add utilities to graph API

src/graph/graph.h

@@ -1029,8 +1029,14 @@ void BaseGraph<NodeIndexType, ArcIndexType, HasReverseArcs>::
                                            t##ArcIterator(*this, node, e));   \
   }
 
-// Adapt our old iteration style to support range-based for loops.
+// Adapt our old iteration style to support range-based for loops. Add typedefs
+// required by std::iterator_traits.
 #define DEFINE_STL_ITERATOR_FUNCTIONS(iterator_class_name)  \
+  using iterator_category = std::input_iterator_tag;        \
+  using difference_type = ptrdiff_t;                        \
+  using pointer = const ArcIndexType*;                      \
+  using reference = const ArcIndexType&;                    \
+  using value_type = ArcIndexType;                          \
   bool operator!=(const iterator_class_name& other) const { \
     return index_ != other.index_;                          \
   }                                                         \
@@ -1152,6 +1158,12 @@ class ListGraph<NodeIndexType, ArcIndexType>::OutgoingArcIterator {
 template <typename NodeIndexType, typename ArcIndexType>
 class ListGraph<NodeIndexType, ArcIndexType>::OutgoingHeadIterator {
  public:
+  using iterator_category = std::input_iterator_tag;
+  using difference_type = ptrdiff_t;
+  using pointer = const NodeIndexType*;
+  using reference = const NodeIndexType&;
+  using value_type = NodeIndexType;
+
   OutgoingHeadIterator(const ListGraph& graph, NodeIndexType node)
       : graph_(graph), index_(graph.start_[node]) {
     DCHECK(graph.IsNodeValid(node));
@@ -1583,13 +1595,8 @@ class ReverseArcListGraph<NodeIndexType, ArcIndexType>::OutgoingHeadIterator {
     DCHECK(Ok());
     index_ = graph_->next_[index_];
   }
-  bool operator!=(
-      const typename ReverseArcListGraph<
-          NodeIndexType, ArcIndexType>::OutgoingHeadIterator& other) const {
-    return index_ != other.index_;
-  }
-  ArcIndexType operator*() const { return Index(); }
-  void operator++() { Next(); }
+
+  DEFINE_STL_ITERATOR_FUNCTIONS(OutgoingHeadIterator);
 
  private:
   const ReverseArcListGraph* graph_;
@@ -2156,6 +2163,23 @@ class CompleteBipartiteGraph
                                                       ArcIndexType from) const;
   IntegerRange<NodeIndexType> operator[](NodeIndexType node) const;
 
+  // Deprecated interface.
+  class OutgoingArcIterator {
+   public:
+    OutgoingArcIterator(const CompleteBipartiteGraph& graph, NodeIndexType node)
+        : index_(graph.right_nodes_ * node),
+          limit_(node >= graph.left_nodes_ ? index_
+                                           : graph.right_nodes_ * (node + 1)) {}
+
+    bool Ok() const { return index_ < limit_; }
+    ArcIndexType Index() const { return index_; }
+    void Next() { index_++; }
+
+   private:
+    ArcIndexType index_;
+    const ArcIndexType limit_;
+  };
+
  private:
   const NodeIndexType left_nodes_;
   const NodeIndexType right_nodes_;
@@ -2165,7 +2189,7 @@ template <typename NodeIndexType, typename ArcIndexType>
 NodeIndexType CompleteBipartiteGraph<NodeIndexType, ArcIndexType>::Head(
     ArcIndexType arc) const {
   DCHECK(this->IsArcValid(arc));
-  return arc % right_nodes_;
+  return left_nodes_ + arc % right_nodes_;
 }
 
 template <typename NodeIndexType, typename ArcIndexType>
@@ -2207,7 +2231,7 @@ template <typename NodeIndexType, typename ArcIndexType>
 IntegerRange<NodeIndexType> CompleteBipartiteGraph<
     NodeIndexType, ArcIndexType>::operator[](NodeIndexType node) const {
   if (node < left_nodes_) {
-    return IntegerRange<NodeIndexType>(0, right_nodes_);
+    return IntegerRange<NodeIndexType>(left_nodes_, left_nodes_ + right_nodes_);
   } else {
     return IntegerRange<NodeIndexType>(0, 0);
   }

src/graph/hamiltonian_path.h

@@ -27,15 +27,14 @@
 // anywhere, but you have to return to your start location.
 //
 // By complete we mean that the algorithm guarantees to compute the optimal
-// solution.
-// The algorithm uses dynamic programming. Its time complexity is
-// O(n * 2 ^ (n - 1)), where n is the number of nodes to be visited, and '^'
-// denotes exponentiation. Its space complexity is also O(n * 2 ^ (n - 1)).
+// solution. The algorithm uses dynamic programming. Its time complexity is
+// O(n^2 * 2^(n-1)), where n is the number of nodes to be visited, and '^'
+// denotes exponentiation. Its space complexity is O(n * 2 ^ (n - 1)).
 //
 // Note that the naive implementation of the SHPP
 // exploring all permutations without memorizing intermediate results would
 // have a complexity of (n - 1)! (factorial of (n - 1) ), which is much higher
-// than n * 2 ^ (n - 1). To convince oneself of this, just use Stirling's
+// than n^2 * 2^(n-1). To convince oneself of this, just use Stirling's
 // formula: n! ~ sqrt(2 * pi * n)*( n / exp(1)) ^ n.
 // Because of these complexity figures, the algorithm is not practical for
 // problems with more than 20 nodes.

src/graph/util.h

@@ -22,6 +22,7 @@
 #include <string>
 
 #include "base/numbers.h"
+#include "base/hash.h"
 #include "base/split.h"
 #include "base/join.h"
 #include "base/murmur.h"
@@ -34,8 +35,7 @@ namespace operations_research {
 
 // Diagnoses whether a graph is symmetric. A graph is symmetric iff
 // for all (a, b), the number of arcs a->b is equal to the number of arcs b->a.
-// If it returns "false", the graph is certainly not symmetric; if it returns
-// "true" then the graph is most likely symmetric. It works in O(graph size).
+// Works in O(graph size).
 template <class Graph>
 bool GraphIsSymmetric(const Graph& graph);
 
@@ -47,6 +47,16 @@ template <class Graph>
 util::StatusOr<Graph*> RemapGraph(const Graph& graph,
                                   const std::vector<int>& new_node_index);
 
+// Returns a std::string representation of a graph: one arc per line. Eg.:
+// "1->2\n3->3" for a graph with 4 nodes and 2 arcs (1->2) and (3->3).
+// Arcs are sorted by their tail, then by the order of OutgoingArcs().
+template <class Graph>
+std::string GraphToString(const Graph& graph);
+
+// Returns a copy of "graph", without self-arcs and duplicate arcs.
+template <class Graph>
+std::unique_ptr<Graph> RemoveSelfArcsAndDuplicateArcs(const Graph& graph);
+
 // Read a graph file in the simple ".g" format: the file should be a text file
 // containing only space-separated integers, whose first line is:
 //   <num nodes> <num edges> [<num_colors> <index of first node with color #1>
@@ -166,6 +176,36 @@ util::StatusOr<Graph*> RemapGraph(const Graph& old_graph,
   return new_graph.release();
 }
 
+template <class Graph>
+std::string GraphToString(const Graph& graph) {
+  std::string out;
+  for (const typename Graph::NodeIndex node : graph.AllNodes()) {
+    for (const typename Graph::ArcIndex arc : graph.OutgoingArcs(node)) {
+      if (!out.empty()) out += '\n';
+      StrAppend(&out, node, "->", graph.Head(arc));
+    }
+  }
+  return out;
+}
+
+template <class Graph>
+std::unique_ptr<Graph> RemoveSelfArcsAndDuplicateArcs(const Graph& graph) {
+  std::unique_ptr<Graph> g(new Graph(graph.num_nodes(), graph.num_arcs()));
+  typedef typename Graph::ArcIndex ArcIndex;
+  typedef typename Graph::NodeIndex NodeIndex;
+  hash_set<std::pair<NodeIndex, NodeIndex>> arcs;
+  for (const NodeIndex tail : graph.AllNodes()) {
+    for (const ArcIndex arc : graph.OutgoingArcs(tail)) {
+      const NodeIndex head = graph.Head(arc);
+      if (head != tail && arcs.insert({tail, head}).second) {
+        g->AddArc(tail, head);
+      }
+    }
+  }
+  g->Build();
+  return g;
+}
+
 template <class Graph>
 util::StatusOr<Graph*> ReadGraphFile(
     const std::string& filename, bool directed,