/// <summary>
/// Dijkstra's algorithm, shortest paths between nodes in a graph
/// https://en.wikipedia.org/wiki/Dijkstra%27s_algorithm
/// Computes the distance table between two nodes in a graph
/// All positions have 8 degrees of freedom, and the cost of moving between positions are equal to
/// 1 + the absolute difference in value.
/// </summary>
/// <param name="items">2D value array</param>
/// <param name="initial">Start position</param>
/// <param name="end">End positions</param>
public static int[,] Dijkstra(IMoveCost[,] items, Position initial, Position end)
{
// Let the node at which we are starting be called the initial node. Let the distance of node Y be the distance from the initial node to Y.
// Dijkstra's algorithm will assign some initial distance values and will try to improve them step by step.
int W = items.GetLength(0);
int H = items.GetLength(1);
int[,] distance = new int[W, H];
bool[,] visited = new bool[W, H];
// 1. Assign to every node a tentative distance value: set it to zero for our initial node and to infinity for all other nodes.
// 2. Set the initial node as current. Mark all other nodes unvisited. Create a set of all the unvisited nodes called the unvisited set.
List<Position> unvisited = new List<Position>();
for (int x = 0; x < W; x++)
for (int y = 0; y < H; y++)
{
distance[x, y] = int.MaxValue;
visited[x, y] = false;
if (!(initial.x == x && initial.y == y))
unvisited.Add(new Position(x,y));
}
distance[initial.x, initial.y] = 0;
Position current = initial;
while (true) {
// For the current node, consider all of its unvisited neighbors and calculate their tentative distances.
// Compare the newly calculated tentative distance to the current assigned value and assign the smaller one.
// For example, if the current node A is marked with a distance of 6, and the edge connecting it with a neighbor B
// has length 2, then the distance to B (through A) will be 6 + 2 = 8. If B was previously marked with a distance
// greater than 8 then change it to 8. Otherwise, keep the current value.
var neighbors = GetUnvisitedNeighbors(visited, current);
foreach (Position neighbor in neighbors)
{
// get the cost of moving to the neighbor
int cost = items[neighbor.x, neighbor.y].MoveCost(items[current.x, current.y]);
// if the path is shorter than a previous path to the neighbor, update it
int path = MOVE_PENALTY + cost + distance[current.x, current.y];
if (path < distance[neighbor.x, neighbor.y])
distance[neighbor.x, neighbor.y] = path;
}
// When we are done considering all of the neighbors of the current node, mark the current node as visited
// and remove it from the unvisited set. A visited node will never be checked again.
unvisited.Remove(current);
visited[current.x, current.y] = true;
// If the destination node has been marked visited (when planning a route between two specific nodes)
// or if the smallest tentative distance among the nodes in the unvisited set is infinity
// (when planning a complete traversal; occurs when there is no connection between the initial node and
// remaining unvisited nodes), then stop. The algorithm has finished.
if (visited[end.x, end.y])
break;
// Otherwise, select the unvisited node that is marked with the smallest tentative distance,
// set it as the new "current node", and go back to step 3.
current = (from p in unvisited
orderby distance[p.x, p.y] ascending
select new Position(p.x, p.y)).First();
}
return distance;
}
/// <summary>
/// Return a list of all the unvisited positions around the given position
/// </summary>
/// <param name="visited">An bool array of visited positions</param>
/// <param name="current">Position to get neighbors of</param>
/// <returns></returns>
internal static List<Position> GetUnvisitedNeighbors(bool[,] visited, Position current)
{
// get a list of all neighbors
var list = GetNeighbors(current, visited.GetLength(0), visited.GetLength(1));
// return the list without all visited neighbors
return list.Where(p => !visited[p.x, p.y]).ToList();
}
/// <summary>
/// Computes the shortest path between two positions, in a 2D distance table.
/// All positions have 8 degrees of freedom
/// </summary>
/// <param name="values">2D Integer distance array</param>
/// <param name="initial">Start position</param>
/// <param name="end">End positions</param>
/// <returns></returns>
public static List<Position> GetShortestPathFromDistance(int[,] dist, Position initial, Position end)
{
// width and height of the given array
int width = dist.GetLength(0);
int height = dist.GetLength(1);
// We have to make sure we don't backtrack when moving between neighbors.
// This can be ignored if we make the assumption of movement cost to be at least 1.
bool[,] visited = new bool[width, height];
for (int x = 0; x < width; x++)
for (int y = 0; y < height; y++)
visited[x, y] = false;
List<Position> path = new List<Position>();
// We start at the end
Position current = end;
// while we are no in the initial position,
while (!(current.x == initial.x && current.y == initial.y))
{
// add the position to the list
path.Add(current);
// set the position as visited
visited[current.x, current.y] = true;
// find all unvisited neighbors
var list = GetUnvisitedNeighbors(visited, current);
// the next position is the neighbor with the lowest value
current = (from item in list
orderby dist[item.x, item.y] ascending
select new Position(item.x, item.y)).First();
}
path.Add(current);
path.Reverse();
return path;
}
/// <summary>
/// Return a list of all the positions around the given position
/// </summary>
/// <param name="visited">An bool array of visited positions</param>
/// <param name="current">Position to get neighbors of</param>
/// <returns></returns>
internal static List<Position> GetNeighbors(Position current, int width, int height)
{
// list of neighbors
List<Position> neighbors = new List<Position>();
// loop over a 3x3 square with the current position in the middle
for (int x = current.x - 1; x <= current.x + 1; x++)
{
for (int y = current.y - 1; y <= current.y + 1; y++)
{
// skip position if it's an edge or middle position
if (x < 0 || x >= width || y < 0 || y >= height || (current.x == x && current.y == y))
continue;
neighbors.Add(new Position(x, y));
}
}
return neighbors;
}
/// <summary>
/// Computes the shortest path between two positions, in a 2D table.
/// All positions have 8 degrees of freedom, and the cost of moving between positions are equal to
/// 1 + the absolute difference in value.
/// </summary>
/// <param name="values">2D Cell array</param>
/// <param name="initial">Start position</param>
/// <param name="end">End positions</param>
/// <returns></returns>
public static List<Position> GetShortestPath(IMoveCost[,] values, Position initial, Position end)
{
// Use Dijkstra's algorithm to compute the distance table for the given values
int[,] dist = Dijkstra(values, initial, end);
// traverse the shortest path and return list
return GetShortestPathFromDistance(dist, initial, end);
}
