aStarLibrary.c

#include <stdlib.h>
#include "aStarLibrary.h"

/*
;===================================================================
;A* Pathfinder (Version 1.71a) by Patrick Lester. Used by permission.
;===================================================================
;Last updated 06/16/03 -- Visual C++ version
 */

//Declare constants

int onClosedList = 10;

//Create needed arrays
int walkability[mapWidth][mapHeight];
int openList[mapWidth*mapHeight+2]; //1 dimensional array holding ID# of open list items
int whichList[mapWidth+1][mapHeight+1];  //2 dimensional array used to record
//whether a cell is on the open list or on the closed list.
int openX[mapWidth*mapHeight+2]; //1d array stores the x location of an item on the open list
int openY[mapWidth*mapHeight+2]; //1d array stores the y location of an item on the open list
int parentX[mapWidth+1][mapHeight+1]; //2d array to store parent of each cell (x)
int parentY[mapWidth+1][mapHeight+1]; //2d array to store parent of each cell (y)
int Fcost[mapWidth*mapHeight+2];	//1d array to store F cost of a cell on the open list
int Gcost[mapWidth+1][mapHeight+1]; 	//2d array to store G cost for each cell.
int Hcost[mapWidth*mapHeight+2];	//1d array to store H cost of a cell on the open list
int pathLength[numberPeople+1];     //stores length of the found path for critter
int pathLocation[numberPeople+1];   //stores current position along the chosen path for critter
int pathBank [numberPeople+1][maxPathLength*2];

//Path reading variables
int pathStatus[numberPeople+1];
int xPath[numberPeople+1];
int yPath[numberPeople+1];

//-----------------------------------------------------------------------------
// Function Prototypes: where needed
//-----------------------------------------------------------------------------
int ReadPathX(int pathfinderID,int pathLocation);
int ReadPathY(int pathfinderID,int pathLocation);


//-----------------------------------------------------------------------------
// Name: InitializePathfinder
// Desc: Allocates memory for the pathfinder.
//-----------------------------------------------------------------------------
void InitializePathfinder ()
{
  int x = 0;
	memset(pathStatus, 0, sizeof(pathStatus));
	memset(pathBank, 0, sizeof(pathBank));
}


//-----------------------------------------------------------------------------
// Name: EndPathfinder
// Desc: Frees memory used by the pathfinder.
//-----------------------------------------------------------------------------
void EndPathfinder (void)
{
}


//-----------------------------------------------------------------------------
// Name: FindPath
// Desc: Finds a path using A*
//-----------------------------------------------------------------------------
int FindPath (int pathfinderID,int startingX, int startingY,
			  int targetX, int targetY)
{
	int onOpenList=0, parentXval=0, parentYval=0,
	a=0, b=0, m=0, u=0, v=0, temp=0, corner=0, numberOfOpenListItems=0,
	addedGCost=0, tempGcost = 0, path = 0,
	tempx, pathX, pathY, cellPosition,
	newOpenListItemID=0;

//1. Convert location data (in pixels) to coordinates in the walkability array.
	int startX = startingX/tileSize;
	int startY = startingY/tileSize;
	targetX = targetX/tileSize;
	targetY = targetY/tileSize;

//2.Quick Path Checks: Under the some circumstances no path needs to
//	be generated ...

//	If starting location and target are in the same location...
	if (startX == targetX && startY == targetY && pathLocation[pathfinderID] > 0)
		return found;
	if (startX == targetX && startY == targetY && pathLocation[pathfinderID] == 0)
		return nonexistent;

//	If target square is unwalkable, return that it's a nonexistent path.
	if (walkability[targetX][targetY] == unwalkable)
		goto noPath;

//3.Reset some variables that need to be cleared
	if (onClosedList > 1000000) //reset whichList occasionally
	{
    int x = 0, y =0;
		for (; x < mapWidth;x++) {
			for (; y < mapHeight;y++)
				whichList [x][y] = 0;
		}
		onClosedList = 10;
	}
	onClosedList = onClosedList+2; //changing the values of onOpenList and onClosed list is faster than redimming whichList() array
	onOpenList = onClosedList-1;
	pathLength [pathfinderID] = notStarted;//i.e, = 0
	pathLocation [pathfinderID] = notStarted;//i.e, = 0
	Gcost[startX][startY] = 0; //reset starting square's G value to 0

//4.Add the starting location to the open list of squares to be checked.
	numberOfOpenListItems = 1;
	openList[1] = 1;//assign it as the top (and currently only) item in the open list, which is maintained as a binary heap (explained below)
	openX[1] = startX ; openY[1] = startY;

//5.Do the following until a path is found or deemed nonexistent.
	do
	{

//6.If the open list is not empty, take the first cell off of the list.
//	This is the lowest F cost cell on the open list.
	if (numberOfOpenListItems != 0)
	{

//7. Pop the first item off the open list.
	parentXval = openX[openList[1]];
	parentYval = openY[openList[1]]; //record cell coordinates of the item
	whichList[parentXval][parentYval] = onClosedList;//add the item to the closed list

//	Open List = Binary Heap: Delete this item from the open list, which
//  is maintained as a binary heap. For more information on binary heaps, see:
//	http://www.policyalmanac.org/games/binaryHeaps.htm
	numberOfOpenListItems = numberOfOpenListItems - 1;//reduce number of open list items by 1

//	Delete the top item in binary heap and reorder the heap, with the lowest F cost item rising to the top.
	openList[1] = openList[numberOfOpenListItems+1];//move the last item in the heap up to slot #1
	v = 1;

//	Repeat the following until the new item in slot #1 sinks to its proper spot in the heap.
	do
	{
	u = v;
	if (2*u+1 <= numberOfOpenListItems) //if both children exist
	{
	 	//Check if the F cost of the parent is greater than each child.
		//Select the lowest of the two children.
		if (Fcost[openList[u]] >= Fcost[openList[2*u]])
			v = 2*u;
		if (Fcost[openList[v]] >= Fcost[openList[2*u+1]])
			v = 2*u+1;
	}
	else
	{
		if (2*u <= numberOfOpenListItems) //if only child #1 exists
		{
	 	//Check if the F cost of the parent is greater than child #1
			if (Fcost[openList[u]] >= Fcost[openList[2*u]])
				v = 2*u;
		}
	}

	if (u != v) //if parent's F is > one of its children, swap them
	{
		temp = openList[u];
		openList[u] = openList[v];
		openList[v] = temp;
	}
	else
		break; //otherwise, exit loop

	}
	while (1);//reorder the binary heap


//7.Check the adjacent squares. (Its "children" -- these path children
//	are similar, conceptually, to the binary heap children mentioned
//	above, but don't confuse them. They are different. Path children
//	are portrayed in Demo 1 with grey pointers pointing toward
//	their parents.) Add these adjacent child squares to the open list
//	for later consideration if appropriate (see various if statements
//	below).
	for (b = parentYval-1; b <= parentYval+1; b++){
	for (a = parentXval-1; a <= parentXval+1; a++){

//	If not off the map (do this first to avoid array out-of-bounds errors)
	if (a != -1 && b != -1 && a != mapWidth && b != mapHeight){

//	If not already on the closed list (items on the closed list have
//	already been considered and can now be ignored).
	if (whichList[a][b] != onClosedList) {

//	If not a wall/obstacle square.
	if (walkability [a][b] != unwalkable) {

//	Don't cut across corners
	corner = walkable;
	if (a == parentXval-1)
	{
		if (b == parentYval-1)
		{
			if (walkability[parentXval-1][parentYval] == unwalkable
				|| walkability[parentXval][parentYval-1] == unwalkable) \
				corner = unwalkable;
		}
		else if (b == parentYval+1)
		{
			if (walkability[parentXval][parentYval+1] == unwalkable
				|| walkability[parentXval-1][parentYval] == unwalkable)
				corner = unwalkable;
		}
	}
	else if (a == parentXval+1)
	{
		if (b == parentYval-1)
		{
			if (walkability[parentXval][parentYval-1] == unwalkable
				|| walkability[parentXval+1][parentYval] == unwalkable)
				corner = unwalkable;
		}
		else if (b == parentYval+1)
		{
			if (walkability[parentXval+1][parentYval] == unwalkable
				|| walkability[parentXval][parentYval+1] == unwalkable)
				corner = unwalkable;
		}
	}
	if (corner == walkable) {

//	If not already on the open list, add it to the open list.
	if (whichList[a][b] != onOpenList)
	{

		//Create a new open list item in the binary heap.
		newOpenListItemID = newOpenListItemID + 1; //each new item has a unique ID #
		m = numberOfOpenListItems+1;
		openList[m] = newOpenListItemID;//place the new open list item (actually, its ID#) at the bottom of the heap
		openX[newOpenListItemID] = a;
		openY[newOpenListItemID] = b;//record the x and y coordinates of the new item

		//Figure out its G cost
		if (abs(a-parentXval) == 1 && abs(b-parentYval) == 1)
			addedGCost = 14;//cost of going to diagonal squares
		else
			addedGCost = 10;//cost of going to non-diagonal squares
		int cost = walkability[a][b];
		if(cost == 0) cost = 1;
		Gcost[a][b] = Gcost[parentXval][parentYval] + addedGCost * cost;
		// printf("gcost [%d][%d] = %d\n", a,b, Gcost[a][b]);

		//Figure out its H and F costs and parent
		Hcost[openList[m]] = 10*(abs(a - targetX) + abs(b - targetY));
		Fcost[openList[m]] = Gcost[a][b] + Hcost[openList[m]];
		parentX[a][b] = parentXval ; parentY[a][b] = parentYval;

		//Move the new open list item to the proper place in the binary heap.
		//Starting at the bottom, successively compare to parent items,
		//swapping as needed until the item finds its place in the heap
		//or bubbles all the way to the top (if it has the lowest F cost).
		while (m != 1) //While item hasn't bubbled to the top (m=1)
		{
			//Check if child's F cost is < parent's F cost. If so, swap them.
			if (Fcost[openList[m]] <= Fcost[openList[m/2]])
			{
				temp = openList[m/2];
				openList[m/2] = openList[m];
				openList[m] = temp;
				m = m/2;
			}
			else
				break;
		}
		numberOfOpenListItems = numberOfOpenListItems+1;//add one to the number of items in the heap

		//Change whichList to show that the new item is on the open list.
		whichList[a][b] = onOpenList;
	}

//8.If adjacent cell is already on the open list, check to see if this
//	path to that cell from the starting location is a better one.
//	If so, change the parent of the cell and its G and F costs.
	else //If whichList(a,b) = onOpenList
	{

		//Figure out the G cost of this possible new path
		if (abs(a-parentXval) == 1 && abs(b-parentYval) == 1)
			addedGCost = 14;//cost of going to diagonal tiles
		else
			addedGCost = 10;//cost of going to non-diagonal tiles
		int cost = walkability[a][b];
		if(cost == 0) cost = 1;
		tempGcost = Gcost[parentXval][parentYval] + addedGCost * cost;

		//If this path is shorter (G cost is lower) then change
		//the parent cell, G cost and F cost.
		if (tempGcost < Gcost[a][b]) //if G cost is less,
		{
			parentX[a][b] = parentXval; //change the square's parent
			parentY[a][b] = parentYval;
			Gcost[a][b] = tempGcost;//change the G cost

			//Because changing the G cost also changes the F cost, if
			//the item is on the open list we need to change the item's
			//recorded F cost and its position on the open list to make
			//sure that we maintain a properly ordered open list.
      int x = 1;
			for (; x <= numberOfOpenListItems; x++) //look for the item in the heap
			{
			if (openX[openList[x]] == a && openY[openList[x]] == b) //item found
			{
				Fcost[openList[x]] = Gcost[a][b] + Hcost[openList[x]];//change the F cost

				//See if changing the F score bubbles the item up from it's current location in the heap
				m = x;
				while (m != 1) //While item hasn't bubbled to the top (m=1)
				{
					//Check if child is < parent. If so, swap them.
					if (Fcost[openList[m]] < Fcost[openList[m/2]])
					{
						temp = openList[m/2];
						openList[m/2] = openList[m];
						openList[m] = temp;
						m = m/2;
					}
					else
						break;
				}
				break; //exit for x = loop
			} //If openX(openList(x)) = a
			} //For x = 1 To numberOfOpenListItems
		}//If tempGcost < Gcost(a,b)

	}//else If whichList(a,b) = onOpenList
	}//If not cutting a corner
	}//If not a wall/obstacle square.
	}//If not already on the closed list
	}//If not off the map
	}//for (a = parentXval-1; a <= parentXval+1; a++){
	}//for (b = parentYval-1; b <= parentYval+1; b++){

	}//if (numberOfOpenListItems != 0)

//9.If open list is empty then there is no path.
	else
	{
		path = nonexistent; break;
	}

	//If target is added to open list then path has been found.
	if (whichList[targetX][targetY] == onOpenList)
	{
		path = found; break;
	}

	}
	while (1);//Do until path is found or deemed nonexistent

//10.Save the path if it exists.
	if (path == found)
	{

//a.Working backwards from the target to the starting location by checking
//	each cell's parent, figure out the length of the path.
	pathX = targetX; pathY = targetY;
	do
	{
		//Look up the parent of the current cell.
		tempx = parentX[pathX][pathY];
		pathY = parentY[pathX][pathY];
		pathX = tempx;

		//Figure out the path length
		pathLength[pathfinderID] = pathLength[pathfinderID] + 1;
	}
	while (pathX != startX || pathY != startY);

//b.Resize the data bank to the right size in bytes
	if(pathLength[pathfinderID] > maxPathLength)
		goto noPath;

//c. Now copy the path information over to the databank. Since we are
//	working backwards from the target to the start location, we copy
//	the information to the data bank in reverse order. The result is
//	a properly ordered set of path data, from the first step to the
//	last.
	pathX = targetX ; pathY = targetY;
	cellPosition = pathLength[pathfinderID]*2;//start at the end
	do
	{
	cellPosition = cellPosition - 2;//work backwards 2 integers
	pathBank[pathfinderID] [cellPosition] = pathX;
	pathBank[pathfinderID] [cellPosition+1] = pathY;

//d.Look up the parent of the current cell.
	tempx = parentX[pathX][pathY];
	pathY = parentY[pathX][pathY];
	pathX = tempx;

//e.If we have reached the starting square, exit the loop.
	}
	while (pathX != startX || pathY != startY);

//11.Read the first path step into xPath/yPath arrays
	ReadPath(pathfinderID,startingX,startingY,1);

	}
	return path;


//13.If there is no path to the selected target, set the pathfinder's
//	xPath and yPath equal to its current location and return that the
//	path is nonexistent.
noPath:
	xPath[pathfinderID] = startingX;
	yPath[pathfinderID] = startingY;
	return nonexistent;
}




//==========================================================
//READ PATH DATA: These functions read the path data and convert
//it to screen pixel coordinates.
int ReadPath(int pathfinderID,int currentX,int currentY,
			  int pixelsPerFrame)
{
/*
;	Note on PixelsPerFrame: The need for this parameter probably isn't
;	that obvious, so a little explanation is in order. This
;	parameter is used to determine if the pathfinder has gotten close
;	enough to the center of a given path square to warrant looking up
;	the next step on the path.
;
;	This is needed because the speed of certain sprites can
;	make reaching the exact center of a path square impossible.
;	In Demo #2, the chaser has a velocity of 3 pixels per frame. Our
;	tile size is 50 pixels, so the center of a tile will be at location
;	25, 75, 125, etc. Some of these are not evenly divisible by 3, so
;	our pathfinder has to know how close is close enough to the center.
;	It calculates this by seeing if the pathfinder is less than
;	pixelsPerFrame # of pixels from the center of the square.

;	This could conceivably cause problems if you have a *really* fast
;	sprite and/or really small tiles, in which case you may need to
;	adjust the formula a bit. But this should almost never be a problem
;	for games with standard sized tiles and normal speeds. Our smiley
;	in Demo #4 moves at a pretty fast clip and it isn't even close
;	to being a problem.
*/

	int ID = pathfinderID; //redundant, but makes the following easier to read

	//If a path has been found for the pathfinder	...
	if (pathStatus[ID] == found)
	{

		//If path finder is just starting a new path or has reached the
		//center of the current path square (and the end of the path
		//hasn't been reached), look up the next path square.
		if (pathLocation[ID] < pathLength[ID])
		{
			//if just starting or if close enough to center of square
			if (pathLocation[ID] == 0 ||
				(abs(currentX - xPath[ID]) < pixelsPerFrame && abs(currentY - yPath[ID]) < pixelsPerFrame))
					pathLocation[ID] = pathLocation[ID] + 1;
		}

		//Read the path data.
		xPath[ID] = ReadPathX(ID,pathLocation[ID]);
		yPath[ID] = ReadPathY(ID,pathLocation[ID]);

		//If the center of the last path square on the path has been
		//reached then reset.
		if (pathLocation[ID] == pathLength[ID])
		{
			// printf("goal %d %d (%d)\n", abs(currentX - xPath[ID]), abs(currentY - yPath[ID]), pixelsPerFrame);
			if (abs(currentX - xPath[ID]) < pixelsPerFrame
				&& abs(currentY - yPath[ID]) < pixelsPerFrame) {  //if close enough to center of square
					pathStatus[ID] = notStarted;
					return 1;
				}
		}
	}

	//If there is no path for this pathfinder, simply stay in the current
 	//location.
	else
	{
		xPath[ID] = currentX;
		yPath[ID] = currentY;
	}
	return 0;
}


//The following two functions read the raw path data from the pathBank.
//You can call these functions directly and skip the readPath function
//above if you want. Make sure you know what your current pathLocation
//is.

//-----------------------------------------------------------------------------
// Name: ReadPathX
// Desc: Reads the x coordinate of the next path step
//-----------------------------------------------------------------------------
int ReadPathX(int pathfinderID,int pathLocation)
{
	int x;
	if (pathLocation <= pathLength[pathfinderID])
	{

	//Read coordinate from bank
	x = pathBank[pathfinderID] [pathLocation*2-2];

	//Adjust the coordinates so they align with the center
	//of the path square (optional). This assumes that you are using
	//sprites that are centered -- i.e., with the midHandle command.
	//Otherwise you will want to adjust this.
	x = tileSize*x + .5*tileSize;

	}
	return x;
}


//-----------------------------------------------------------------------------
// Name: ReadPathY
// Desc: Reads the y coordinate of the next path step
//-----------------------------------------------------------------------------
int ReadPathY(int pathfinderID,int pathLocation)
{
	int y;
	if (pathLocation <= pathLength[pathfinderID])
	{

	//Read coordinate from bank
	y = pathBank[pathfinderID] [pathLocation*2-1];

	//Adjust the coordinates so they align with the center
	//of the path square (optional). This assumes that you are using
	//sprites that are centered -- i.e., with the midHandle command.
	//Otherwise you will want to adjust this.
	y = tileSize*y + .5*tileSize;

	}
	return y;
}