""" Code example from Complexity and Computation, a book about
exploring complexity science with Python.  Available free from

http://greenteapress.com/complexity

Copyright 2011 Allen B. Downey.
Distributed under the GNU General Public License at gnu.org/licenses/gpl.html.
"""
import itertools


class GraphError(Exception):
    """Exception for Graph errors"""
    pass


class Vertex(object):
    """A Vertex is a node in a graph."""

    def __init__(self, label=''):
        self.label = label

    def __repr__(self):
        """Returns a string representation of this object that can
        be evaluated as a Python expression."""
        return 'Vertex(%s)' % repr(self.label)

    __str__ = __repr__
    """The str and repr forms of this object are the same."""


class Edge(tuple):
    """An Edge is a list of two vertices."""

    def __new__(cls, *vs):
        """The Edge constructor takes two vertices."""
        if len(vs) != 2:
            raise ValueError, 'Edges must connect exactly two vertices.'
        return tuple.__new__(cls, vs)

    def __repr__(self):
        """Return a string representation of this object that can
        be evaluated as a Python expression."""
        return 'Edge(%s, %s)' % (repr(self[0]), repr(self[1]))

    __str__ = __repr__
    """The str and repr forms of this object are the same."""


class Graph(dict):
    """A Graph is a dictionary of dictionaries.  The outer
    dictionary maps from a vertex to an inner dictionary.
    The inner dictionary maps from other vertices to edges.

    For vertices a and b, graph[a][b] maps
    to the edge that connects a->b, if it exists."""

    def __init__(self, vs=None, es=None):
        """Creates a new graph.
        vs: list of vertices;
        es: list of edges.
        """
        if vs:
            for v in vs:
                self.add_vertex(v)

        if es:
            for e in es:
                self.add_edge(e)

    def add_vertex(self, v):
        """Add a vertex to the graph."""
        self[v] = {}

    def add_edge(self, e):
        """Adds and edge to the graph by adding an entry in both directions.

        If there is already an edge connecting these Vertices, the
        new edge replaces it.
        """
        v, w = e
        self[v][w] = e
        self[w][v] = e

    def get_edge(self, v, w):
        """Returns the edge object that exists between the two vertices v & w,
        or None if no such edge exists.

        """
        try:
            return self[v][w]
        except KeyError:
            return None

    def remove_edge(self, e):
        """Removes the edge e from the graph."""

        v, w = e
        del self[v][w]
        del self[w][v]

    def vertices(self):
        """Returns a list of the vertices in the graph."""

        return self.keys()

    def edges(self):
        """"Returns a list of the edges in the graph."""

        edge_set = set()
        for d in self.itervalues():
            edge_set.update(d.itervalues())

        return list(edge_set)

    def out_vertices(self, v):
        """Returns a list of vertices that are adjacent to the given vertex v.

        """
        return self[v].keys()

    def out_edges(self, v):
        """Returns a list of edges connected to a given vertex v."""

        return self[v].values()

    def remove_all_edges(self):
        """Removes all edges in the graph."""
        for v in self.iterkeys():
            self[v] = {}

    def add_all_edges(self):
        """Makes the graph complete by adding edges between all pairs of
        vertices.

        """
        # Clear all edges first
        self.remove_all_edges()

        # For each combination of 2 vertices, create an edge between them:
        for v, w in itertools.combinations(self.iterkeys(), 2):
            self.add_edge(Edge(v, w))

    def add_regular_edges(self, k):
        """Makes the graph regular by making every vertex have k edges.

        It is not always possible to create a regular graph with a given degree.
        If a graph has n vertices, then a regular graph can be constructed with
        degree k if n >= k + 1 and n * k is even. If these conditions are not
        met a GraphError exception is raised.

        """
        n = len(self.vertices())
        if n < k + 1:
            raise GraphError("Can't make a regular graph with degree >= number"
                             " of vertices")
        if (n * k) % 2 != 0:
            raise GraphError("Can't make a regular graph of degree k and"
                             " order n where k * n is odd")

        # Remove all edges first
        self.remove_all_edges()

        if k % 2 != 0:      # if k is odd
            self._add_regular_edges_even(k - 1)
            self._add_regular_edges_odd()
        else:
            self._add_regular_edges_even(k)

    def _add_regular_edges_even(self, k):
        """Make a regular graph with degree k. k must be even."""

        vs = self.vertices()
        vs2 = vs * 2

        for i, v in enumerate(vs):
            for j in range(1, k / 2 + 1):
                w = vs2[i + j]
                self.add_edge(Edge(v, w))

    def _add_regular_edges_odd(self):
        """Adds an extra edge across the graph to finish off a regular graph
        with odd degree. The number of vertices must be even.

        """
        vs = self.vertices()
        vs2 = vs * 2
        n = len(vs)

        for i in range(n / 2):
            v = vs2[i]
            w = vs2[i + n / 2]
            self.add_edge(Edge(v, w))


def main(script, *args):
    import pprint

    v = Vertex('v')
    print v
    w = Vertex('w')
    print w
    e = Edge(v, w)
    print e
    g = Graph([v,w], [e])
    pprint.pprint(g)

    print "g.get_edge(v, w): ", g.get_edge(v, w)
    x = Vertex('x')
    print "g.get_edge(v, x): ", g.get_edge(v, x)

    g.remove_edge(e)
    pprint.pprint(g)

    print "vertices: ", g.vertices()
    print "edges: ", g.edges()

    g.add_edge(e)
    u = Vertex('u')
    e1 = Edge(u, v)
    e2 = Edge(u, w)
    g.add_vertex(u)
    g.add_edge(e1)
    g.add_edge(e2)
    print "Adding vertex u and edges:"
    pprint.pprint(g)
    print "vertices: ", g.vertices()
    print "edges: ", g.edges()

    print "Out vertices for v: ", g.out_vertices(v)
    print "Out edges for v: ", g.out_edges(v)

    x = Vertex('x')
    g.add_vertex(x)
    g.add_all_edges()
    pprint.pprint(g)


if __name__ == '__main__':
    import sys
    main(*sys.argv)