Removing the obstacle that yields the best path from a map after A* traversal

Question

I traverse a 16x16 maze using my own A* implementation.

All is well. However, after the traversal, I would like to find out what wall would give me the best alternative path. Apart from removing every block and re-running A* on the maze, what's a more clever and elegant solution?

I was thinking give every wall node (ignored by A*), a tentative F-value, and change the node structure to also have a n-sized list of node *tentative_parent where n is the number of walls in the maze. Could this be viable?

Answer 1

When you add a node to the list of nodes to consider, also add a flag whether the path through that node has already gone through a wall.

possibleNode.heuristic = currentNode.distance + heuristicEstimate(possibleNode)
possibleNode.goneThroughWall = currentNode.goneThroughWall || possibleNode.isAWall
allPossiblePaths.insert(possibleNode) // Priority queue sorted on Node.heuristic

Then when considering possible paths from a node, if it hasn't gone through a wall, consider adjacent walls to be fair paths.

foreach neighborNode in neighbors(someNode)
    if !(someNode.goneThroughWall && neighborNode.isAWall)
        addToPossiblePaths(neighborNode)

You already have to maintain distance from the start node to the current node being processed, and it uses what you already have in place.

Full proof of concept:

We see that operator== is defined to also consider whether or not the path has hit a wall yet. This allows us to consider the a node twice if needed, when we look in the closed set to see if we have already encountered this node. This is the case in the center hallway in the example maze in the source below.

The portions of code marked with #ifdef I_AM_ALLOWED_TO_GO_THROUGH_A_WALL show the portions needed to augment a normal A* algorithm to be able to go through a single wall.

#include <cassert>
#include <algorithm>
#include <cstdlib>
#include <iostream>
#include <set>
#include <vector>

#define I_AM_ALLOWED_TO_GO_THROUGH_A_WALL

const int width  = 5;
const int height = 5;

// Define maze
const int maze[height][width] =
  { { 0, 1, 1, 0, 0 },
    { 0, 1, 0, 1, 0 },
    { 0, 1, 0, 1, 0 },
    { 0, 1, 0, 1, 0 },
    { 0, 0, 0, 0, 0 } };

// Square represents the actual structure of the maze
// Here, we only care about where it is, and whether it is a wall
struct Square {
  Square(int _x, int _y)
    : x(_x), y(_y) {}

  bool operator==(const Square& rhs) const {
    return x == rhs.x && y == rhs.y;
  }

  bool isAWall() const {
    return maze[y][x];
  }

  int distance(const Square& goal) const {
    return std::abs(x - goal.x) + std::abs(y - goal.y);
  }

  int x;
  int y;
};

// Define start and end of maze
const Square goalSquare(3, 0);
const Square startSquare(0, 0);

// A PathNode describes the A* algorithm's path through the maze
// It keeps track of its associated Square
//                   its previous PathNode in the path
//                   the length of the path up to the current PathNode
//                   whether the path so far has goneThroughWall
struct PathNode {
  explicit PathNode(const Square& s, int length = 0, const PathNode* _prev = NULL)
    : square(s),
      prev(_prev),
      pathLength(length) {
    heuristic = pathLength + square.distance(goalSquare);

#ifdef I_AM_ALLOWED_TO_GO_THROUGH_A_WALL
    if(prev)
      goneThroughWall = prev->goneThroughWall || square.isAWall();
    else
      goneThroughWall = false;

    // Sanity check, these should be the same
    assert(goneThroughWall == hasGoneThroughAWall());
#endif
  }

  bool operator<(const PathNode& rhs) const {
    return heuristic < rhs.heuristic;
  }

  // This is very important. When examining the closed set to see
  // if we've already evaulated this node, we want to differentiate
  // from whether we got to that node using a path through a wall.
  //
  // This is especially important in the given example maze.
  // Without this differentiation, paths going up column 3 will hit
  // old, closed paths going through the walls in column 2, and not
  // find the best path.
  bool operator==(const PathNode& rhs) const {
    return square == rhs.square
#ifdef I_AM_ALLOWED_TO_GO_THROUGH_A_WALL
      && goneThroughWall == rhs.goneThroughWall
#endif
      ;
  }

  bool weakEquals(const PathNode& rhs) const {
    return square == rhs.square;
  }

  bool weakEquals(const Square& rhs) const {
    return square == rhs;
  }

  // Sanity checker
  bool hasGoneThroughAWall() const {
    if(square.isAWall()) return true;

    const PathNode* p = prev;
    while(p) {
      if(p->square.isAWall())
        return true;
      p = p->prev;
    }

    return false;
  }

  Square square;
  const PathNode* prev;
  int heuristic, pathLength;
#ifdef I_AM_ALLOWED_TO_GO_THROUGH_A_WALL
  bool goneThroughWall;
#endif
};

std::ostream& operator<<(std::ostream& ostr, const PathNode& p) {
  ostr << "(" << p.square.x << ", " << p.square.y << ")";
  if(p.square.isAWall())
    ostr << " <- Wall!";
  return ostr;
}

std::vector<Square> getNeighbors(const Square& s) {
  std::vector<Square> neighbors;

  if(s.x > 0)
    neighbors.push_back(Square(s.x - 1, s.y));
  if(s.x < width - 1)
    neighbors.push_back(Square(s.x + 1, s.y));
  if(s.y > 0)
    neighbors.push_back(Square(s.x, s.y - 1));
  if(s.y < height - 1)
    neighbors.push_back(Square(s.x, s.y + 1));

  return neighbors;
}

void printList(const PathNode* p, int i = 0) {
  if(p) {
    printList(p->prev, i + 1);
    std::cout << *p << std::endl;
  } else {
    std::cout << "Length: " << i << std::endl;
  }
}

typedef std::multiset<PathNode> Set;

int main(int argc, char** argv) {
  // Print out maze
  for(int j = 0; j < height; ++j) {
    for(int i = 0; i < width; ++i) {
      std::cout << " " << maze[j][i];
    }
    std::cout << std::endl;
  }
  std::cout << std::endl;

  Set closedSet;
  Set openSet;
  openSet.insert(PathNode(startSquare)); // Start node (defined at line ~42)

  int processedCount = 0;
  while(!openSet.empty()) {
    Set::iterator currentNode = openSet.begin();
    ++processedCount;

    // We've found the goal, so print solution.
    if(currentNode->weakEquals(goalSquare)) {
      std::cout << "Processed nodes: " << processedCount << std::endl;
      printList(&*currentNode);
      return 0;
    }

    {
      // Move from open set to closed set
      Set::iterator del = currentNode;
      currentNode = closedSet.insert(*currentNode);
      openSet.erase(del);
    }

    std::vector<Square> neighbors = getNeighbors(currentNode->square);
    for(unsigned int i = 0; i < neighbors.size(); ++i) {
      PathNode currentNeighbor(neighbors[i], currentNode->pathLength + 1, &*currentNode);

      // Skip if it is 2nd wall
      if(
#ifdef I_AM_ALLOWED_TO_GO_THROUGH_A_WALL
        currentNode->goneThroughWall &&
#endif
        currentNeighbor.square.isAWall()
      )
        continue;

      // Skip if it is already in the closed set
      // Note: Using std::find here instead of std::multiset::find because
      // std::multiset::find uses the definition !(a < b) && !(b < a) for
      // searching. I want it to use my overloaded a == b.
      if(find(closedSet.begin(), closedSet.end(), currentNeighbor) != closedSet.end())
        continue;

      // Skip if we were just there
      const PathNode* p = currentNode->prev;
      if(p && p->weakEquals(currentNeighbor))
        continue;

      // See if there is a better alternative in the open set
      // Note: Using std::find here instead of std::multiset::find. See above.
      Set::iterator contender = find(openSet.begin(), openSet.end(), currentNeighbor);
      if(contender == openSet.end())
        openSet.insert(currentNeighbor);
      else if(currentNeighbor.pathLength < contender->pathLength) {
        // We've found a better alternative, so replace
        openSet.erase(contender);
        openSet.insert(currentNeighbor);
      }
    }
  }

  std::cout << "Failure." << std::endl;
  return 1;
}