# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this
# file, You can obtain one at http://mozilla.org/MPL/2.0/.

from __future__ import absolute_import, print_function, unicode_literals

import collections

class Graph(object):
    """
    Generic representation of a directed acyclic graph with labeled edges
    connecting the nodes.  Graph operations are implemented in a functional
    manner, so the data structure is immutable.

    It permits at most one edge of a given name between any set of nodes.  The
    graph is not checked for cycles, and methods may hang or otherwise fail if
    given a cyclic graph.

    The `nodes` and `edges` attributes may be accessed in a read-only fashion.
    The `nodes` attribute is a set of node names, while `edges` is a set of
    `(left, right, name)` tuples representing an edge named `name` going from
    node `left` to node `right..
    """

    def __init__(self, nodes, edges):
        """
        Create a graph.  Nodes and edges are both as described in the class
        documentation.  Both values are used by reference, and should not be
        modified after building a graph.
        """
        assert isinstance(nodes, set)
        assert isinstance(edges, set)
        self.nodes = nodes
        self.edges = edges

    def __eq__(self, other):
        return self.nodes == other.nodes and self.edges == other.edges

    def __repr__(self):
        return "<Graph nodes={!r} edges={!r}>".format(self.nodes, self.edges)

    def transitive_closure(self, nodes):
        """
        Return the transitive closure of <nodes>: the graph containing all
        specified nodes as well as any nodes reachable from them, and any
        intervening edges.
        """
        assert isinstance(nodes, set)
        assert nodes <= self.nodes

        # generate a new graph by expanding along edges until reaching a fixed
        # point
        new_nodes, new_edges = nodes, set()
        nodes, edges = set(), set()
        while (new_nodes, new_edges) != (nodes, edges):
            nodes, edges = new_nodes, new_edges
            add_edges = set((left, right, name) for (left, right, name) in self.edges if left in nodes)
            add_nodes = set(right for (_, right, _) in add_edges)
            new_nodes = nodes | add_nodes
            new_edges = edges | add_edges
        return Graph(new_nodes, new_edges)

    def visit_postorder(self):
        """
        Generate a sequence of nodes in postorder, such that every node is
        visited *after* any nodes it links to.

        Behavior is undefined (read: it will hang) if the graph contains a
        cycle.
        """
        queue = collections.deque(sorted(self.nodes))
        links_by_node = self.links_dict()
        seen = set()
        while queue:
            node = queue.popleft()
            if node in seen:
                continue
            links = links_by_node[node]
            if all((n in seen) for n in links):
                seen.add(node)
                yield node
            else:
                queue.extend(n for n in links if n not in seen)
                queue.append(node)

    def links_dict(self):
        """
        Return a dictionary mapping each node to a set of its downstream
        nodes (omitting edge names)
        """
        links = collections.defaultdict(set)
        for left, right, _ in self.edges:
            links[left].add(right)
        return links

    def reverse_links_dict(self):
        """
        Return a dictionary mapping each node to a set of its upstream
        nodes (omitting edge names)
        """
        links = collections.defaultdict(set)
        for left, right, _ in self.edges:
            links[right].add(left)
        return links
