A* (A-star) implementation in AS3
- by Bryan Hare
Hey,
I am putting together a project for a class that requires me to put AI in a top down Tactical Strategy game in Flash AS3.
I decided that I would use a node based path finding approach because the game is based on a circular movement scheme. When a player moves a unit he essentially draws a series of line segments that connect that a player unit will follow along.
I am trying to put together a similar operation for the AI units in our game by creating a list of nodes to traverse to a target node. Hence my use of Astar (the resulting path can be used to create this line).
Here is my Algorithm
function findShortestPath (startN:node, goalN:node)
{
var openSet:Array = new Array();
var closedSet:Array = new Array();
var pathFound:Boolean = false;
startN.g_score = 0;
startN.h_score = distFunction(startN,goalN);
startN.f_score = startN.h_score;
startN.fromNode = null;
openSet.push (startN);
var i:int = 0
for(i= 0; i< nodeArray.length; i++)
{
for(var j:int =0; j<nodeArray[0].length; j++)
{
if(!nodeArray[i][j].isPathable)
{
closedSet.push(nodeArray[i][j]);
}
}
}
while (openSet.length != 0)
{
var cNode:node = openSet.shift();
if (cNode == goalN)
{
resolvePath (cNode);
return true;
}
closedSet.push (cNode);
for (i= 0; i < cNode.dirArray.length; i++)
{
var neighborNode:node = cNode.nodeArray[cNode.dirArray[i]];
if (!(closedSet.indexOf(neighborNode) == -1))
{
continue;
}
neighborNode.fromNode = cNode;
var tenativeg_score:Number = cNode.gscore + distFunction(neighborNode.fromNode,neighborNode);
if (openSet.indexOf(neighborNode) == -1)
{
neighborNode.g_score = neighborNode.fromNode.g_score + distFunction(neighborNode,cNode);
if (cNode.dirArray[i] >= 4)
{
neighborNode.g_score -= 4;
}
neighborNode.h_score=distFunction(neighborNode,goalN);
neighborNode.f_score=neighborNode.g_score+neighborNode.h_score;
insertIntoPQ (neighborNode, openSet);
//trace(" F Score of neighbor: " + neighborNode.f_score + " H score of Neighbor: " + neighborNode.h_score + " G_score or neighbor: " +neighborNode.g_score);
}
else if (tenativeg_score <= neighborNode.g_score)
{
neighborNode.fromNode=cNode;
neighborNode.g_score=cNode.g_score+distFunction(neighborNode,cNode);
if (cNode.dirArray[i]>=4)
{
neighborNode.g_score-=4;
}
neighborNode.f_score=neighborNode.g_score+neighborNode.h_score;
openSet.splice (openSet.indexOf(neighborNode),1);
//trace(" F Score of neighbor: " + neighborNode.f_score + " H score of Neighbor: " + neighborNode.h_score + " G_score or neighbor: " +neighborNode.g_score);
insertIntoPQ (neighborNode, openSet);
}
}
}
trace ("fail");
return false;
}
Right now this function creates paths that are often not optimal or wholly inaccurate given the target and this generally happens when I have nodes that are not path able, and I am not quite sure what I am doing wrong right now.
If someone could help me correct this I would appreciate it greatly.
Some Notes
My OpenSet is essentially a Priority Queue, so thats how I sort my nodes by cost.
Here is that function
function insertIntoPQ (iNode:node, pq:Array)
{
var inserted:Boolean=true;
var iterater:int=0;
while (inserted)
{
if (iterater==pq.length)
{
pq.push (iNode);
inserted=false;
}
else if (pq[iterater].f_score >= iNode.f_score)
{
pq.splice (iterater,0,iNode);
inserted=false;
}
++iterater;
}
}
Thanks!
