# Ticket #7527: trac_7527-reviewer.patch

File trac_7527-reviewer.patch, 10.5 KB (added by mvngu, 11 years ago)

reviewer patch

• ## sage/graphs/graph_coloring.py

# HG changeset patch
# User Minh Van Nguyen <nguyenminh2@gmail.com>
# Date 1260298074 28800
# Node ID 81b01e7a0099b3e9d9b920bae9e12352500b8c60
# Parent  8ca939870ef50e8f8ec3d94a0b9508d32489867c
trac 7527: graph colouring: reviewer patch

diff -r 8ca939870ef5 -r 81b01e7a0099 sage/graphs/graph_coloring.py
 a """ Graph Coloring Functions Graph Coloring AUTHORS: into an implementation of the Dancing Links algorithm described by Knuth (who attributes the idea to Hitotumatu and Noshita). The construction works as follows: (columns) * The first |V| columns correspond to a vertex -- a 1 in this column indicates that that vertex has a color. * After those |V| columns, we add n*|E| columns -- a 1 in these columns indicate that a particular edge is incident to a vertex with a certain color. The construction works as follows. Columns: (rows) * For each vertex, add n rows; one for each color c.  Place a 1 in the column corresponding to the vertex, and a 1 in the appropriate column for each edge incident to the vertex, indicating that that edge is incident to the color c. * If n > 2, the above construction cannot be exactly covered since each edge will be incident to only two vertices (and hence two colors) - so we add n*|E| rows, each one containing a 1 for each of the n*|E| columns.  These get added to the cover solutions "for free" during the backtracking. * The first |V| columns correspond to a vertex -- a 1 in this column indicates that that vertex has a color. * After those |V| columns, we add n*|E| columns -- a 1 in these columns indicate that a particular edge is incident to a vertex with a certain color. Rows: * For each vertex, add n rows; one for each color c.  Place a 1 in the column corresponding to the vertex, and a 1 in the appropriate column for each edge incident to the vertex, indicating that that edge is incident to the color c. * If n > 2, the above construction cannot be exactly covered since each edge will be incident to only two vertices (and hence two colors) - so we add n*|E| rows, each one containing a 1 for each of the n*|E| columns.  These get added to the cover solutions "for free" during the backtracking. Note that this construction results in n*|V| + 2*n*|E| + n*|E| entries in the matrix.  The Dancing Links algorithm uses a sparse representation, so if the graph is simple, |E| \leq |V|^2 and n <= |V|, this construction runs in O(|V|^3) time. Back-conversion to a coloring solution is a simple scan of the solutions, which will contain |V| + (n-2)*|E| entries,  so solutions, which will contain |V| + (n-2)*|E| entries,  so runs in O(|V|^3) time also.  For most graphs, the conversion will be much faster -- for example, a planar graph will be transformed for 4-coloring in linear time since |E| = O(|V|). REFERENCES: http://www-cs-staff.stanford.edu/~uno/papers/dancing-color.ps.gz http://www-cs-staff.stanford.edu/~uno/papers/dancing-color.ps.gz EXAMPLES:: sage: from sage.graphs.graph_coloring import chromatic_number sage: G = Graph({0:[1,2,3],1:}) sage: chromatic_number(G) 3 3 sage: G = graphs.PetersenGraph() sage: G.chromatic_number() 3 """ o = G.order() if o == 0: return m for n in range(m,o+1): for C in all_graph_colorings(G,n): return n return n def vertex_coloring(g, k=None, value_only=False, hex_colors=False, log=0): r""" Computes the chromatic number of the given graph or tests its k-colorability. See http://en.wikipedia.org/wiki/Graph_coloring for further details on graph coloring. INPUT: - g -- a graph. be no message printed by the solver. OUTPUT: - If k=None and value_only=False, then return a partition of the vertex set into the minimum possible of independent sets. - If k=None and value_only=True, return the chromatic number. - If k is set and value_only=None, return False if the graph is not k-colorable, and a partition of the vertex set into k independent sets otherwise. k-colorable, and return True or False accordingly. EXAMPLE:: sage: from sage.graphs.graph_coloring import vertex_coloring sage: g = graphs.PetersenGraph() sage: vertex_coloring(g, value_only=True) # optional - requires GLPK or CBC return dict(zip(rainbow(k), value)) else: return value # Degeneracy # Vertices whose degree is less than k are of no importance in # the coloring. return dict(zip(rainbow(k), value)) else: return value p = MixedIntegerLinearProgram(maximization=True) color = p.new_variable(dim=2) # a vertex has exactly one color [p.add_constraint(sum([color[v][i] for i in xrange(k)]), min=1, max=1) for v in g.vertices()] # adjacent vertices have different colors [p.add_constraint(color[u][i] + color[v][i], max=1) [p.add_constraint(color[u][i] + color[v][i], max=1) for (u, v) in g.edge_iterator(labels=None) for i in xrange(k)] # anything is good as an objective value as long as it is satisfiable color = p.get_values(color) # builds the color classes classes = [[] for i in xrange(k)] [classes[i].append(v) for i in xrange(k) for v in g.vertices() [classes[i].append(v) for i in xrange(k) for v in g.vertices() if color[v][i] == 1] if hex_colors: return dict(zip(rainbow(len(classes)), classes)) Properly colors the edges of a graph. See the URL http://en.wikipedia.org/wiki/Edge_coloring for further details on edge coloring. INPUT: - g -- a graph. In the following, \Delta is equal to the maximum degree in the graph g. - If vizing=True and value_only=False, return a partition of the edge set into \Delta + 1 matchings. - If vizing=False and value_only=False, return a partition of the edge set into the minimum number of matchings. - If vizing=True and value_only=True, should return something, but mainly you are just trying to compute the maximum degree of the graph, and this is not the easiest way. By Vizing's theorem, a graph has a chromatic index equal to \Delta or to \Delta + 1. EXAMPLE:: sage: from sage.graphs.graph_coloring import edge_coloring sage: g = graphs.PetersenGraph() sage: edge_coloring(g, value_only=True) # optional - requires GLPK or CBC if hex_colors: return zip(rainbow(len(classes)), classes) else: return classes return classes p = MixedIntegerLinearProgram(maximization=True) color = p.new_variable(dim=2) e = g.edge_iterator().next() p.set_objective(color[R(e)]) p.set_binary(color) try: try: if value_only: p.solve(objective_only=True, log=log) else: # Builds the color classes color = p.get_values(color) classes = [[] for i in xrange(k)] [classes[i].append(e) for e in g.edge_iterator() for i in xrange(k) [classes[i].append(e) for e in g.edge_iterator() for i in xrange(k) if color[R(e)][i] == 1] # if needed, builds a dictionary from the color classes adding colors if hex_colors: sage: sum([Set([e for e in g.edges_incident(v)]).cardinality() for v in g]) == 2*g.size() True sage: Set([e for e in g.edge_iterator()]).cardinality() 9 9 """ if n <= 1: raise ValueError("There must be at least two vertices in the graph.") class Test: r""" This class performs randomized testing for all_graph_colorings. Since everything else in this file is derived from Since everything else in this file is derived from all_graph_colorings, this is a pretty good randomized tester for the entire file.  Note that for a graph G, G.chromatic_polynomial() uses an entirely different algorithm, so we provide a good, def random(self,tests = 1000): r""" Calls self.random_all_graph_colorings().  In the future, if Calls self.random_all_graph_colorings().  In the future, if other methods are added, it should call them, too. TESTS:: sage: from sage.graphs.graph_coloring import Test def random_all_graph_colorings(self,tests = 1000): r""" Verifies the results of all_graph_colorings in three ways: 1) all colorings are unique 2) number of m-colorings is P(m) (where P is the chromatic polynomial of the graph being tested) 3) colorings are valid -- that is, that no two vertices of the same color share an edge. Verifies the results of all_graph_colorings() in three ways: #. all colorings are unique #. number of m-colorings is P(m) (where P is the chromatic polynomial of the graph being tested) #. colorings are valid -- that is, that no two vertices of the same color share an edge. TESTS::