c_cpp BFS（宽度优先搜索算法）-C ++

Posted 2021-05-14

/*
 Sample code from https://www.redblobgames.com/pathfinding/a-star/
 Copyright 2014 Red Blob Games <[e-mail]>
 
 Feel free to use this code in your own projects, including commercial projects
 License: Apache v2.0 <http://www.apache.org/licenses/LICENSE-2.0.html>
*/

#include <iostream>
#include <iomanip>
#include <unordered_map>
#include <unordered_set>
#include <array>
#include <vector>
#include <utility>
#include <queue>
#include <tuple>
#include <algorithm>
#include <cstdlib>

struct SimpleGraph {
  std::unordered_map<char, std::vector<char> > edges;

  std::vector<char> neighbors(char id) {
    return edges[id];
  }
};

SimpleGraph example_graph {{
    {'A', {'B'}},
    {'B', {'A', 'C', 'D'}},
    {'C', {'A'}},
    {'D', {'E', 'A'}},
    {'E', {'B'}}
  }};

struct GridLocation {
  int x, y;
};

namespace std {
/* implement hash function so we can put GridLocation into an unordered_set */
template <> struct hash<GridLocation> {
  typedef GridLocation argument_type;
  typedef std::size_t result_type;
  std::size_t operator()(const GridLocation& id) const noexcept {
    return std::hash<int>()(id.x ^ (id.y << 4));
  }
};
}


struct SquareGrid {
  static std::array<GridLocation, 4> DIRS;

  int width, height;
  std::unordered_set<GridLocation> walls;

  SquareGrid(int width_, int height_)
     : width(width_), height(height_) {}

  bool in_bounds(GridLocation id) const {
    return 0 <= id.x && id.x < width
        && 0 <= id.y && id.y < height;
  }

  bool passable(GridLocation id) const {
    return walls.find(id) == walls.end();
  }

  std::vector<GridLocation> neighbors(GridLocation id) const {
    std::vector<GridLocation> results;

    for (GridLocation dir : DIRS) {
      GridLocation next{id.x + dir.x, id.y + dir.y};
      if (in_bounds(next) && passable(next)) {
        results.push_back(next);
      }
    }

    if ((id.x + id.y) % 2 == 0) {
      // aesthetic improvement on square grids
      std::reverse(results.begin(), results.end());
    }

    return results;
  }
};

std::array<GridLocation, 4> SquareGrid::DIRS =
  {GridLocation{1, 0}, GridLocation{0, -1}, GridLocation{-1, 0}, GridLocation{0, 1}};

// Helpers for GridLocation

bool operator == (GridLocation a, GridLocation b) {
  return a.x == b.x && a.y == b.y;
}

bool operator != (GridLocation a, GridLocation b) {
  return !(a == b);
}

bool operator < (GridLocation a, GridLocation b) {
  return std::tie(a.x, a.y) < std::tie(b.x, b.y);
}

std::basic_iostream<char>::basic_ostream& operator<<(std::basic_iostream<char>::basic_ostream& out, const GridLocation& loc) {
  out << '(' << loc.x << ',' << loc.y << ')';
  return out;
}

// This outputs a grid. Pass in a distances map if you want to print
// the distances, or pass in a point_to map if you want to print
// arrows that point to the parent location, or pass in a path vector
// if you want to draw the path.
template<class Graph>
void draw_grid(const Graph& graph, int field_width,
               std::unordered_map<GridLocation, double>* distances=nullptr,
               std::unordered_map<GridLocation, GridLocation>* point_to=nullptr,
               std::vector<GridLocation>* path=nullptr) {
  for (int y = 0; y != graph.height; ++y) {
    for (int x = 0; x != graph.width; ++x) {
      GridLocation id {x, y};
      std::cout << std::left << std::setw(field_width);
      if (graph.walls.find(id) != graph.walls.end()) {
        std::cout << std::string(field_width, '#');
      } else if (point_to != nullptr && point_to->count(id)) {
        GridLocation next = (*point_to)[id];
        if (next.x == x + 1) { std::cout << "> "; }
        else if (next.x == x - 1) { std::cout << "< "; }
        else if (next.y == y + 1) { std::cout << "v "; }
        else if (next.y == y - 1) { std::cout << "^ "; }
        else { std::cout << "* "; }
      } else if (distances != nullptr && distances->count(id)) {
        std::cout << (*distances)[id];
      } else if (path != nullptr && find(path->begin(), path->end(), id) != path->end()) {
        std::cout << '@';
      } else {
        std::cout << '.';
      }
    }
    std::cout << '\n';
  }
}

void add_rect(SquareGrid& grid, int x1, int y1, int x2, int y2) {
  for (int x = x1; x < x2; ++x) {
    for (int y = y1; y < y2; ++y) {
      grid.walls.insert(GridLocation{x, y});
    }
  }
}

SquareGrid make_diagram1() {
  SquareGrid grid(30, 15);
  add_rect(grid, 3, 3, 5, 12);
  add_rect(grid, 13, 4, 15, 15);
  add_rect(grid, 21, 0, 23, 7);
  add_rect(grid, 23, 5, 26, 7);
  return grid;
}

struct GridWithWeights: SquareGrid {
  std::unordered_set<GridLocation> forests;
  GridWithWeights(int w, int h): SquareGrid(w, h) {}
  double cost(GridLocation from_node, GridLocation to_node) const {
    return forests.find(to_node) != forests.end()? 5 : 1;
  }
};

GridWithWeights make_diagram4() {
  GridWithWeights grid(10, 10);
  add_rect(grid, 1, 7, 4, 9);
  typedef GridLocation L;
  grid.forests = std::unordered_set<GridLocation> {
    L{3, 4}, L{3, 5}, L{4, 1}, L{4, 2},
    L{4, 3}, L{4, 4}, L{4, 5}, L{4, 6},
    L{4, 7}, L{4, 8}, L{5, 1}, L{5, 2},
    L{5, 3}, L{5, 4}, L{5, 5}, L{5, 6},
    L{5, 7}, L{5, 8}, L{6, 2}, L{6, 3},
    L{6, 4}, L{6, 5}, L{6, 6}, L{6, 7},
    L{7, 3}, L{7, 4}, L{7, 5}
  };
  return grid;
}

template<typename T, typename priority_t>
struct PriorityQueue {
  typedef std::pair<priority_t, T> PQElement;
  std::priority_queue<PQElement, std::vector<PQElement>,
                 std::greater<PQElement>> elements;

  inline bool empty() const {
     return elements.empty();
  }

  inline void put(T item, priority_t priority) {
    elements.emplace(priority, item);
  }

  T get() {
    T best_item = elements.top().second;
    elements.pop();
    return best_item;
  }
};

template<typename Location, typename Graph>
void dijkstra_search
  (Graph graph,
   Location start,
   Location goal,
   std::unordered_map<Location, Location>& came_from,
   std::unordered_map<Location, double>& cost_so_far)
{
  PriorityQueue<Location, double> frontier;
  frontier.put(start, 0);

  came_from[start] = start;
  cost_so_far[start] = 0;
  
  while (!frontier.empty()) {
    Location current = frontier.get();

    if (current == goal) {
      break;
    }

    for (Location next : graph.neighbors(current)) {
      double new_cost = cost_so_far[current] + graph.cost(current, next);
      if (cost_so_far.find(next) == cost_so_far.end()
          || new_cost < cost_so_far[next]) {
        cost_so_far[next] = new_cost;
        came_from[next] = current;
        frontier.put(next, new_cost);
      }
    }
  }
}

template<typename Location>
std::vector<Location> reconstruct_path(
   Location start, Location goal,
   std::unordered_map<Location, Location> came_from
) {
  std::vector<Location> path;
  Location current = goal;
  while (current != start) {
    path.push_back(current);
    current = came_from[current];
  }
  path.push_back(start); // optional
  std::reverse(path.begin(), path.end());
  return path;
}

inline double heuristic(GridLocation a, GridLocation b) {
  return std::abs(a.x - b.x) + std::abs(a.y - b.y);
}

template<typename Location, typename Graph>
void a_star_search
  (Graph graph,
   Location start,
   Location goal,
   std::unordered_map<Location, Location>& came_from,
   std::unordered_map<Location, double>& cost_so_far)
{
  PriorityQueue<Location, double> frontier;
  frontier.put(start, 0);

  came_from[start] = start;
  cost_so_far[start] = 0;
  
  while (!frontier.empty()) {
    Location current = frontier.get();

    if (current == goal) {
      break;
    }

    for (Location next : graph.neighbors(current)) {
      double new_cost = cost_so_far[current] + graph.cost(current, next);
      if (cost_so_far.find(next) == cost_so_far.end()
          || new_cost < cost_so_far[next]) {
        cost_so_far[next] = new_cost;
        double priority = new_cost + heuristic(next, goal);
        frontier.put(next, priority);
        came_from[next] = current;
      }
    }
  }
}