/// <summary>
/// Assembles a path from the data stored in the AStarTileGrid, tracing the tiles using each tile's parent location.
/// </summary>
/// <param name="aStarTileGrid"></param>
/// <param name="endTile"></param>
/// <param name="startingLocation"></param>
/// <returns></returns>
private AStarPath BuildPath(ref AStarTileGrid aStarTileGrid, AStarTile endTile, Vector2Int startingLocation)
{
AStarTile priorTile = endTile;
AStarPath path = new AStarPath();
//Trace through each parent, as the path is effectively stored as a linked list, where the endTile is the start of the list.
path.AddTile(tiles[endTile.location]);
while (priorTile.location != startingLocation)
{
path.AddTile(tiles[priorTile.location]);
priorTile = aStarTileGrid[priorTile.parentLocation];
}
//The starting location is not added in the above loop, so we manually add it after
path.AddTile(tiles[startingLocation]);
return(path);
}
/// Loosely based on https://www.redblobgames.com/pathfinding/a-star/introduction.html
///From Red Blob Games.
/// And http://theory.stanford.edu/~amitp/GameProgramming/Heuristics.html#breaking-ties
///
///More detailed breakdown from https://medium.com/@nicholas.w.swift/easy-a-star-pathfinding-7e6689c7f7b2
///From Nicholas W. Swift
/// <summary>
/// Uses A* to find a path from the starting location, to the target location.
/// Returns true if a path was found, which is stored in the out variable of path.
/// </summary>
/// <param name="startingLocation"></param>
/// <param name="targetLocation"></param>
/// <param name="path"></param>
/// <returns></returns>
public bool FindPath(Vector2Int startingLocation, Vector2Int targetLocation, out AStarPath path)
{
//The AStarTileGrid stores the state information for each node (each tile in the grid)
AStarTileGrid aStarTileGrid = new AStarTileGrid(length, width, ref tiles);
//Acts as a priority queue to deterimine which tile to move through next
List <AStarTile> openTiles = new List <AStarTile>();
//Add the starting tile to the open list, to start with.
openTiles.Add(new AStarTile(0, Vector2Int.Distance(startingLocation, targetLocation)));
AStarTile currentTile = openTiles[0];
bool pathFound = false;
//Main loop
while (openTiles.Count != 0)
{
//As openTiles is a sorted list based on the F scores, the first item is tile with the lowest cost.
currentTile = openTiles[0];
//If the current tile is at the targetLocation, a path is found and we can terminate our search
if (currentTile.location == targetLocation)
{
pathFound = true;
break;
}
//For each adjacent tile that is not an obstacle
foreach (AStarTile neighbour in aStarTileGrid.GetAdjacentEmptyTiles(currentTile.location))
{
float routeCost = currentTile.G + Vector2Int.Distance(currentTile.location, neighbour.location);
//Explore the neighbouring tile, and update it's costs
if (!neighbour.hasBeenExplored)
{
neighbour.G = routeCost;
neighbour.H = CalculateH(neighbour.location, targetLocation);
neighbour.hasBeenExplored = true;
neighbour.parentLocation = currentTile.location;
openTiles.Add(neighbour);
continue;
}
//Neighbour has been explored. Have we found a short root to this tile?
if (routeCost < neighbour.G)
{
//A new path was found.
neighbour.G = routeCost;
neighbour.parentLocation = currentTile.location;
//Reopen the tile if necessary
if (!openTiles.Contains(neighbour))
{
openTiles.Add(neighbour);
}
}
}
//Remove the tile from the open list, so that the A* examines the next tile. Required for termination of the main loop.
openTiles.Remove(currentTile);
//TODO: Replace this with a more optimized method of updating the priority queue. Note that using a lambda here is not efficient.
openTiles.Sort((x, y) => x.F.CompareTo(y.F));
}
if (pathFound)
{
path = BuildPath(ref aStarTileGrid, currentTile, startingLocation);
return(true);
}
//Unable to find a path
path = null;
return(false);
}