edmonds-matching-algorithm/src/graph.cpp

#include "graph.hpp" // always include corresponding header first

/**
 * Note this included everything from the header similar to copy-pasting it here,
 * including our two classes, the function declarations and all the includes.
 * In this file we will actually implement the in- and output routines though,
 * so we need to include the actual implementation of std::istream and std::ostream.
 */
#include <iostream>

/**
 * We are also going to use stringstream in order to treat a line,
 * which we have already read from the input, like an input stream.
 */
#include <sstream>

/**
 * The execption header is used to terminate our program
 * in the case of unexpected input,
 * which is often the best way to handle such input.
 * More complex programs may want to catch exceptions in
 * surrouding code and either try to recover or help debug them.
 */
#include <stdexcept>

#include <cassert>

// Anonymous name spaces may be used to show the reader
// that a function will only be used in the current file.
namespace
{

// Using a function for converting the DIMACS node ids to and from our node ids
// makes the in and output code more understandable.
ED::NodeId from_dimacs_id(ED::size_type dimacs_node_id)
{
   if (dimacs_node_id <= 0)
   {
      throw std::runtime_error("Non-positive DIMACS node id can not be converted.");
   }
   return dimacs_node_id - 1;
}

ED::size_type to_dimacs_id(ED::NodeId node_id)
{
   return node_id + 1;
}

// Returns the first line which is not a comment, i.e. does not start with c.
std::string read_next_non_comment_line(std::istream & input)
{
   std::string line;
   do
   {
      if (!std::getline(input, line))
      {
         throw std::runtime_error("Unexpected end of DIMACS stream.");
      }
   }
   while (line[0] == 'c');
   return line;
}

} // end of anonymous namespace

namespace ED
{
/////////////////////////////////////////////
//! \c Node definitions
/////////////////////////////////////////////

void Node::add_neighbor(NodeId const id)
{
   _neighbors.push_back(id);
}

/////////////////////////////////////////////
//! \c Graph definitions
/////////////////////////////////////////////

// Whenever reasonably possible you should prefer to use `:`
// to initalize the members of your class, instead of
// assigning values to them after they were default initialized.
// Note you should initialize them in the same order
// they were declare in back in the class body!
Graph::Graph(NodeId const num_nodes)
: _nodes(num_nodes)
, _num_edges(0)
{}

void Graph::add_edge(NodeId node1_id, NodeId node2_id)
{
   // It is ok if your program crashes for garbage input,
   // but it should be an explicit, deliberate choice, e.g. like this.
   if (node1_id == node2_id)
   {
      throw std::runtime_error("ED::Graph class does not support loops!");
   }

   _nodes[node1_id].add_neighbor(node2_id);
   _nodes[node2_id].add_neighbor(node1_id);
   ++_num_edges;
}

Graph Graph::read_dimacs(std::istream & input)
{   
   // Unfortunatley the common std input functions require us to first declare
   // our variables and assign them the correct values only later.
   // Because we want to avoid unitizalized variables, we use a new syntax
   // added in c++17 to call the constuctor with no arguments,
   // often called the default constructor: We write {} behind the variable name.

   // When parsing the DIMACS format, there are some words we are not interested in.
   // We read them into this variable and never use the afterwards.
   std::string unused_word{};

   // As we need to watch out for comments, we first need to read the input by line.
   // In order to split non-comment lines into multiple variables we use a std::stringstream.
   std::stringstream first_buffering_stream{};
 
   // Note if you do not plan to modify a variable, always declare it as constant.
   // This does not only prevent you from doing so accidently,
   // but also helps anybody reading your code understand what you are doing,
   // as there are less possiblities what can happen.
   std::string const first_line = read_next_non_comment_line(input);

   size_type num_nodes{};
   size_type num_edges{};
   first_buffering_stream << first_line;
   first_buffering_stream >> unused_word >> unused_word >> num_nodes >> num_edges;

   // Now we successively add edges to our graph;
   Graph graph(num_nodes);
   for (size_type i = 1; i <= num_edges; ++i)
   {
      // This works just as parsing the first line!
      std::stringstream ith_buffering_stream{};
      std::string const ith_line = read_next_non_comment_line(input);
      size_type dimacs_node1{};
      size_type dimacs_node2{};
      ith_buffering_stream << ith_line;
      ith_buffering_stream >> unused_word >> dimacs_node1 >> dimacs_node2;
      graph.add_edge(from_dimacs_id(dimacs_node1), from_dimacs_id(dimacs_node2)); 
   }

   return graph;
}

std::ostream & operator<<(std::ostream & output, Graph const & graph)
{
   // We use std::endl to write new lines here.
   // If you prefer the new line character, \n on linux, that one works fine, too.
   output << "p edge " << graph.num_nodes() << " " << graph.num_edges() << std::endl;

   // We will need the id of the node we are at, so we write a plain old loop here.
   for (NodeId node_id = 0; node_id < graph.num_nodes(); ++node_id)
   {
      Node const & node = graph.node(node_id);
      // We do not need to keep track of the index neighbor_id has in node,
      // so we can use this cool loop syntax introduced in c++11.
      for (NodeId const & neighbor_id : node.neighbors())
      {
	 // Note we iterate over each edge two times, so we use the following
	 // comparism to check if the edge was not yet written to str!
         if (node_id < neighbor_id)
         {
            output << "e " << to_dimacs_id(node_id) << " " << to_dimacs_id(neighbor_id) << std::endl;
         }
      }
   }

   // Streams sometimes buffer their output.
   // Once one is done with some output routine, it can make sense to flush them,
   // which clears the buffer and writes the remaining output.
   output << std::flush;
   return output;
}


} // namespace ED