lib: rename graphs to graph

[nit.git] / lib / graph / digraph.nit

# This file is part of NIT (http://www.nitlanguage.org).
#
# Copyright 2015 Alexandre Blondin Massé <blondin_masse.alexandre@uqam.ca>
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Implementation of directed graphs, also called digraphs.
#
# Overview
# ========
#
# This module provides a simple interface together with a concrete
# implementation of directed graphs (or digraphs).
#
# The upper level interface is `Digraph` and contains all methods for digraphs
# that do not depend on the underlying data structure. More precisely, if basic
# operations such as `predecessors`, `successors`, `num_vertices`, etc.  are
# implemented, then high level operations (such as computing the connected
# components or a shortest path between two vertices) can be easily derived.
# Also, all methods found in `Digraph` do no modify the graph. For mutable
# methods, one needs to check the `MutableDigraph` child class. Vertices can be
# any `Object`, but there is no information stored in the arcs, which are
# simple arrays of the form `[u,v]`, where `u` is the source of the arc and `v`
# is the target.
#
# There is currently only one concrete implementation named `HashDigraph` that
# makes use of the HashMap class for storing the predecessors and successors.
# It is therefore simple to provide another implementation: One only has to
# create a concrete specialization of either `Digraph` or `MutableDigraph`.
#
# Basic methods
# =============
#
# To create an (empty) new graph whose keys are integers, one simply type
# ~~~
# import digraph
# var g = new HashDigraph[Int]
# ~~~
#
# Then we can add vertices and arcs. Note that if an arc is added whose source
# and target are not already in the digraph, the vertices are added beforehand.
# ~~~
# import digraph
# var g = new HashDigraph[Int]
# g.add_vertex(0)
# g.add_vertex(1)
# g.add_arc(0,1)
# g.add_arc(1,2)
# assert g.to_s == "Digraph of 3 vertices and 2 arcs"
# ~~~
#
# One might also create digraphs with strings in vertices, for instance to
# represent some directed relation. However, it is currently not possible to
# store values in the arcs.
# ~~~
# import digraph
# var g = new HashDigraph[String]
# g.add_vertex("Amy")
# g.add_vertex("Bill")
# g.add_vertex("Chris")
# g.add_vertex("Diane")
# g.add_arc("Amy", "Bill")    # Amy likes Bill
# g.add_arc("Bill", "Amy")    # Bill likes Amy
# g.add_arc("Chris", "Diane") # and so on
# g.add_arc("Diane", "Amy")   # and so on
# ~~~
#
# `HashDigraph`s are mutable, i.e. one might remove arcs and/or vertices:
# ~~~
# import digraph
# var g = new HashDigraph[Int]
# g.add_arc(0,1)
# g.add_arc(0,2)
# g.add_arc(1,2)
# g.add_arc(2,3)
# g.add_arc(2,4)
# g.remove_vertex(1)
# g.remove_arc(2, 4)
# assert g.to_s == "Digraph of 4 vertices and 2 arcs"
# ~~~
#
# If one has installed [Graphviz](http://graphviz.org), it is easy to produce a
# *dot* file which Graphviz process into a picture:
# ~~~
# import digraph
# var g = new HashDigraph[Int]
# g.add_arcs([[0,1],[0,2],[1,2],[2,3],[2,4]])
# print g.to_dot
# # Then call "dot -Tpng -o graph.png"
# ~~~
#
# ![A graph drawing produced by Graphviz](https://github.com/nitlang/nit/blob/master/lib/graph.png)
#
# Other methods
# =============
#
# There exist other methods available for digraphs and many other will be
# implemented in the future. For more details, one should look at the methods
# directly. For instance, the [strongly connected components]
# (https://en.wikipedia.org/wiki/Strongly_connected_component) of a digraph are
# returned as a [disjoint set data structure]
# (https://en.wikipedia.org/wiki/Disjoint-set_data_structure) (i.e. a set of
# sets):
# ~~~
# import digraph
# var g = new HashDigraph[Int]
# g.add_arcs([[1,2],[2,1],[2,3],[3,4],[4,5],[5,3]])
# for component in g.strongly_connected_components.to_partitions
# do
#       print component
# end
# # Prints [1,2] and [3,4,5]
# ~~~
#
# It is also possible to compute a shortest (directed) path between two
# vertices:
# ~~~
# import digraph
# var g = new HashDigraph[Int]
# g.add_arcs([[1,2],[2,1],[2,3],[3,4],[4,5],[5,3]])
# var path = g.a_shortest_path(2, 4)
# if path != null then print path else print "No path"
# # Prints [2,3,4]
# path = g.a_shortest_path(4, 2)
# if path != null then print path else print "No path"
# # Prints "No path"
# ~~~
#
# Extending the library
# =====================
#
# There are at least two ways of providing new methods on digraphs. If the
# method is standard and could be useful to other users, you should consider
# including your implementation directly in this library.
#
# Otherwise, for personal use, you should simply define a new class inheriting
# from `HashDigraph` and add the new services.
module digraph

# Interface for digraphs
interface Digraph[V: Object]

        ## ---------------- ##
        ## Abstract methods ##
        ## ---------------- ##

        # The number of vertices in this graph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_vertex(0)
        # g.add_vertex(1)
        # assert g.num_vertices == 2
        # g.add_vertex(0)
        # assert g.num_vertices == 2
        # ~~~
        fun num_vertices: Int is abstract

        # The number of arcs in this graph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(0, 1)
        # assert g.num_arcs == 1
        # g.add_arc(0, 1)
        # assert g.num_arcs == 1
        # g.add_arc(2, 3)
        # assert g.num_arcs == 2
        # ~~~
        fun num_arcs: Int is abstract

        # Returns true if and only if `u` exists in this graph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_vertex(1)
        # assert g.has_vertex(1)
        # assert not g.has_vertex(0)
        # g.add_vertex(1)
        # assert g.has_vertex(1)
        # assert not g.has_vertex(0)
        # ~~~
        fun has_vertex(u: V): Bool is abstract

        # Returns true if and only if `(u,v)` is an arc in this graph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(0, 1)
        # g.add_arc(1, 2)
        # assert g.has_arc(0, 1)
        # assert g.has_arc(1, 2)
        # assert not g.has_arc(0, 2)
        # ~~~
        fun has_arc(u, v: V): Bool is abstract

        # Returns the predecessors of `u`.
        #
        # If `u` does not exist, then it returns null.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(0, 1)
        # g.add_arc(1, 2)
        # g.add_arc(0, 2)
        # assert g.predecessors(2).has(0)
        # assert g.predecessors(2).has(1)
        # assert not g.predecessors(2).has(2)
        # ~~~
        fun predecessors(u: V): Collection[V] is abstract

        # Returns the successors of `u`.
        #
        # If `u` does not exist, then an empty collection is returned.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(0, 1)
        # g.add_arc(1, 2)
        # g.add_arc(0, 2)
        # assert not g.successors(0).has(0)
        # assert g.successors(0).has(1)
        # assert g.successors(0).has(2)
        # ~~~
        fun successors(u: V): Collection[V] is abstract

        # Returns an iterator over the vertices of this graph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(0, 1)
        # g.add_arc(0, 2)
        # g.add_arc(1, 2)
        # var vs = new HashSet[Int]
        # for v in g.vertices_iterator do vs.add(v)
        # assert vs == new HashSet[Int].from([0,1,2])
        # ~~~
        fun vertices_iterator: Iterator[V] is abstract

        ## -------------------- ##
        ## Non abstract methods ##
        ## -------------------- ##

        ## ------------- ##
        ## Basic methods ##
        ## ------------- ##

        # Returns true if and only if this graph is empty.
        #
        # An empty graph is a graph without vertex and arc.
        #
        # ~~~
        # import digraph
        # assert (new HashDigraph[Int]).is_empty
        # ~~~
        fun is_empty: Bool do return num_vertices == 0 and num_arcs == 0

        # Returns an array containing the vertices of this graph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_vertices([0,2,4,5])
        # assert g.vertices.length == 4
        # ~~~
        fun vertices: Array[V] do return [for u in vertices_iterator do u]

        # Returns an iterator over the arcs of this graph
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(0, 1)
        # g.add_arc(0, 2)
        # g.add_arc(1, 2)
        # for arc in g.arcs_iterator do
        #       assert g.has_arc(arc[0], arc[1])
        # end
        # ~~~
        fun arcs_iterator: Iterator[Array[V]] do return new ArcsIterator[V](self)

        # Returns the arcs of this graph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 3)
        # g.add_arc(2, 3)
        # assert g.arcs.length == 2
        # ~~~
        fun arcs: Array[Array[V]] do return [for arc in arcs_iterator do arc]

        # Returns the incoming arcs of vertex `u`.
        #
        # If `u` is not in this graph, an empty array is returned.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 3)
        # g.add_arc(2, 3)
        # for arc in g.incoming_arcs(3) do
        #       assert g.is_predecessor(arc[0], arc[1])
        # end
        # ~~~
        fun incoming_arcs(u: V): Collection[Array[V]]
        do
                if has_vertex(u) then
                        return [for v in predecessors(u) do [v, u]]
                else
                        return new Array[Array[V]]
                end
        end

        # Returns the outgoing arcs of vertex `u`.
        #
        # If `u` is not in this graph, an empty array is returned.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 3)
        # g.add_arc(2, 3)
        # g.add_arc(1, 2)
        # for arc in g.outgoing_arcs(1) do
        #       assert g.is_successor(arc[1], arc[0])
        # end
        # ~~~
        fun outgoing_arcs(u: V): Collection[Array[V]]
        do
                if has_vertex(u) then
                        return [for v in successors(u) do [u, v]]
                else
                        return new Array[Array[V]]
                end
        end

        ## ---------------------- ##
        ## String representations ##
        ## ---------------------- ##

        redef fun to_s
        do
                var vertex_word = "vertices"
                var arc_word = "arcs"
                if num_vertices <= 1 then vertex_word = "vertex"
                if num_arcs <= 1 then arc_word = "arc"
                return "Digraph of {num_vertices} {vertex_word} and {num_arcs} {arc_word}"
        end

        # Returns a GraphViz string representing this digraph.
        fun to_dot: String
        do
                var s = "digraph \{\n"
                var id_set = new HashMap[V, Int]
                # Writing the vertices
                for u in vertices_iterator, i in [0 .. vertices.length[ do
                        id_set[u] = i
                        s += "   \"{i}\" "
                        s += "[label=\"{u.to_s.escape_to_dot}\"];\n"
                end
                # Writing the arcs
                for arc in arcs do
                        s += "   {id_set[arc[0]]} "
                        s += "-> {id_set[arc[1]]};"
                end
                s += "\}"
                return s
        end

        # Open Graphviz with `self.to_dot`.
        #
        # Mainly used for debugging.
        fun show_dot do
                var f = new ProcessWriter("dot", "-Txlib")
                f.write to_dot
                f.close
        end

        ## ------------ ##
        ## Neighborhood ##
        ## ------------ ##

        # Returns true if and only if `u` is a predecessor of `v`.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 3)
        # assert g.is_predecessor(1, 3)
        # assert not g.is_predecessor(3, 1)
        # ~~~
        fun is_predecessor(u, v: V): Bool do return has_arc(u, v)

        # Returns true if and only if `u` is a successor of `v`.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 3)
        # assert not g.is_successor(1, 3)
        # assert g.is_successor(3, 1)
        # ~~~
        fun is_successor(u, v: V): Bool do return has_arc(v, u)

        # Returns the number of arcs whose target is `u`.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 3)
        # g.add_arc(2, 3)
        # assert g.in_degree(3) == 2
        # assert g.in_degree(1) == 0
        # ~~~
        fun in_degree(u: V): Int do return predecessors(u).length

        # Returns the number of arcs whose source is `u`.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 2)
        # g.add_arc(1, 3)
        # g.add_arc(2, 3)
        # assert g.out_degree(3) == 0
        # assert g.out_degree(1) == 2
        # ~~~
        fun out_degree(u: V): Int do return successors(u).length

        # ------------------ #
        # Paths and circuits #
        # ------------------ #

        # Returns true if and only if `vertices` is a path of this digraph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 2)
        # g.add_arc(2, 3)
        # g.add_arc(3, 4)
        # assert g.has_path([1,2,3])
        # assert not g.has_path([1,3,3])
        # ~~~
        fun has_path(vertices: SequenceRead[V]): Bool
        do
                for i in [0..vertices.length - 1[ do
                        if not has_arc(vertices[i], vertices[i + 1]) then return false
                end
                return true
        end

        # Returns true if and only if `vertices` is a circuit of this digraph.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 2)
        # g.add_arc(2, 3)
        # g.add_arc(3, 1)
        # assert g.has_circuit([1,2,3,1])
        # assert not g.has_circuit([1,3,2,1])
        # ~~~
        fun has_circuit(vertices: SequenceRead[V]): Bool
        do
                return vertices.is_empty or (has_path(vertices) and vertices.first == vertices.last)
        end

        # Returns a shortest path from vertex `u` to `v`.
        #
        # If no path exists between `u` and `v`, it returns `null`.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 2)
        # g.add_arc(2, 3)
        # g.add_arc(3, 4)
        # assert g.a_shortest_path(1, 4).length == 4
        # g.add_arc(1, 3)
        # assert g.a_shortest_path(1, 4).length == 3
        # assert g.a_shortest_path(4, 1) == null
        # ~~~
        fun a_shortest_path(u, v: V): nullable Sequence[V]
        do
                var queue = new List[V].from([u]).as_fifo
                var pred = new HashMap[V, nullable V]
                var visited = new HashSet[V]
                var w: nullable V = null
                pred[u] = null
                while not queue.is_empty do
                        w = queue.take
                        if not visited.has(w) then
                                visited.add(w)
                                if w == v then break
                                for wp in successors(w) do
                                        if not pred.keys.has(wp) then
                                                queue.add(wp)
                                                pred[wp] = w
                                        end
                                end
                        end
                end
                if w != v then
                        return null
                else
                        var path = new List[V]
                        path.add(v)
                        w = v
                        while pred[w] != null do
                                path.unshift(pred[w].as(not null))
                                w = pred[w]
                        end
                        return path
                end
        end

        # Returns the distance between `u` and `v`
        #
        # If no path exists between `u` and `v`, it returns null. It is not
        # symmetric, i.e. we may have `dist(u, v) != dist(v, u)`.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 2)
        # g.add_arc(2, 3)
        # g.add_arc(3, 4)
        # assert g.distance(1, 4) == 3
        # g.add_arc(1, 3)
        # assert g.distance(1, 4) == 2
        # assert g.distance(4, 1) == null
        # ~~~
        fun distance(u, v: V): nullable Int
        do
                var queue = new List[V].from([u]).as_fifo
                var dist = new HashMap[V, Int]
                var visited = new HashSet[V]
                var w: nullable V
                dist[u] = 0
                while not queue.is_empty do
                        w = queue.take
                        if not visited.has(w) then
                                visited.add(w)
                                if w == v then break
                                for wp in successors(w) do
                                        if not dist.keys.has(wp) then
                                                queue.add(wp)
                                                dist[wp] = dist[w] + 1
                                        end
                                end
                        end
                end
                return dist.get_or_null(v)
        end

        # -------------------- #
        # Connected components #
        # -------------------- #

        # Returns the weak connected components of this digraph.
        #
        # The weak connected components of a digraph are the usual
        # connected components of its associated undirected graph,
        # i.e. the graph obtained by replacing each arc by an edge.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 2)
        # g.add_arc(2, 3)
        # g.add_arc(4, 5)
        # assert g.weakly_connected_components.number_of_subsets == 2
        # ~~~
        fun weakly_connected_components: DisjointSet[V]
        do
                var components = new DisjointSet[V]
                components.add_all(vertices)
                for arc in arcs_iterator do
                        components.union(arc[0], arc[1])
                end
                return components
        end

        # Returns the strongly connected components of this digraph.
        #
        # Two vertices `u` and `v` belong to the same strongly connected
        # component if and only if there exists a path from `u` to `v`
        # and there exists a path from `v` to `u`.
        #
        # This is computed in linear time (Tarjan's algorithm).
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(1, 2)
        # g.add_arc(2, 3)
        # g.add_arc(3, 1)
        # g.add_arc(3, 4)
        # g.add_arc(4, 5)
        # g.add_arc(5, 6)
        # g.add_arc(6, 5)
        # assert g.strongly_connected_components.number_of_subsets == 3
        # ~~~
        fun strongly_connected_components: DisjointSet[V]
        do
                var tarjanAlgorithm = new TarjanAlgorithm[V](self)
                return tarjanAlgorithm.strongly_connected_components
        end
end

# Computing the strongly connected components using Tarjan's algorithm
private class TarjanAlgorithm[V: Object]
        # The graph whose strongly connected components will be computed
        var graph: Digraph[V]
        # The strongly connected components computed in Tarjan's algorithm
        var sccs = new DisjointSet[V]
        # An index used for Tarjan's algorithm
        var index = 0
        # A stack used for Tarjan's algorithm
        var stack: Queue[V] = (new Array[V]).as_lifo
        # A map associating with each vertex its index
        var vertex_to_index = new HashMap[V, Int]
        # A map associating with each vertex its ancestor in Tarjan's algorithm
        var ancestor = new HashMap[V, Int]
        # True if and only if the vertex is in the stack
        var in_stack = new HashSet[V]

        # Returns the strongly connected components of a graph
        fun strongly_connected_components: DisjointSet[V]
        do
                for u in graph.vertices_iterator do sccs.add(u)
                for v in graph.vertices_iterator do
                        visit(v)
                end
                return sccs
        end

        # The recursive part of Tarjan's algorithm
        fun visit(u: V)
        do
                vertex_to_index[u] = index
                ancestor[u] = index
                index += 1
                stack.add(u)
                in_stack.add(u)
                for v in graph.successors(u) do
                        if not vertex_to_index.keys.has(v) then
                                visit(v)
                                ancestor[u] = ancestor[u].min(ancestor[v])
                        else if in_stack.has(v) then
                                ancestor[u] = ancestor[u].min(vertex_to_index[v])
                        end
                end
                if vertex_to_index[u] == ancestor[u] then
                        var v
                        loop
                                v = stack.take
                                in_stack.remove(v)
                                sccs.union(u, v)
                                if u == v then break
                        end
                end
        end
end

# Arcs iterator
class ArcsIterator[V: Object]
        super Iterator[Array[V]]

        # The graph whose arcs are iterated over
        var graph: Digraph[V]
        # Attributes
        #
        private var sources_iterator: Iterator[V] is noinit
        private var targets_iterator: Iterator[V] is noinit
        init
        do
                sources_iterator = graph.vertices_iterator
                if sources_iterator.is_ok then
                        targets_iterator = graph.successors(sources_iterator.item).iterator
                        if not targets_iterator.is_ok then update_iterators
                end
        end

        redef fun is_ok do return sources_iterator.is_ok and targets_iterator.is_ok

        redef fun item do return [sources_iterator.item, targets_iterator.item]

        redef fun next
        do
                targets_iterator.next
                update_iterators
        end

        private fun update_iterators
        do
                while not targets_iterator.is_ok and sources_iterator.is_ok
                do
                        sources_iterator.next
                        if sources_iterator.is_ok then
                                targets_iterator = graph.successors(sources_iterator.item).iterator
                        end
                end
        end
end

# Mutable digraph
abstract class MutableDigraph[V: Object]
        super Digraph[V]

        ## ---------------- ##
        ## Abstract methods ##
        ## ---------------- ##

        # Adds the vertex `u` to this graph.
        #
        # If `u` already belongs to the graph, then nothing happens.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_vertex(0)
        # assert g.has_vertex(0)
        # assert not g.has_vertex(1)
        # g.add_vertex(1)
        # assert g.num_vertices == 2
        # ~~~
        fun add_vertex(u: V) is abstract

        # Removes the vertex `u` from this graph and all its incident arcs.
        #
        # If the vertex does not exist in the graph, then nothing happens.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_vertex(0)
        # g.add_vertex(1)
        # assert g.has_vertex(0)
        # g.remove_vertex(0)
        # assert not g.has_vertex(0)
        # ~~~
        fun remove_vertex(u: V) is abstract

        # Adds the arc `(u,v)` to this graph.
        #
        # If there is already an arc from `u` to `v` in this graph, then
        # nothing happens. If vertex `u` or vertex `v` do not exist in the
        # graph, they are added.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(0, 1)
        # g.add_arc(1, 2)
        # assert g.has_arc(0, 1)
        # assert g.has_arc(1, 2)
        # assert not g.has_arc(1, 0)
        # g.add_arc(1, 2)
        # assert g.num_arcs == 2
        # ~~~
        fun add_arc(u, v: V) is abstract

        # Removes the arc `(u,v)` from this graph.
        #
        # If the arc does not exist in the graph, then nothing happens.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_arc(0, 1)
        # assert g.num_arcs == 1
        # g.remove_arc(0, 1)
        # assert g.num_arcs == 0
        # g.remove_arc(0, 1)
        # assert g.num_arcs == 0
        # ~~~
        fun remove_arc(u, v: V) is abstract

        ## -------------------- ##
        ## Non abstract methods ##
        ## -------------------- ##

        # Adds all vertices of `vertices` to this digraph.
        #
        # If vertices appear more than once, they are only added once.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # g.add_vertices([0,1,2,3])
        # assert g.num_vertices == 4
        # g.add_vertices([2,3,4,5])
        # assert g.num_vertices == 6
        # ~~~
        fun add_vertices(vertices: Collection[V])
        do
                for u in vertices do add_vertex(u)
        end

        # Adds all arcs of `arcs` to this digraph.
        #
        # If arcs appear more than once, they are only added once.
        #
        # ~~~
        # import digraph
        # var g = new HashDigraph[Int]
        # var arcs = [[0,1], [1,2], [1,2]]
        # g.add_arcs(arcs)
        # assert g.num_arcs == 2
        # ~~~
        fun add_arcs(arcs: Collection[Array[V]])
        do
                for a in arcs do add_arc(a[0], a[1])
        end
end
# A directed graph represented by hash maps
class HashDigraph[V: Object]
        super MutableDigraph[V]

        # Attributes
        #
        private var incoming_vertices_map = new HashMap[V, Array[V]]
        private var outgoing_vertices_map = new HashMap[V, Array[V]]
        private var number_of_arcs = 0

        redef fun num_vertices do return outgoing_vertices_map.keys.length end

        redef fun num_arcs do return number_of_arcs end

        redef fun add_vertex(u)
        do
                if not has_vertex(u) then
                        incoming_vertices_map[u] = new Array[V]
                        outgoing_vertices_map[u] = new Array[V]
                end
        end

        redef fun has_vertex(u) do return outgoing_vertices_map.keys.has(u)

        redef fun remove_vertex(u)
        do
                if has_vertex(u) then
                        for v in successors(u) do
                                remove_arc(u, v)
                        end
                        for v in predecessors(u) do
                                remove_arc(v, u)
                        end
                        incoming_vertices_map.keys.remove(u)
                        outgoing_vertices_map.keys.remove(u)
                end
        end

        redef fun add_arc(u, v)
        do
                if not has_vertex(u) then add_vertex(u)
                if not has_vertex(v) then add_vertex(v)
                if not has_arc(u, v) then
                        incoming_vertices_map[v].add(u)
                        outgoing_vertices_map[u].add(v)
                        number_of_arcs += 1
                end
        end

        redef fun has_arc(u, v)
        do
                return outgoing_vertices_map[u].has(v)
        end

        redef fun remove_arc(u, v)
        do
                if has_arc(u, v) then
                        outgoing_vertices_map[u].remove(v)
                        incoming_vertices_map[v].remove(u)
                        number_of_arcs -= 1
                end
        end

        redef fun predecessors(u): Array[V]
        do
                if incoming_vertices_map.keys.has(u) then
                        return incoming_vertices_map[u].clone
                else
                        return new Array[V]
                end
        end

        redef fun successors(u): Array[V]
        do
                if outgoing_vertices_map.keys.has(u) then
                        return outgoing_vertices_map[u].clone
                else
                        return new Array[V]
                end
        end

        redef fun vertices_iterator: Iterator[V] do return outgoing_vertices_map.keys.iterator
end