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+-- Copyright 2012 by Till Tantau
+--
+-- This file may be distributed and/or modified
+--
+-- 1. under the LaTeX Project Public License and/or
+-- 2. under the GNU Public License
+--
+-- See the file doc/generic/pgf/licenses/LICENSE for more information
+
+-- @release $Header$
+
+
+local declare = require("pgf.gd.interface.InterfaceToAlgorithms").declare
+
+
+---
+-- @section subsection {Cycle Removal}
+--
+-- The Sugiyama method works only on directed \emph{acyclic}
+-- graphs. For this reason, if the input graph is not (yet) acyclic, a
+-- number of edges need to be redirected so that acyclicity arises. In
+-- the following, the different options that allow you to fine-tune
+-- this process are documented.
+--
+-- @end
+
+
+
+---
+
+declare {
+ key = "depth first cycle removal",
+ algorithm = require "pgf.gd.layered.CycleRemovalGansnerKNV1993",
+ phase = "cycle removal",
+ phase_default = true,
+
+ summary = [["
+ Selects a cycle removal algorithm that is especially
+ appropriate for graphs specified ``by hand''.
+ "]],
+ documentation = [["
+ When graphs are created by humans manually, one can
+ make assumptions about the input graph that would otherwise not
+ be possible. For instance, it seems reasonable to assume that the
+ order in which nodes and edges are entered by the user somehow
+ reflects the natural flow the user has had in mind for the graph.
+
+ In order to preserve the natural flow of the input graph, Gansner
+ et al.\ propose to remove cycles by performing a series of
+ depth-first searches starting at individual nodes in the order they
+ appear in the graph. This algorithm implicitly constructs a spanning
+ tree of the nodes reached during the searches. It thereby partitions
+ the edges of the graph into tree edges and non-tree edges. The
+ non-tree edges are further subdivided into forward edges, cross edges,
+ and back edges. Forward edges point from a tree nodes to one of their
+ descendants. Cross edges connect unrelated branches in the search tree.
+ Back edges connect descendants to one of their ancestors. It is not
+ hard to see that reversing back edges will not only introduce no new
+ cycles but will also make any directed graph acyclic.
+ Gansner et al.\ argue that this approach is more stable than others
+ in that fewer inappropriate edges are reversed compared to other
+ methods, despite the lack of a provable upper bound for the number
+ of reversed edges.
+
+ See section~4.1.1 of Pohlmann's Diplom thesis for more details.
+
+ This is the default algorithm for cycle removals.
+ "]]
+ }
+
+---
+
+declare {
+ key = "prioritized greedy cycle removal",
+ algorithm = "pgf.gd.layered.CycleRemovalEadesLS1993",
+ phase = "cycle removal",
+
+ summary = [["
+ This algorithm implements a greedy heuristic of Eades et al.\ for
+ cycle removal that prioritizes sources and sinks.
+ "]],
+ documentation = [["
+ See section~4.1.1 of Pohlmann's Diploma theses for details.
+ "]]
+}
+
+
+---
+
+declare {
+ key = "greedy cycle removal",
+ algorithm = "pgf.gd.layered.CycleRemovalEadesLS1993",
+ phase = "cycle removal",
+
+ summary = [["
+ This algorithm implements a greedy heuristic of Eades et al.\ for
+ cycle removal that prioritizes sources and sinks.
+ "]],
+ documentation = [["
+ See section~4.1.1 of Pohlmann's Diploma theses for details.
+ "]]
+ }
+
+---
+
+declare {
+ key = "naive greedy cycle removal",
+ algorithm = "pgf.gd.layered.CycleRemovalBergerS1990a",
+ phase = "cycle removal",
+
+ summary = [["
+ This algorithm implements a greedy heuristic of Berger and Shor for
+ cycle removal. It is not really compared to the other heuristics and
+ only included for demonstration purposes.
+ "]],
+ documentation = [["
+ See section~4.1.1 of Pohlmann's Diploma theses for details.
+ "]]
+ }
+
+---
+
+declare {
+ key = "random greedy cycle removal",
+ algorithm = "pgf.gd.layered.CycleRemovalBergerS1990b",
+ phase = "cycle removal",
+
+ summary = [["
+ This algorithm implements a randomized greedy heuristic of Berger
+ and Shor for cycle removal. It, too, is not really compared to
+ the other heuristics and only included for demonstration purposes.
+ "]],
+ documentation = [["
+ See section~4.1.1 of Pohlmann's Diploma theses for details.
+ "]]
+ } \ No newline at end of file