Source

dictionary_switches / src / Projects.rst

Full commit

Projects

Note

This chapter has not had any significant translation yet.

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:

# Projects/ratsAndMazes/Maze.py

class Maze(Canvas):
    lines = [] # a line is a char array
    width = -1
    height = -1
    main(self):
        if (args.length < 1):
            print("Enter filename")
            System.exit(0)

        Maze m = Maze()
        m.load(args[0])
        Frame f = Frame()
        f.setSize(m.width*20, m.height*20)
        f.add(m)
        Rat r = Rat(m, 0, 0)
        f.setVisible(1)

    def __init__(self):
        lines = Vector()
        setBackground(Color.lightGray)

    def isEmptyXY(x, 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]==' '

    def setXY(x, 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()

    def  load(String filename):
        String currentLine = null
        BufferedReader br = BufferedReader(
          FileReader(filename))
        for(currentLine = br.readLine()
            currentLine != null
            currentLine = br.readLine()) :
            lines.addElement(currentLine.getBytes())
            if(width < 0 ||
               currentLine.getBytes().length > width)
                width = currentLine.getBytes().length

        height = len(lines)
        br.close()

    def update(self, Graphics g): paint(g)
    def paint(Graphics g):
        canvasHeight = self.getBounds().height
        canvasWidth  = self.getBounds().width
        if (height < 1 || width < 1)
            return # nothing to do
        width =
            ((byte[])(lines.elementAt(0))).length
        for (int y = 0 y < len(lines) 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::


# Projects/ratsAndMazes/Rat.py

class Rat:
    ratCount = 0
    prison = Maze()
    vertDir = 0
    horizDir = 0
    x = 0, y = 0
    myRatNo = 0
    def __init__(self, maze, xStart, yStart):
        myRatNo = ratCount++
        print ("Rat no." + myRatNo + " ready to scurry.")
        prison = maze
        x = xStart
        y = yStart
        prison.setXY(x,y, (byte)'R')

    def run(self): scurry().start()

    def scurry(self):
        # Try and maintain direction if possible.
        # Horizontal backward
        boolean ratCanMove = 1
        while(ratCanMove):
            ratCanMove = 0
            # South
            if (prison.isEmptyXY(x, y + 1)):
                vertDir = 1 horizDir = 0
                ratCanMove = 1

            # North
            if (prison.isEmptyXY(x, y - 1))
                if (ratCanMove)
                    Rat(prison, x, y-1)
                    # Rat can move already, so give
                    # this choice to the next rat.
                else:
                    vertDir = -1 horizDir = 0
                    ratCanMove = 1

            # West
            if (prison.isEmptyXY(x-1, y))
                if (ratCanMove)
                    Rat(prison, x-1, y)
                    # Rat can move already, so give
                    # this choice to the next rat.
                else:
                    vertDir = 0 horizDir = -1
                    ratCanMove = 1

            # East
            if (prison.isEmptyXY(x+1, y))
                if (ratCanMove)
                    Rat(prison, x+1, y)
                    # Rat can move already, so give
                    # this choice to the next rat.
                else:
                    vertDir = 0 horizDir = 1
                    ratCanMove = 1

            if (ratCanMove): # Move original rat.
                x += horizDir
                y += vertDir
                prison.setXY(x,y,(byte)'R')
                # If not then the rat will die.
            Thread.sleep(2000)

        print ("Rat no." + myRatNo +
          " can't move..dying..aarrgggh.")

The maze initialization file:

# Projects/ratsAndMazes/Amaze.txt
   * **      *  * **      *
 ***    * *******    * ****
     ***          ***
 *****   **********   *****
 * * * * **  ** * * * **  *
   * * *  * **  * * *  * **
 *     **     *     **
   * **   * **  * **   * **
 *** *  *** ***** *  *** **
 *      *   * *      *   *
   * ** * *     * ** * *

Other Maze Resources

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

http://www.mazeworks.com/mazegen/mazegen.htm

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

http://www.red3d.com/cwr/steer/