depth_first_search.h

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*/

#ifndef DEPTH_FIRST_SEARCH_INCLUDED
#define DEPTH_FIRST_SEARCH_INCLUDED

#include <functional>
#include <set>
#include <stack>
#include <tuple>

/**
  @file include/depth_first_search.h
*/

/**
  Performs a Depth First Search algorithm on a graph defined by a set of
  vertices being an subset of universum of values of TVertex type and, and set
  of directed edges generated on demand by get_neighbors() method supplied.
  Search starts with a selected vertex, for each vertex that is encountered a
  visitor_start() method supplied is called and when the DFS finishes traversal
  from that vertex, a visitor_end() method is called. The set of visited nodes
  is maintained, so the input graph don't have to be a Directed Acyclic Graph.

  @param start_vertex A vertex to start a search from.
  @param visitor_start A method or lambda that will be called when a specified
    vertex starts to be processed, the vertex is supplied as the only argument.
  @param visitor_end A method or lambda that will be called when a specified
    vertex ends to be processed, i.e. all its neighbors were already processed,
    the vertex is supplied as the only argument.
  @param get_neighbors A method or lambda that takes a vertex as an argument and
    returns a enumerable list of all vertices to which the edges exists.
  @param [in,out] visited_set A set of vertices that were visited. This can be
    used to run multiple DFS runs and assure any vertex will not be visited more
    than once.
*/
template <typename TVertex, typename TVisit_start, typename TVisit_end,
          typename TGet_neighbors, typename TVertex_less = std::less<TVertex>>
void depth_first_search(TVertex start_vertex, TVisit_start visitor_start,
                        TVisit_end visitor_end, TGet_neighbors get_neighbors,
                        std::set<TVertex> &visited_set = std::set<TVertex>{}) {
  /* A constant for a index denoting if the search from specified vertex has
    just been started of just ends. */
  constexpr int START_VISITING = 0;
  /* A constant for a index denoting the vertex. */
  constexpr int CURRENT_VERTEX = 1;

  /* An actual stack for search. */
  std::stack<std::tuple<bool, TVertex>> stack;
  /* Check if this vertex was not already visited - this may happen if
    the visited_set was returned by another DFS run. */
  if (!visited_set.insert(start_vertex).second) {
    return;
  }
  stack.emplace(true, start_vertex);

  while (!stack.empty()) {
    std::tuple<bool, TVertex> &elem = stack.top();
    if (std::get<START_VISITING>(elem)) {
      visitor_start(std::get<CURRENT_VERTEX>(elem));

      std::get<START_VISITING>(elem) = false;
      for (TVertex neighbor : get_neighbors(std::get<CURRENT_VERTEX>(elem))) {
        /* Check if this vertex was not visited yet in this or previous runs. */
        if (visited_set.insert(neighbor).second) {
          stack.emplace(true, neighbor);
        }
      }
    } else {
      visitor_end(std::get<CURRENT_VERTEX>(elem));
      stack.pop();
    }
  }
}

#endif