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diff --git a/Master/texmf-dist/doc/generic/pgf/pgfmanual-en-gd-algorithm-layer.tex b/Master/texmf-dist/doc/generic/pgf/pgfmanual-en-gd-algorithm-layer.tex new file mode 100644 index 00000000000..c4f8c4ed6d2 --- /dev/null +++ b/Master/texmf-dist/doc/generic/pgf/pgfmanual-en-gd-algorithm-layer.tex @@ -0,0 +1,832 @@ +% Copyright 2019 by Renée Ahrens, Olof Frahm, Jens Kluttig, Matthias Schulz, Stephan Schuster +% Copyright 2019 by Till Tantau +% Copyright 2019 by Jannis Pohlmann +% +% 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{The Algorithm Layer} +\label{section-gd-algorithm-layer} + +\noindent{\emph{by Till Tantau}} + +\ifluatex +\else + This section of the manual can only be typeset using Lua\TeX. + \expandafter\endinput +\fi + + +\subsection{Overview} + +The present section is addressed at readers interested in implementing new +graph drawing algorithms for the graph drawing system. Obviously, in order to +do so, you need to have an algorithm in mind and also some programming skills; +but fortunately only in the Lua programming language: Even though the graph +drawing system was originally developed as an extension of \tikzname, is has +been restructured so that the ``algorithm layer'' where you define algorithms +is scrupulously separated from \tikzname. In particular, an algorithm declared +and implemented on this layer can be used in with every ``display layers'', see +Section~\ref{section-gd-display-layer}, without change. Nevertheless, in the +following we will use the \tikzname\ display layer and syntax in our examples. + +Normally, new graph drawing algorithms can and must be implemented in the Lua +programming language, which is a small, easy-to-learn (and quite beautiful) +language integrated into current versions of \TeX. However, as explained in +Section~\ref{section-algorithms-in-c}, you can also implement algorithms in C +or C++ (and, possibly, in the future also in other languages), but this comes +at a great cost concerning portability. In the present section, I assume that +you are only interested in writing an algorithm using Lua. + +In the following, after a small ``hello world'' example of graph drawing and a +discussion of technical details like how to name files so that \TeX\ will find +them, we have a look at the main parts of the algorithm layer: +% +\begin{itemize} + \item Section~\ref{section-gd-namespaces} gives and overview of the + available namespaces and also of naming conventions used in the graph + drawing system. + \item Section~\ref{section-gd-gd-scope} explores what graph drawing scopes + ``look like on the algorithm layer''. As the graph of a graph drawing + scope is being parsed on the display layer, a lot of information is + gathered: The nodes and edges of the graph are identified and the + object-oriented model is built, but other information is also + collected. For instance, a sequence of \emph{events} is created during + the parsing process. As another example, numerous kinds of + \emph{collections} may be identified by the parser. The parsed graph + together with the event sequence and the collections are all gathered + in a single table, called the \emph{scope table} of the current graph + drawing scope. Algorithms can access this table to retrieve information + that goes beyond the ``pure'' graph model. + + One entry in this table is of particular importance: The + \emph{syntactic digraph.} While most graph drawing algorithms are not + really interested in the ``details'' of how a graph was specified, for + some algorithms it makes a big difference whether you write |a -> b| or + |b <- a| in your specification of the graph. These algorithms can + access the ``fine details'' of how the input graph was specified + through the syntactic digraph; all other algorithms can access their + |digraph| or |ugraph| fields and do not have to worry about the + difference between |a -> b| and |b <- a|. + \item Section~\ref{section-gd-models} explains the object-oriented model of + graphs used throughout the graph drawing system. Graph drawing + algorithms do not get the ``raw'' specification used by the user to + specify a graph (like |{a -> {b,c}}| in the |graph| syntax). Instead, + what a graph drawing algorithm sees is ``just'' a graph object that + provides methods for accessing the vertices and arcs. + \item Section~\ref{section-gd-transformations} explains how the information + in the graph drawing scope is processed. One might expect that we + simply run the algorithm selected by the user; however, things are more + involved in practice. When the layout of a graph needs to be computed, + only very few algorithms will actually be able to compute positions for + the nodes of \emph{every} graph. For instance, most algorithms + implicitly assume that the input graph is connected; algorithms for + computing layouts for trees assume that the input is, well, a tree; and + so on. For this reason, graph drawing algorithms will not actually need + the original input graph as their input, but some \emph{transformed} + version of it. Indeed, \emph{all} graph drawing algorithms are treated + as graph transformations by the graph drawing engine. + + This section explains how transformations are chosen and which + transformations are applied by default. + \item Section~\ref{section-gd-interface-to-algorithms} documents the + interface-to-algorithm class. This interface encapsulates all that an + algorithm ``sees'' of the graph drawing system (apart from the classes + in |model| and |lib|). + \item Section~\ref{section-gd-examples} provides a number of complete + examples that show how graph drawing algorithms can, actually, be + implemented. + \item Section~\ref{section-gd-libs} documents the different libraries + functions that come with the graph drawing engine. For instance, there + are library functions for computing the (path) distance of nodes in a + graph; a parameter that is needed by some algorithms. +\end{itemize} + + +\subsection{Getting Started} + +In this section, a ``hello world'' example of a graph drawing algorithm is +given, followed by an overview of the organization of the whole engine. + + +\subsubsection{The Hello World of Graph Drawing} + +Let us start our tour of the algorithm layer with a ``hello world'' version of +graph drawing: An algorithm that simply places all nodes of a graph in a circle +of a fixed radius. Naturally, this is not a particularly impressive or +intelligent graph drawing algorithm; but neither is the classical ``hello +world''$\dots$\ Here is a minimal version of the needed code (this is not the +typical way of formulating the code, but it is the shortest; we will have a +look at the more standard and verbose way in a moment): +% +\begin{codeexample}[code only, tikz syntax=false] +pgf.gd.interface.InterfaceToAlgorithms.declare { + key = "very simple demo layout", + algorithm = { + run = + function (self) + local alpha = (2 * math.pi) / #self.ugraph.vertices + for i,vertex in ipairs(self.ugraph.vertices) do + vertex.pos.x = math.cos(i * alpha) * 25 + vertex.pos.y = math.sin(i * alpha) * 25 + end + end + } +} +\end{codeexample} +\directlua{ +pgf.gd.interface.InterfaceToAlgorithms.declare { + key = "very simple demo layout", + algorithm = { + run = + function (self) + local alpha = (2 * math.pi) / \luaescapestring{#}self.ugraph.vertices + for i,vertex in ipairs(self.ugraph.vertices) do + vertex.pos.x = math.cos(i * alpha) * 25 + vertex.pos.y = math.sin(i * alpha) * 25 + end + end + } +} +} + +This code \emph {declares} a new algorithm (|very simple demo layout|) and +includes an implementation of the algorithm (through the |run| field of the +|algorithm| field). When the |run| method is called, the |self| parameter will +contain the to-be-drawn graph in its |ugraph| field. It is now the job of the +code to modify the positions of the vertices in this graph (in the example, +this is done by assigning values to |vertex.pos.x| and |vertex.pos.y|). + +In order to actually \emph{use} the algorithm, the above code first needs to be +executed somehow. For \tikzname, one can just call |\directlua| on it or put it +in a file and then use |\directlua| plus |require| (a better alternative) or +you put it in a file like |simpledemo.lua| and use |\usegdlibrary{simpledemo}| +(undoubtedly the ``best'' way). For another display layer, like a graphical +editor, the code could also be executed through the use of |require|. + +Executing the code ``just'' declares the algorithm, this is what the |declare| +function does. Inside some internal tables, the algorithm layer will store the +fact that a |very simple demo layout| is now available. The algorithm layer +will also communicate with the display layer through the binding layer to +advertise this fact to the ``user''. In the case of \tikzname, this means that +the option key |very simple demo layout| becomes available at this point and we +can use it like this: +% +\begin{codeexample}[] +\tikz [very simple demo layout] + \graph { f -> c -> e -> a -> {b -> {c, d, f}, e -> b}}; +\end{codeexample} + +It turns out, that our little algorithm is already more powerful than one might +expect. Consider the following example: +% +\begin{codeexample}[] +\tikz [very simple demo layout, componentwise] + \graph { + 1 -> 2 ->[orient=right] 3 -> 1; + a -- b --[orient=45] c -- d -- a; + }; +\end{codeexample} + +Note that, in our algorithm, we ``just'' put all nodes on a circle around the +origin. Nevertheless, the graph gets decomposed into two connected components, +the components are rotated so that the edge from node |2| to node |3| goes from +left to right and the edge from |b| to |c| goes up at an angle of $45^\circ$, +and the components are placed next to each other so that some spacing is +achieved. + +The ``magic'' that achieves all this behind the scenes is called ``graph +transformations''. They will heavily pre- and postprocess the input and output +of graph drawing algorithms to achieve the above results. + +Naturally, some algorithms may not wish their inputs and/or outputs to be +``tampered'' with. An algorithm can easily configure which transformations +should be applied, by passing appropriate options to |declare|. + + +\subsubsection{Declaring an Algorithm} + +Let us now have a look at how one would ``really'' implement the example +algorithm. First of all, we place our algorithm in a separate file called, say, +|ExampleLayout.lua|. This way, by putting it in a separate file, all display +layers can easily install the algorithm at runtime by saying +|require "ExampleLayout"|. + +Next, the |declare| function is needed quite often, so it makes sense to create +a short local name for it: +% +\begin{codeexample}[code only, tikz syntax=false] +-- This is the file ExampleLayout.lua +local declare = require "pgf.gd.interface.InterfaceToAlgorithms".declare +\end{codeexample} + +The |declare| function is the work-horse of the algorithm layer. It takes a +table that contains at least a |key| field, which must be a unique string, and +some other fields that specify in more detail what kind of key is declared. +Once declared through a call of |declare|, the ``key'' can be used on the +display layer. + +For declaring an algorithm, the table passed to |declare| must contain a field +|algorithm|. This field, in turn, must (normally) be set to a table that will +become the algorithm class. In the above example, our algorithm was so simple +that we could place the whole definition of the class inside the call of +|declare|, but normally the class is defined in more detail after the call to +|declare|: +% +\begin{codeexample}[code only, tikz syntax=false] +local ExampleClass = {} -- A local variable holding the class table + +declare { + key = "very simple demo layout", + algorithm = ExampleClass +} + +function ExampleClass:run () + local alpha = (2 * math.pi) / #self.ugraph.vertices + ... +end +\end{codeexample} + +The effect of the |declare| will be that the table stored in |ExampleClass| is +setup to form a class in the sense of object-oriented programming. In +particular, a static |new| function is installed. + +Now, whenever the user uses the key |very simple demo layout| on a graph, at +some point the graph drawing engine will create a new instance of the +|ExampleClass| using |new| and will then call the |run| method of this class. +The class can have any number of other methods, but |new| and |run| are the +only ones directly called by the graph drawing system. + + +\subsubsection{The Run Method} + +The |run| method of an algorithm classes lies at the heart of any graph drawing +algorithm. This method will be called whenever a graph needs to be laid out. +Upon this call, the |self| object will have some important fields set: +% +\begin{itemize} + \item |ugraph| This stands for ``undirected graph'' and is the + ``undirected'' version of the to-be-laid out graph. In this graph, + whenever there is an arc between $u$ and $v$, there is also an arc + between $v$ and $u$. It is obtained by considering the syntactic + digraph and then ``forgetting'' about the actual direction of the + edges. + + When you have set certain |preconditions| in your algorithm class, like + |connected=true|, the |ugraph| will satisfy these conditions. In + particular, the |ugraph| typically will not be the underlying + undirected graph of the complete syntactic digraph, but rather of some + part of it. The use of (sub)layouts will also modify the syntactic + digraph is fancy ways. + + Refer to this graph whenever your algorithm is ``most comfortable'' + with an undirected graph, as is the case for instance for most + force-base algorithms. + \item |digraph| This stands for ``directed graph'' and is the + ``semantically directed'' version of the to-be-laid out graph. + Basically, when happens is that reverse edges in the syntactic digraph + (an edge like |b <- a|) will yield an |Arc| from |a| to |b| in the + |digraph| while they yield a |b| to |a| arc and edge in the syntactic + digraph. Also, undirected edges like |a -- b| are replaced by directed + edges in both directions between the vertices. + \item |scope| The graph drawing scope. + \item |layout| The layout object for this graph. This is a collection of + kind |layout|. +\end{itemize} + + +\subsubsection{Loading Algorithms on Demand} + +In order to use the |very simple demo layout| on the display layer, |declare| +must have been called for this key. However, we just saw that the |declare| +function takes the actual class table as parameter and, thus, whenever an +algorithm is declared, it is also completely loaded and compiled at this point. + +This is not always desirable. A user may wish to include a number of libraries +in order to declare a large number of potentially useful algorithms, but will +not actually use all of them. Indeed, at least for large, complex algorithms, +it is preferable that the algorithm's code is loaded only when the algorithm is +used for the first time. + +Such a ``loading of algorithms on demand'' is supported through the option of +setting the |algorithm| field in a |declare| to a string. This string must now +be the file name of a Lua file that contains the code of the actual algorithm. +When the key is actually used for the first time, this file will be loaded. It +must return a table that will be plugged into the |algorithm| field; so +subsequent usages of the key will not load the file again. + +The net effect of all this is that you can place implementations of algorithms +in files separate from interface files that just contain the |declare| commands +for these algorithms. You will typically do this only for rather large +algorithms. + +For our example, the code would look like this: +% +\begin{codeexample}[code only, tikz syntax=false] +-- File ExampleLayout.lua +local declare = require "pgf.gd.interface.InterfaceToAlgorithms".declare +declare { + key = "very simple demo layout", + algorithm = "ExampleLayoutImplementation" +} +\end{codeexample} + +\begin{codeexample}[code only, tikz syntax=false] +-- File ExampleLayoutImplementation.lua +local ExampleClass = {} +function ExampleClass:run () + local alpha = (2 * math.pi) / #self.ugraph.vertices + ... +end +return ExampleClass +\end{codeexample} + + +\subsubsection{Declaring Options} + +Let us now make our example algorithm a bit more ``configurable''. For this, we +use |declare| once more, but instead of the |algorithm| field, we use a |type| +field. This tells the display layer that the key is not used to select an +algorithm, but to configure ``something'' about the graph or about nodes or +edges. + +In our example, we may wish to configure the radius of the graph. So, we +introduce a |radius| key (actually, this key already exists, so we would not +need to declare it, but let us do so anyway for example purposes): +% +\begin{codeexample}[code only, tikz syntax=false] +declare { + key = "radius", + type = "length", + initial = "25pt" +} +\end{codeexample} + +This tells the display layer that there is now an option called |radius|, that +users set it to some ``length'', and that if it is not set at all, then the +25pt should be used. + +To access what the user has specified for this key, an algorithm can access the +|options| field of a graph, vertex, or arc at the key's name: +% +\begin{codeexample}[code only, tikz syntax=false] + vertex.pos.x = math.cos(i * alpha) * vertex.options.radius + vertex.pos.y = math.sin(i * alpha) * vertex.options.radius +\end{codeexample} + + +\subsubsection{Adding Inline Documentation} + +You should always document the keys you |declare|. For this, the |declare| +function allows you to add three fields to its argument table: +% +\begin{itemize} + \item |summary| This should be a string that succinctly summarizes the + effect this key has. The idea is that this text will be shown as a + ``tooltip'' in a graphical editor or will be printed out by a command + line tool when a user requests help about the key. You can profit from + using Lua's |[[| and |]]| syntax for specifying multi-line strings. + + Also, when the file containing the key is parsed for this manual, this + text will be shown. + \item |documentation| When present, this field contains a more extensive + documentation of the key. It will also be shown in this manual, but + typically not as a tool tip. + \item |examples| This should either be a single string or an array of + strings. Each string should be an example demonstrating how the key is + used in \tikzname. They will all be included in the manual, each + surrounded by a |codeexample| environment. +\end{itemize} + +Let us augment our |radius| key with some documentation. The three dashes +before the |declare| are only needed when the declaration is part of this +manual and they will trigger an inclusion of the key in the manual. +% +\begin{codeexample}[code only, tikz syntax=false] +--- +declare { + key = "radius", + type = "length", + initial = "25pt", + summary = [[ + Specifies the radius of a circle on which the nodes are placed when + the |very simple example layout| is used. Each vertex can have a + different radius. + ]], + examples = [[ + \tikz \graph [very simple example layout, radius=2cm] { + a -- b -- c -- d -- e; + }; + ]] +} +\end{codeexample} + +As a courtesy, all of the strings given in the documentation can start and end +with quotation marks, which will be removed. (This helps syntax highlighting +with editors that do not recognize the |[[| to |]]| syntax.) Also, the +indentation of the strings is removed (we compute the minimum number of leading +spaces on any line and remove this many spaces from all lines). + + +\subsubsection{Adding External Documentation} +\label{section-gd-documentation-in} + +As an alternative to inlining documentation, you can also store the +documentation of keys in a separate file that is loaded only when the +documentation is actually accessed. Since this happens only rarely (for +instance, not at all, when \tikzname\ is run, except for this manual), this +will save time and space. Also, for C code, it is impractical to store +multi-line documentation strings directly in the C file. + +In order to store documentation externally, instead of the |summary|, +|documentation|, and |examples| keys, you provide the key |documentation_in|. +The |documentation_in| key must be set to a string that is input using +|require|. + +In detail, when someone tries to access the |summary|, |documentation|, or +|examples| field of a key and these keys are not (yet) defined, the system +checks whether the |documentation_in| key is set. If so, we apply |require| to +the string stored in this field. The file loaded in this way can now setup the +missing fields of the current key and, typically, also of all other keys +defined in the same file as the current key. For this purpose, it is advisable +to use the |pgf.gd.doc| class: + +\includeluadocumentationof{pgf.gd.doc} + +As a longer example, consider the following declarations: +% +\begin{codeexample}[code only, tikz syntax=false] +--- +declare { + key = "very simple demo layout", + algorithm = ExampleClass, + documentation_in = "documentation_file" +} + +--- +declare { + key = "radius", + type = "length", + initial = "25", + documentation_in = "documentation_file" +} +\end{codeexample} + +The file |documentation_file.lua| would look like this: +% +\begin{codeexample}[code only, tikz syntax=false] +-- File documentation_file.lua +local key = require 'pgf.gd.doc'.key +local documentation = require 'pgf.gd.doc'.documentation +local summary = require 'pgf.gd.doc'.summary +local example = require 'pgf.gd.doc'.example + +key "very simple demo layout" +documentation "This layout is a very simple layout that, ..." + +key "radius" +summary "Specifies the radius of a circle on which the nodes are placed." +documentation +[[ +This key can be used together with |very simple example layout|. An +important feature ist that... +]] +example +[[ +\tikz \graph [very simple example layout, radius=2cm] +{ a -- b -- c -- d -- e; }; +]] +\end{codeexample} + + +\subsection{Namespaces and File Names} +\label{section-gd-namespaces} + +\subsubsection{Namespaces} + +All parts of the |graphdrawing| library reside in the Lua ``namespace'' +|pgf.gd|, which is itself a ``sub-namespace'' of |pgf|. For your own +algorithms, you are free to place them in whatever namespace you like; only for +the official distribution of \pgfname\ everything has been put into the correct +namespace. + +Let us now have a more detailed look at these namespaces. A namespace is just a +Lua table, and sub-namespaces are just subtables of namespace tables. Following +the Java convention, namespaces are in lowercase letters. The following +namespaces are part of the core of the graph drawing engine: +% +\begin{itemize} + \item |pgf| This namespace is the main namespace of \pgfname. Other parts + of \pgfname\ and \tikzname\ that also employ Lua should put an entry + into this table. Since, currently, only the graph drawing engine + adheres to this rule, this namespace is declared inside the graph + drawing directory, but this will change. + + The |pgf| table is the \emph{only} entry into the global table of Lua + generated by the graph drawing engine (or, \pgfname, for that matter). + If you intend to extend the graph drawing engine, do not even + \emph{think} of polluting the global namespace. You will be fined. + \item |pgf.gd| This namespace is the main namespace of the graph drawing + engine, including the object-oriented models of graphs and the layout + pipeline. Algorithms that are part of the distribution are also inside + this namespace, but if you write your own algorithms you do not need + place them inside this namespace. (Indeed, you probably should not + before they are made part of the official distribution.) + \item |pgf.gd.interface| This namespace handles, on the one hand, the + communication between the algorithm layer and the binding layer and, on + the other hand, the communication between the display layer (\tikzname) + and the binding layer. + \item |pgf.gd.binding| So-called ``bindings'' between display layers and + the graph drawing system reside in this namespace. + \item |pgf.gd.lib| Numerous useful classes that ``make an algorithm's your + life easier'' are collected in this namespace. Examples are a class for + decomposing a graph into connected components or a class for computing + the ideal distance between two sibling nodes in a tree, taking all + sorts of rotations and separation parameters into account. + \item |pgf.gd.model| This namespace contains all Lua classes that are part + of the object-oriented model of graphs employed throughout the graph + drawing engine. For readers familiar with the model--view--controller + pattern: This is the namespace containing the model-part of this + pattern. + \item |pgf.gd.control| This namespace contains the ``control logic'' of the + graph drawing system. It will transform graphs according to rules, + disassemble layouts and sublayouts and will call the appropriate + algorithms. For readers still familiar with the model--view--controller + pattern: This is the namespace containing the control-part of this + pattern. + \item |pgf.gd.trees| This namespace contains classes that are useful for + dealing with graphs that are trees. In particular, it contains a class + for computing a spanning tree of an arbitrary connected graph; an + operation that is an important preprocessing step for many algorithms. + + In addition to providing ``utility functions for trees'', the namespace + \emph{also} includes actual algorithms for computing graph layouts like + |pgf.gd.trees.ReingoldTilford1981|. It may seem to be a bit of an + ``impurity'' that a namespace mixes utility classes and ``real'' + algorithms, but experience has shown that it is better to keep things + together in this way. + + Concluding the analogy to the model--view--controller pattern, a graph + drawing algorithm is, in a loose sense, the ``view'' part of the + pattern. + \item |pgf.gd.layered| This namespace provides classes and functions for + ``layered'' layouts; the Sugiyama layout method being the most + well-known one. Again, the namespace contains both algorithms to be + used by a user and utility functions. + \item |pgf.gd.force| Collects force-based algorithms and, again, also + utility functions and classes. + \item |pgf.gd.examples| Contains some example algorithms. They are + \emph{not} intended to be used directly, rather they should serve as + inspirations for readers wishing to implement their own algorithms. +\end{itemize} + +There are further namespaces that also reside in the |pgf.gd| namespace, these +namespaces are used to organize different graph drawing algorithms into +categories. + +In Lua, similarly to Java, when a class |SomeClass| is part of, say, the +namespace |pgf.gd.example|, it is customary to put the class's code in a file +|SomeClass.lua| and then put this class in a directory |example|, that is a +subdirectory of a directory |gd|, which is in turn a subdirectory of a +directory |pgf|. When you write \texttt{require "pgf.gd.example.SomeClass"} the +so-called \emph{loader} will turn this into a request for the file +\texttt{pgf/gd/example/SomeClass.lua} (for Unix systems). + + +\subsubsection{Defining and Using Namespaces and Classes} + +There are a number of rules concerning the structure and naming of namespaces +as well as the naming of files. Let us start with the rules for naming +namespaces, classes, and functions. They follow the ``Java convention'': +% +\begin{enumerate} + \item A namespace is a short lowercase |word|. + \item A function in a namespace is in + |lowercase_with_underscores_between_words|. + \item A class name is in |CamelCaseWithAnUppercaseFirstLetter|. + \item A class method name is in |camelCaseWithALowercaseFirstLetter|. +\end{enumerate} + +From Lua's point of view, every namespace and every class is just a table. +However, since these tables will be loaded using Lua's |require| function, each +namespace and each class must be placed inside a separate file (unless you +modify the |package.loaded| table, but, then, you know what you are doing +anyway). Inside such a file, you should first declare a local variable whose +name is the name of the namespace or class that you intend to define and then +assign a (possibly empty) table to this variable: +% +\begin{codeexample}[code only, tikz syntax=false] +-- File pgf.gd.example.SomeClass.lua: +local SomeClass = {} +\end{codeexample} +% +Next, you should add your class to the encompassing namespace. This is achieved +as follows: +% +\begin{codeexample}[code only, tikz syntax=false] +require("pgf.gd.example").SomeClass = SomeClass +\end{codeexample} +% +The reason this works is that the |require| will return the table that is the +namespace |pgf.gd.example|. So, inside this namespace, the |SomeClass| field +will be filled with the table stored in the local variable of the same name -- +which happens to be the table representing the class. + +At the end of the file, you must write +% +\begin{codeexample}[code only, tikz syntax=false] +return SomeClass +\end{codeexample} +% +This ensures that the table that is defined in this file gets stored by Lua in +the right places. Note that you need and should not use Lua's |module| command. +The reason is that this command has disappeared in the new version of Lua and +that it is not really needed. + +Users of your class can import and use your class by writing: +% +\begin{codeexample}[code only, tikz syntax=false] +... +local SomeClass = require "pgf.gd.examples.SomeClass" +... +\end{codeexample} + + +\subsection{The Graph Drawing Scope} +\label{section-gd-gd-scope} + +\includeluadocumentationof{pgf.gd.interface.Scope} + + +\subsection{The Model Classes} +\label{section-gd-models} + +All that a graph drawing algorithm will ``see'' of the graph specified by the +user is a ``graph object''. Such an object is an object-oriented model of the +user's graph that no longer encodes the specific way in which the user +specified the graph; it only encodes which nodes and edges are present. For +instance, the \tikzname\ graph specification +% +\begin{codeexample}[code only] +graph { a -- {b, c} } +\end{codeexample} +% +\noindent and the graph specification +% +\begin{codeexample}[code only] +node (a) { a } +child { node (b) {b} } +child { node (c) {c} } +\end{codeexample} +% +will generate exactly the same graph object. + +\begin{luanamespace}{pgf.gd.}{model} + This namespace contains the classes modeling graphs, nodes, and edges. + Also, the |Coordinate| class is found here, since coordinates are also part + of the modeling. +\end{luanamespace} + + +\subsubsection{Directed Graphs (Digraphs)} + +Inside the graph drawing engine, the only model of a graph that is available +treats graphs as +% +\begin{enumerate} + \item directed (all edges have a designated head and a designated tail) and + \item simple (there can be at most one edge between any pair of nodes). +\end{enumerate} +% +These two properties may appear to be somewhat at odds with what users can +specify as graphs and with what some graph drawing algorithms might expect as +input. For instance, suppose a user writes +% +\begin{codeexample}[code only] +graph { a -- b --[red] c, b --[green, bend right] c } +\end{codeexample} +% +In this case, it seems that the input graph for a graph drawing algorithm +should actually be an \emph{undirected} graph in which there are +\emph{multiple} edges (namely $2$) between |b| and~|c|. Nevertheless, the graph +drawing engine will turn the user's input a directed simple graph in ways +described later. You do not need to worry that information gets lost during +this process: The \emph{syntactic digraph,} which is available to graph drawing +algorithms on request, stores all the information about which edges are present +in the original input graph. + +The main reasons for only considering directed, simple graphs are speed and +simplicity: The implementation of these graphs has been optimized so that all +operations on these graphs have a guaranteed running time that is small in +practice. + +\includeluadocumentationof{pgf.gd.model.Digraph} + + +\subsubsection{Vertices} + +\includeluadocumentationof{pgf.gd.model.Vertex} + + +\subsubsection{Arcs} +\label{section-gd-arc-model} + +\includeluadocumentationof{pgf.gd.model.Arc} + + +\subsubsection{Edges} + +\includeluadocumentationof{pgf.gd.model.Edge} + + +\subsubsection{Collections} + +\includeluadocumentationof{pgf.gd.model.Collection} + + +\subsubsection{Coordinates, Paths, and Transformations} + +\includeluadocumentationof{pgf.gd.model.Coordinate} +\includeluadocumentationof{pgf.gd.model.Path} +\includeluadocumentationof{pgf.gd.lib.Transform} + + +\subsubsection{Options and Data Storages for Vertices, Arcs, and Digraphs} + +Many objects in the graph drawing system have an |options| table attached to +them. These tables will contain the different kinds options specified by the +user for the object. For efficiency reasons, many objects may share the same +options table (since, more often than not, almost all objects have exactly the +same |options| table). For this reason, you cannot store anything in an options +table, indeed, you should never attempt to write anything into an options +table. Instead, you should use a |Storage|. + +\includeluadocumentationof{pgf.gd.lib.Storage} + + +\subsubsection{Events} + +\includeluadocumentationof{pgf.gd.lib.Event} + + +\subsection{Graph Transformations} +\label{section-gd-transformations} + +\subsubsection{The Layout Pipeline} + +\includeluadocumentationof{pgf.gd.control.LayoutPipeline} + + +\subsubsection{Hints For Edge Routing} + +\includeluadocumentationof{pgf.gd.routing.Hints} + + +\subsection{The Interface To Algorithms} +\label{section-gd-interface-to-algorithms} + +\includeluadocumentationof{pgf.gd.interface.InterfaceToAlgorithms} + + +\subsection{Examples of Implementations of Graph Drawing Algorithms} +\label{section-gd-examples} + +\includeluadocumentationof{pgf.gd.examples.library} +\includeluadocumentationof{pgf.gd.examples.SimpleDemo} +\includeluadocumentationof{pgf.gd.examples.SimpleEdgeDemo} +\includeluadocumentationof{pgf.gd.examples.SimpleHuffman} + + +\subsection{Support Libraries} +\label{section-gd-libs} + +The present section lists a number of general-purpose libraries that are used +by different algorithms. + +\subsubsection{Basic Functions} + +\includeluadocumentationof{pgf} + +\includeluadocumentationof{pgf.gd.lib} + + +\subsubsection{Lookup Tables} + +\includeluadocumentationof{pgf.gd.lib.LookupTable} + + +\subsubsection{Computing Distances in Graphs} + +\emph{Still needs to be ported to digraph classes!} + +%\includeluadocumentationof{pgf.gd.lib.PathLengths} + + +\subsubsection{Priority Queues} + +\includeluadocumentationof{pgf.gd.lib.PriorityQueue} |