No Fly Zone (Commodore 64) by Sander Alsema

--- No Fly Zone ---

Written by Sander Alsema for the 10-liner competition 2024.

Language: Commodore 64 Basic v2

Category: PUR-80

--- Description & Instructions ---

Up till now you had it fairly easy. Stationed in sector ZZ9 Plural Z

Alpha, your job mainly consisted in drinking coffee while watching

pitch-black computer screens monitoring open space. Sometimes you even

wondered whether they were switched on at all.

In times of peace, being a peace keeper for the United Galaxies can be

quite boring. Therefore you couldn't be more surprised, when suddenly all

alarms went off!

According to the message on your screen, one of the parties the UG tries

to keep from tearing each other apart has violated a ceasefire that has

lasted for millennia. Hordes of fly shaped military space craft are

heading through your sector to wage war.

It is your mission to intercept them before they reach their target. All

you have at your disposal is an outdated High Energy Lithium Laser (HELL)

system that fires one shot at a time. To make matters worse, you can only

take aim from your space station in four directions.

Use your joystick in port II to shoot DIAGONALLY at the flies, before

they reach the other side of the screen, or it's game over. With every

fly intercepted, new ones will appear increasingly sooner, making your

mission progressively harder.

Press any key to restart the game.

--- Code Explanation ---

line 0: Read data for the fly-string from elsewhere in the program.

        In a loop create a vertical orientation-string.

        Set character colour for the entire screen to green.

        Do this by clearing the screen and display the satellite.

Line 1: Set background colour to black.

        Create a string with quotation marks.

        Use this string twice to give the fly feet, immediately erasing

        the second pair upon display.

        Create a string that erases a fly upon interception.

        Prevent flashing of the topleft character.

        Set a variable with the improbability value.

        Set a variable to screen memory.

Line 2: In a loop process the activities of all four flies.

        The horizontal position of each fly is stored in an array.

        Use a variable with a short name to access these in order to

        reduce code size.

        If this position is larger than zero (meaning that the fly is

        activated) then display or erase the fly.

        If the fly was intercepted (and therefore erased) then stop

        the laser and reset the horizontal position.

Line 3: Set a variable with the values of joystick II.

        Set a variable with all directional joystick values.

        Calculate the horizontal direction of the laser using the

        horizontal joystick values.

        Calculate the vertical direction of the laser using the

        vertical joystick values.

        Use both of these directions to determine the appropriate

        laser character screen code.

        Reset the display-string to the fly-string.

Line 4: Move the fly forward if it was activated or decide using the

        improbability value whether to activate it or not.

        Override the laser character screen code with a neutral value

        if the joystick was untouched.

        Store the new horizontal value in the array.

        If the other side of the screen has been reached then jump

        to line 9.

Line 5: If a fly had been intercepted then increase the score.

        Clear the laser.

        If the firebutton is pressed, the joystick points in a

        direction and the laser is not yet fired then start the laser,

        using the calculated directions, screen code and start

        position.

Line 6: Clear the interception variable.

        Display the score in the topleft corner.

        If the laser was fired then move it forward.

        Display the laser.

        If the top or bottom has been reached then stop the laser.

Line 7: If the laser was fired and the position in front of it is not

        clear (you hit something) then set the display-string to the

        erasure-string.

        Manipulate the loop index so that in the next iteration the

        intercepted fly is processed.

        Set the interception variable.

Line 8: Display the body of the satellite using the inverted laser

        character screen code.

        Manipulate the loop index so that there can never be a fifth

        fly in the middle.

        Come out of the loop or iterate.

        Jump to line 2.

        This line also contains data for the fly-string.

Line 9: Display "GAME OVER" in the centre of the screen.

        Clear the keyboard buffer.

        Wait for a key to be pressed.

        Restart the game.