Nibbles in Ruby Qt

In this part of the Ruby Qt programming tutorial, we will create a Nibbles game clone.

Nibbles is an older classic video game. It was first created in late 70s. Later it was brought to PCs. In this game the player controls a snake. The objective is to eat as many apples as possible. Each time the snake eats an apple, its body grows. The snake must avoid the walls and its own body.

Development

The size of each of the joints of a snake is 10px. The snake is controlled with the cursor keys. Initially, the snake has three joints. The game starts immediately. When the game is finished, we display "Game Over" message in the center of the window.

Board.rb
WIDTH = 300
HEIGHT = 300
DOT_SIZE = 10
ALL_DOTS = WIDTH * HEIGHT / (DOT_SIZE * DOT_SIZE)
RAND_POS = 29
DELAY = 140

$x = [0] * ALL_DOTS
$y = [0] * ALL_DOTS

class Board < Qt::Widget

    
    def initialize(parent)
        super(parent)
        
        setFocusPolicy Qt::StrongFocus
       
        initGame
    end


    def initGame
    
        @left = false
        @right = true
        @up = false
        @down = false
        @inGame = true
        @dots = 3
        
        begin
            @ball = Qt::Image.new "dot.png"
            @apple = Qt::Image.new "apple.png"
            @head = Qt::Image.new "head.png"
        rescue
            puts "cannot load images"
        end

        for i in (0..@dots)
            $x[i] = 50 - i * 10
            $y[i] = 50
        end

        locateApple
        setStyleSheet "QWidget { background-color: #000000 }"
        
        @timer = Qt::BasicTimer.new 
        @timer.start(140, self)
        
    end


    def paintEvent event

        painter = Qt::Painter.new
        painter.begin self

        if @inGame
            drawObjects painter
        else 
            gameOver painter
        end

        painter.end
    end

    def drawObjects painter

        painter.drawImage @apple_x, @apple_y, @apple

        for z in (0..@dots)
            if z == 0
                painter.drawImage $x[z], $y[z], @head
            else 
                painter.drawImage $x[z], $y[z], @ball
            end
        end
    end

    def gameOver painter
        msg = "Game Over"
        small = Qt::Font.new "Helvetica", 12,
            Qt::Font::Bold.value
        
        metr = Qt::FontMetrics.new small
        
        textWidth = metr.width msg
        h = height
        w = width

        painter.setPen Qt::Color.new Qt::white
        painter.setFont small
        painter.translate Qt::Point.new w/2, h/2
        painter.drawText -textWidth/2, 0, msg
    end


    def checkApple

        if $x[0] == @apple_x and $y[0] == @apple_y 
            @dots = @dots + 1
            locateApple
        end
    end
    
    def move

        z = @dots

        while z > 0
            $x[z] = $x[(z - 1)]
            $y[z] = $y[(z - 1)]
            z = z - 1
        end

        if @left
            $x[0] -= DOT_SIZE
        end

        if @right 
            $x[0] += DOT_SIZE
        end

        if @up
            $y[0] -= DOT_SIZE
        end

        if @down
            $y[0] += DOT_SIZE
        end
        
     end


    def checkCollision

        z = @dots
       
        while z > 0
            if z > 4 and $x[0] == $x[z] and $y[0] == $y[z]
                @inGame = false
            end
            z = z - 1
        end

        if $y[0] > HEIGHT
            @inGame = false
        end
        
        if $y[0] < 0
            @inGame = false
        end
        
        if $x[0] > WIDTH
            @inGame = false
        end
        
        if $x[0] < 0
            @inGame = false
        end    
        
    end


    def locateApple
    
        r = rand RAND_POS
        @apple_x = r * DOT_SIZE
        r = rand RAND_POS
        @apple_y = r * DOT_SIZE
    end

    def timerEvent event

        if @inGame 
            checkApple
            checkCollision
            move
        else 
            @timer.stop
        end

        repaint
    end

    def keyPressEvent event
        
        key = event.key
        
        if key == Qt::Key_Left.value and not @right
            @left = true
            @up = false
            @down = false
        end
        
        if key == Qt::Key_Right.value and not @left
            @right = true
            @up = false
            @down = false
        end
        
        if key == Qt::Key_Up.value and not @down
            @up = true
            @right = false
            @left = false            
        end
        
        if key == Qt::Key_Down.value and not @up
            @down = true
            @right = false
            @left = false
        end
    end
end

First we will define some constants used in our game.

The WIDTH and HEIGHT constants determine the size of the Board. The DOT_SIZE is the size of the apple and the dot of the snake. The ALL_DOTS constant defines the maximum number of possible dots on the Board. The RAND_POS constant is used to calculate a random position of an apple. The DELAY constant determines the speed of the game.

$x = [0] * ALL_DOTS
$y = [0] * ALL_DOTS

These two arrays store x, y coordinates of all possible joints of a snake.

The initGame method initialises variables, loads images and starts a timeout function.

if @inGame
    drawObjects painter
else 
    gameOver painter
end

Inside the paintEvent method, we check the @inGame variable. If it is true, we draw our objects. The apple and the snake joints. Otherwise we display "Game over" text.

def drawObjects painter

    painter.drawImage @apple_x, @apple_y, @apple

    for z in (0..@dots)
        if z == 0
            painter.drawImage $x[z], $y[z], @head
        else 
            painter.drawImage $x[z], $y[z], @ball
        end
    end
end

The drawObjects method draws the apple and the joints of the snake. The first joint of a snake is its head, which is represented by a red circle.

def checkApple

    if $x[0] == @apple_x and $y[0] == @apple_y 
        @dots = @dots + 1
        locateApple
    end
end

The checkApple method checks if the snake has hit the apple object. If so, we add another snake joint and call the locateApple method, which randomly places a new apple object.

In the move method we have the key algorithm of the game. To understand it, look at how the snake is moving. You control the head of the snake. You can change its direction with the cursor keys. The rest of the joints move one position up the chain. The second joint moves where the first was, the third joint where the second was etc.

while z > 0
    $x[z] = $x[(z - 1)]
    $y[z] = $y[(z - 1)]
    z = z - 1
end

This code moves the joints up the chain.

if @left
    $x[0] -= DOT_SIZE
end

Move the head to the left.

In the checkCollision method, we determine if the snake has hit itself or one of the walls.

while z > 0
    if z > 4 and $x[0] == $x[z] and $y[0] == $y[z]
        @inGame = false
    end
    z = z - 1
end

We finish the game if the snake hits one of its joints with the head.

if $y[0] > HEIGHT
    @inGame = false
end

We finish the game if the snake hits the bottom of the Board.

The locateApple method locates an apple randomly on the board.

r = rand RAND_POS

We get a random number from 0 to RAND_POS - 1.

@apple_x = r * DOT_SIZE
...
@apple_y = r * DOT_SIZE

These line set the x and y coordinates of the apple object.

if @inGame 
    checkApple
    checkCollision
    move
else 
    @timer.stop
end

Every 140 ms, the timerEvent method is called. If we are in the game, we call three methods that build the logic of the game. Otherwise we stop the timer.

In the keyPressEvent method of the Board class, we determine the keys that were pressed.

if key == Qt::Key_Left.value and not @right
    @left = true
    @up = false
    @down = false
end

If we hit the left cursor key, we set @left variable to true. This variable is used in the move method to change coordinates of the snake object. Notice also that when the snake is heading to the right, we cannot turn immediately to the left.

Nibbles.rb
#!/usr/bin/ruby

# ZetCode Ruby Qt tutorial
#
# In this program, we create
# a Nibbles game clone.
#
# author: Jan Bodnar
# website: www.zetcode.com
# last modified: September 2012

require 'Qt'
require 'Board'

class QtApp < Qt::MainWindow
    
    def initialize
        super
        
        setWindowTitle "Nibbles"
       
        setCentralWidget Board.new(self)
       
        resize 310, 310
        move 300, 300

        show
    end
end

app = Qt::Application.new ARGV
QtApp.new
app.exec

In the Nibbles.rb file, we set up the Nibbles game.

Nibbles
Figure: Nibbles

This was the Nibbles computer game programmed with the Qt library and the Ruby programming language.