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authorNorbert Preining <norbert@preining.info>2023-01-16 03:03:27 +0000
committerNorbert Preining <norbert@preining.info>2023-01-16 03:03:27 +0000
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+% Copyright 2018 by Christian Feuersaenger
+%
+% This file may be distributed and/or modified
+%
+% 1. under the LaTeX Project Public License and/or
+% 2. under the GNU Free Documentation License.
+%
+% See the file doc/generic/pgf/licenses/LICENSE for more details.
+
+
+\section{Adding libraries to \pgfname: temporary registers}
+\label{section-internal-registers}
+
+This section is intended for those who like to write libraries to extend
+\pgfname. Of course, this requires a good deal of knowledge about
+\TeX-programming and the structure of the \pgfname\ basic layer. Besides, one
+will encounter the need of temporary variables and, especially, temporary \TeX\
+registers. This section describes how to use a set of pre-allocated temporary
+registers of the basic layer without needing to allocate more of them.
+
+A part of these internals are already mentioned in
+section~\ref{section-internal-pointcmds}, but the basic layer provides more
+temporaries than |\pgf@x| and |\pgf@y|.
+
+\begin{internallist}[dimen register]{\pgf@x,\pgf@y}
+ These registers are used to process point coordinates in the basic layer of
+ \pgfname, see section~\ref{section-internal-pointcmds}. After a
+ |\pgfpoint|$\dotsc$ command, they contain the final $x$ and $y$ coordinate,
+ respectively.
+
+ The values of |\pgf@x| and |\pgf@y| are set \emph{globally} in contrast to
+ other available \pgfname\ registers. You should never assume anything about
+ their value unless the context defines them explicitly.
+
+ Please prefer the |\pgf@xa|, |\pgf@xb|, $\dotsc$ registers for temporary
+ dimen registers unless you are writing point coordinate commands.
+\end{internallist}
+
+\begin{internallist}[dimen register]{
+ \pgf@xa,
+ \pgf@xb,
+ \pgf@xc,
+ \pgf@ya,
+ \pgf@yb,
+ \pgf@yc%
+}
+ Temporary registers for \TeX\ dimensions which can be modified freely. Just
+ make sure changes occur only within \TeX\ groups.
+
+ \paragraph{Attention:}
+ %
+ \pgfname\ uses these registers to perform path operations. For reasons of
+ efficiency, path commands do not always guard them. As a consequence, the
+ code
+ %
+\begin{codeexample}[code only]
+\pgfpointadd{\pgfpoint{\pgf@xa}{\pgf@ya}}{\pgfpoint{\pgf@xb}{\pgf@yb}}
+\end{codeexample}
+ %
+ \noindent may fail: Inside |\pgfpointadd|, the |\pgf@xa| and friend
+ registers might be modified. In particular, it might happen that |\pgf@xb|
+ is changed before |\pgfpoint{\pgf@xb}{\pgf@yb}| is evaluated. The right
+ thing to do would be to first expand everything using |\edef| and process
+ the values afterwards, resulting in unnecessary expensive operations. Of
+ course, one can avoid this by simply looking into the source code of
+ |\pgfpointadd| to see which registers are used.
+\end{internallist}
+
+\begin{internallist}[dimen register]{\pgfutil@tempdima,\pgfutil@tempdimb}
+ Further multi-purpose temporary dimen registers. For \LaTeX, these
+ registers are already allocated as |\@tempdima| and |\@tempdimb| and are
+ simply |\let| to the |\pgfutil@|$\dotsc$ names.
+\end{internallist}
+
+\begin{internallist}[count register]{
+ \c@pgf@counta,
+ \c@pgf@countb,
+ \c@pgf@countc,
+ \c@pgf@countd%
+}
+ These multiple-purpose count registers are used throughout \pgfname\ to
+ perform integer computations. Feel free to use them as well, just make sure
+ changes are scoped by local \TeX\ groups.
+\end{internallist}
+
+\begin{internallist}[openout handle]{\w@pgf@writea}
+ An |\openout| handle which is used to generate complete output files within
+ locally scoped parts of \pgfname\ (for example, to interact with
+ |gnuplot|). You should always use |\immediate| in front of output
+ operations involving |\w@pgf@writea| and you should always close the file
+ before returning from your code.
+ %
+\begin{codeexample}[code only]
+\immediate\openout\w@pgf@writea=myfile.dat
+\immediate\write\w@pgf@writea{...}%
+\immediate\write\w@pgf@writea{...}%
+\immediate\closeout\w@pgf@writea%
+\end{codeexample}
+ %
+\end{internallist}
+
+\begin{internallist}[openin handle]{\r@pgf@reada}
+ An |\openin| handle which is used to read files within locally scoped parts
+ of \pgfname, for example to check if a file exists or to read data files.
+ You should always use |\immediate| in front of output operations involving
+ |\r@pgf@writea| and you should always close the file before returning from
+ your code.
+ %
+\begin{codeexample}[code only]
+\immediate\openin\r@pgf@reada=myfile.dat
+% do something with \macro
+\ifeof\r@pgf@reada
+ % end of file or it doesn't exist
+\else
+ % loop or whatever
+ \immediate\read\r@pgf@reada to\macro
+ ...
+\fi
+\immediate\closein\r@pgf@reada
+\end{codeexample}
+ %
+\end{internallist}
+
+\begin{internallist}[box]{\pgfutil@tempboxa}
+ A box for temporary use inside of local \TeX\ scopes. For \LaTeX, this box
+ is the same as the already pre-allocated |\@tempboxa|.
+\end{internallist}