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+-- Copyright 2012 by Till Tantau
+--
+-- This file may be distributed an/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: /cvsroot/pgf/pgf/generic/pgf/graphdrawing/lua/pgf/gd/lib/Direct.lua,v 1.2 2013/04/04 20:43:45 tantau Exp $
+
+
+
+--- Direct is a class that collects algorithms for computing new
+-- versions of a graph where arcs point in certain directions.
+
+local Direct = {}
+
+-- Namespace
+require("pgf.gd.lib").Direct = Direct
+
+-- Imports
+local Digraph = require "pgf.gd.model.Digraph"
+
+
+--- Compute a digraph from a syntactic digraph.
+--
+-- This function takes a syntactic digraph and compute a new digraph
+-- where all arrow point in the "semantic direction" of the syntactic
+-- arrows. For instance, while "a <- b" will cause an arc from a to be
+-- to be added to the syntactic digraph, calling this function will
+-- return a digraph in which there is an arc from b to a rather than
+-- the other way round. In detail, "a <- b" is tranlated as just
+-- described, "a -> b" yields an arc from a to b as expected, "a <-> b"
+-- and "a -- b" yield arcs in both directions and, finally, "a -!- b"
+-- yields no arc at all.
+--
+-- @param syntactic_digraph A syntacitic digraph, usually the "input"
+-- graph as specified syntactically be the user.
+--
+-- @return A new "semantic" digraph object.
+
+function Direct.digraphFromSyntacticDigraph(syntactic_digraph)
+ local digraph = Digraph.new(syntactic_digraph) -- copy
+
+ -- Now go over all arcs of the syntactic_digraph and turn them into
+ -- arcs with the correct direction in the digraph:
+ for _,a in ipairs(syntactic_digraph.arcs) do
+ for _,m in ipairs(a.syntactic_edges) do
+ local direction = m.direction
+ if direction == "->" then
+ digraph:connect(a.tail, a.head)
+ elseif direction == "<-" then
+ digraph:connect(a.head, a.tail)
+ elseif direction == "--" or direction == "<->" then
+ digraph:connect(a.tail, a.head)
+ digraph:connect(a.head, a.tail)
+ end
+ -- Case -!-: No edges...
+ end
+ end
+
+ return digraph
+end
+
+
+--- Turn an arbitrary graph into a directed graph
+--
+-- Takes a digraph as input and returns its underlying undirected
+-- graph, coded as a digraph. This means that between any two vertices
+-- if there is an arc in one direction, there is also one in the other.
+--
+-- @param digraph A directed graph
+--
+-- @return The underlying undirected graph of digraph.
+
+function Direct.ugraphFromDigraph(digraph)
+ local ugraph = Digraph.new(digraph)
+
+ -- Now go over all arcs of the syntactic_digraph and turn them into
+ -- arcs with the correct direction in the digraph:
+ for _,a in ipairs(digraph.arcs) do
+ ugraph:connect(a.head,a.tail)
+ ugraph:connect(a.tail,a.head)
+ end
+
+ return ugraph
+end
+
+
+
+
+-- Done
+
+return Direct \ No newline at end of file