_to_fp:n \f@baselineskip } < \f@size
          {
            \fp_set_eq:NN \g_@@_fp \g_@@_linespread_fp
          }
          {  
            \fp_set:Nn \g_@@_linespread_fp
              {
                \f@size / \dim_to_fp:n { \f@baselineskip } * \g_@@_fp
              }
          }
      }
  }
%    \end{macrocode}
% \end{macro}
% \ExplSyntaxOff
%
% \begin{command}{\selectfont}
% We also need to hook into \cs{selectfont} using the general
% \texttt{selectfont} hook to re-calculate the stretch factor after the font
% size has been changed and \opt{byselectfont=true}.
%    \begin{macrocode}
\hook_gput_code:nnn { selectfont } { setspaceenhanced }
  {%
    \bool_if:NT \g_@@_byselectfont_bool
      {
        \cs_if_exist:NT \size@update
          {
            \fp_compare:nNnT \g_@@_linespread_fp = \f@linespread
              {
                \@@_calc_stretch:
                \set@fontsize
                  {
                    \fp_to_decimal:N \g_@@_linespread_fp
                  }
                  \f@size \f@baselineskip
              }
          }
      }
  }
%    \end{macrocode}
% \end{command}
%
% \begin{command}{\setstretch}
% If \opt{keepfontsize=false}, we use the original functionality of
% \pkg{setspace}. But with \opt{keepfontsize=true} we use \cs{linespread}:
%    \begin{macrocode}
\renewcommand*{\setstretch}[1]{
  \bool_if:NTF \g_@@_fontsize_bool
    {
      \linespread{#1}\selectfont
    }
    {
      \def\baselinestretch{#1}%
      \@currsize
    }
}
%    \end{macrocode}
% \end{command}
%
% Almost the end of the package we define all options:
% \changes{v1.01}{2023/09/19}{option \opt{fontsize} renamed to
%   \opt{keepfontsize} because of incompatibility (issue
%   \href{https://github.com/komascript/third-party-enhancements/issues/1}
%        {\#1})}
%    \begin{macrocode}
\DeclareKeys{%
  singlespacing  .code        = \@@_set_spacing:nn {#1} {single},
  singlespacing  .usage       = general,
  onehalfspacing .code        = \@@_set_spacing:nn {#1} {onehalf},
  onehalfspacing .usage       = general,
  doublespacing  .code        = \@@_set_spacing:nn {#1} {double},
  doublespacing  .usage       = general,
  byselectfont   .bool_set:N  = \g_@@_byselectfont_bool,
  byselectfont   .default:n   = true,
  byselectfont   .initial:n   = false,
  keepfontsize   .bool_set:N  = \g_@@_fontsize_bool,
  keepfontsize   .default:n   = true,
  keepfontsize   .initial:n   = false,
}
%    \end{macrocode}
% And process them:
%    \begin{macrocode}
\ProcessKeyOptions\relax
%    \end{macrocode}
%
% \begin{command}{\singlespacing,\onehalfspacing,\doublespacing,\SetSingleSpace}
% We have to redefine the user interface commands for
% singlespacing, onehalfspacing and doublespacing.
%    \begin{macrocode}
\renewcommand*{\singlespacing}{
  \@@_set_spacing:nn {} {single}
}
\renewcommand*{\onehalfspacing}{
  \@@_set_spacing:nn {} {onehalf}
}
\renewcommand*{\doublespacing}{
  \@@_set_spacing:nn {} {double}
}
\renewcommand*{\SetSinglespace}[1]{
  \fp_set:Nn \g_@@_single_factor_fp { #1 }
}
%    \end{macrocode}
% \end{command}
%
% \begin{macro}{\setspace@singlespace}
% \begin{description}
% \item[Note:] Defining this internal macro does not work using
%   \pkg*{setspaceenhanced}.
%   \marginline{\raisebox{\dimexpr\ht\strutbox-\height\relax}{\Huge\textcolor{red}{?}}}^^A
%   Should we add a test for users and package authors, who do not use
%   \cs{SetSinglespace} but redefine the internal macro?
% \end{description}
% \end{macro}
%
% \begin{command}{spacesetup}
% User interface to not need to use \cs{SetKeys}:
%    \begin{macrocode}
\newcommand*{\spacesetup}{\SetKeys[setspaceenhanced]}
%    \end{macrocode}
% \end{command}
%
% \iffalse
%</package>
% \fi
%
% \Finale
% \PrintChanges
%  
% \endinput
% Local Variables:
% mode: doctex
% ispell-local-dictionary: "en_US"
% eval: (flyspell-mode 1)
% TeX-master: t
% End: