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# 13: Projects

A number of more challenging projects for you to solve. [[Some of these may turn into examples in the book, and so at some point might disappear from here]]

## Rats & Mazes

First, create a Blackboard (cite reference) which is an object on which anyone may record information. This particular blackboard draws a maze, and is used as information comes back about the structure of a maze from the rats that are investigating it.

Now create the maze itself. Like a real maze, this object reveals very little information about itself — given a coordinate, it will tell you whether there are walls or spaces in the four directions immediately surrounding that coordinate, but no more. For starters, read the maze in from a text file but consider hunting on the internet for a maze-generating algorithm. In any event, the result should be an object that, given a maze coordinate, will report walls and spaces around that coordinate. Also, you must be able to ask it for an entry point to the maze.

Finally, create the maze-investigating Rat class. Each rat can communicate with both the blackboard to give the current information and the maze to request new information based on the current position of the rat. However, each time a rat reaches a decision point where the maze branches, it creates a new rat to go down each of the branches. Each rat is driven by its own thread. When a rat reaches a dead end, it terminates itself after reporting the results of its final investigation to the blackboard.

The goal is to completely map the maze, but you must also determine whether the end condition will be naturally found or whether the blackboard must be responsible for the decision.

An example implementation by Jeremy Meyer:

```//: c13:Maze.java
import java.util.*;
import java.io.*;
import java.awt.*;

public class Maze extends Canvas {
private Vector lines; // a line is a char array
private int width = -1;
private int height = -1;
public static void main (String [] args)
throws IOException {
if (args.length < 1) {
System.out.println("Enter filename");
System.exit(0);
}
Maze m = new Maze();
Frame f = new Frame();
f.setSize(m.width*20, m.height*20);
Rat r = new Rat(m, 0, 0);
f.setVisible(true);
}
public Maze() {
lines = new Vector();
setBackground(Color.lightGray);
}
synchronized public boolean
isEmptyXY(int x, int y) {
if (x < 0) x += width;
if (y < 0) y += height;
// Use mod arithmetic to bring rat in line:
byte[] by =
(byte[])(lines.elementAt(y%height));
return by[x%width]==' ';
}
synchronized public void
setXY(int x, int y, byte newByte) {
if (x < 0) x += width;
if (y < 0) y += height;
byte[] by =
(byte[])(lines.elementAt(y%height));
by[x%width] = newByte;
repaint();
}
public void
String currentLine = null;
currentLine != null;
if(width < 0 ||
currentLine.getBytes().length > width)
width = currentLine.getBytes().length;
}
height = lines.size();
br.close();
}
public void update(Graphics g) { paint(g); }
public void paint (Graphics g) {
int canvasHeight = this.getBounds().height;
int canvasWidth  = this.getBounds().width;
if (height < 1 || width < 1)
return; // nothing to do
int width =
((byte[])(lines.elementAt(0))).length;
for (int y = 0; y < lines.size(); y++) {
byte[] b;
b = (byte[])(lines.elementAt(y));
for (int x = 0; x < width; x++) {
switch(b[x]) {
case ' ': // empty part of maze
g.setColor(Color.lightGray);
g.fillRect(
x*(canvasWidth/width),
y*(canvasHeight/height),
canvasWidth/width,
canvasHeight/height);
break;
case '*':     // a wall
g.setColor(Color.darkGray);
g.fillRect(
x*(canvasWidth/width),
y*(canvasHeight/height),
(canvasWidth/width)-1,
(canvasHeight/height)-1);
break;
default:      // must be rat
g.setColor(Color.red);
g.fillOval(x*(canvasWidth/width),
y*(canvasHeight/height),
canvasWidth/width,
canvasHeight/height);
break;
}
}
}
}
} ///:~```
```//: c13:Rat.java

public class Rat {
static int ratCount = 0;
private Maze prison;
private int vertDir = 0;
private int horizDir = 0;
private int x,y;
private int myRatNo = 0;
public Rat(Maze maze, int xStart, int yStart) {
myRatNo = ratCount++;
System.out.println("Rat no." + myRatNo +
prison = maze;
x = xStart;
y = yStart;
prison.setXY(x,y, (byte)'R');
public void run(){ scurry(); }
}.start();
}
public void scurry() {
// Try and maintain direction if possible.
// Horizontal backward
boolean ratCanMove = true;
while(ratCanMove) {
ratCanMove = false;
// South
if (prison.isEmptyXY(x, y + 1)) {
vertDir = 1; horizDir = 0;
ratCanMove = true;
}
// North
if (prison.isEmptyXY(x, y - 1))
if (ratCanMove)
new Rat(prison, x, y-1);
// Rat can move already, so give
// this choice to the next rat.
else {
vertDir = -1; horizDir = 0;
ratCanMove = true;
}
// West
if (prison.isEmptyXY(x-1, y))
if (ratCanMove)
new Rat(prison, x-1, y);
// Rat can move already, so give
// this choice to the next rat.
else {
vertDir = 0; horizDir = -1;
ratCanMove = true;
}
// East
if (prison.isEmptyXY(x+1, y))
if (ratCanMove)
new Rat(prison, x+1, y);
// Rat can move already, so give
// this choice to the next rat.
else {
vertDir = 0; horizDir = 1;
ratCanMove = true;
}
if (ratCanMove) { // Move original rat.
x += horizDir;
y += vertDir;
prison.setXY(x,y,(byte)'R');
}  // If not then the rat will die.
try {
} catch(InterruptedException ie) {}
}
System.out.println("Rat no." + myRatNo +
" can't move..dying..aarrgggh.");
}
} ///:~```

The maze initialization file:

```//:! c13:Amaze.txt
* **      *  * **      *
***    * *******    * ****
***          ***
*****   **********   *****
* * * * **  ** * * * **  *
* * *  * **  * * *  * **
*     **     *     **
* **   * **  * **   * **
*** *  *** ***** *  *** **
*      *   * *      *   *
* ** * *     * ** * *
///:~```

### Other maze resources

A discussion of algorithms to create mazes as well as Java source code to implement them:

A discussion of algorithms for collision detection and other individual/group moving behavior for autonomous physical objects:

## XML Decorator

Create a pair of decorators for I/O Readers and Writers that encode (for the Writer decorator) and decode (for the reader decorator) XML