# A* (A-star) implementation in AS3

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.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!

## Answers

Could you explain what's the purpose of these lines:

```if (cNode.dirArray[i] >= 4)
{
neighborNode.g_score -= 4;
}
```

A* is for problems where costs are always positive i.e. cost of paths is monotonically increasing.

Another thing to check, regarding optimality, is that distFunction() is returning always a value which is lesser or equal than the actual cost to reach the goal (i.e. the heuristic is required to be admissible so A* can guarantee it will find optimal solutions).

I don't know anything about AS3 at all - so I can't comment on the priority queue usage.

There is a fast implementation here: https://github.com/tomnewton/AS3AStar

Try out http://script3.blogspot.com/2010/04/star-path-finding-algorthim-in.html you can find there powerful a* pathfinding class with smooth pathfinding option.

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