Air Traffic Control version 1.0

I am unaware of any copyright surrounding the original game. In so far as any existing copyright is not contradicted, this translation is

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Air Traffic Control represents a simple challenge — to guide a number of aeroplanes in or out of their respective airports, to or from their desired destinations.

1 History

I first played Air Traffic Control, or ATC for short, on a TRS-80 in 1981. Whether or not the present incarnation is a faithful representation of the original, I can’t say, as my memory isn’t that good, but I think that the basic ideas are the same. ATC might be considered rather a tame game in these days of blood-stained GUIs and tracker balls, but it can still generate some tense moments when the concentration reaches its limit and the eyes can no longer look at so many different places at once.

For the record, this is my second translation of the game; the first was into a dialect of BASIC for the BBC microcomputer. The great attraction of Emacs Lisp as a programming environment, however, is that it offers not only a stable language but also a consistent visual interface, over a very large range of machines; so it is in this, Emacs Lisp, translation that I hope ATC will find some success.

2 Installing ATC

In order for Emacs to be able to run ATC, it must know where to find the file containing the Emacs Lisp program which defines the game. Unless the name of this file has for some reason been changed, it is called ‘atc.el’.

If you have a standard location in which you put all Emacs Lisp files which didn’t come with the Emacs distribution — which might for example be called ‘/usr/local/emacs/lisp/local’ — the easiest option would be to add ‘atc.el’ to those files. Then, assuming that this location is one of those mentioned in the Emacs Lisp variable load-path, you would load the ATC program into Emacs by typing M-x load-library <RET> atc <RET>. To have this done automatically every time you start up Emacs, insert the line

(load "atc")

If there is no such standard location on your system, or if it is difficult for you to get access to it, then you must keep ‘atc.el’ in your own file-space. You could load in the program every time you want to use it by typing M-x load-file <RET> followed by the pathname of ‘atc.el’. Or you could do this automatically whenever you start Emacs by typing something like

(load "/home/neil/emacs-lisp/atc.el")

in your ‘.emacs’ file. Or you could arrange to load in ATC automatically when you need it, like this:

(autoload 'atc "/home/neil/emacs-lisp/atc.el"
   "Play Air Traffic Control." t)

(again, in your ‘.emacs’ file). This last method avoids using Emacs’ memory unnecessarily as the program will only be loaded in when and if you want to play the game.

3 Firing Up and Suspending ATC

If something distracts your attention while you’re in the middle of a game, or someone walks into the room who ought not to know that you’re playing games, you can suspend ATC and resume playing the same game later on.

To suspend ATC rapidly during a game, hit <ESC>: the game will stop and Emacs will replace the ATC buffer with whatever you were working on previously. This is the quick-change-to-avoid-detection option.

To suspend ATC in a less panic-stricken way, type C-z: the game will be suspended but will remain visible in its window on the screen. This option is for those who like a little breather in mid-game or who have just had a thought that can’t wait. Emacs will ask if you want to resume immediately (i.e. without doing any other Emacs work first): type y or n.

Whichever way you suspend, the game position will be saved and Emacs will remember that a game of ATC is in progress (until you shut down Emacs completely, that is). Then the next time you type M-x atc, Emacs will ask if you want to resume playing the game you started earlier. If you don’t, type n and the board will be swept clean and a new game prepared. If you do, type y and carry on.

Note: during the game the only keys that will evoke a response are <ESC>, C-z, C-g and the keys described below for controlling the aeroplanes; no other Emacs commands will work. But the normal situation is restored as soon as the game ends or is suspended: <ESC> will revert to being the escape prefix and C-z will invoke suspend-emacs if you are not running under a window system.

4 Rules of the Game

You have control over a region of air-space. Inside your region are situated two airports, one of them (#) almost dead centre and the other (*) a little way up towards the north-north-east. The perimeter of your air-space is divided into 10 areas, called 0/1, 1/2, 2/3 and so on up to 9/0.

During the game a number of aeroplanes will fly into your air-space from afar, wanting to land at one of your two airports. As a consequence of the prevailing wind directions, aeroplanes destined for airport # should always land in a westerly direction, while those destined for airport * should land in the north-west direction. Any attempt to land in an incorrect direction will probably do serious damage to the airport buildings and the plane.

Other aeroplanes will request take-off from one of your airports to a place outside your air-space. According to the direction of their ultimate destination, these aeroplanes are designated to leave via one of the 10 areas around the perimeter. A plane which leaves through the wrong area will not crash, but it will waste a lot of time and money. When a plane leaves your air-space it must have reached altitude 5 — this is a general requirement for aeroplanes in uncontrolled air-space.

Finally, there will be a few aeroplanes acting as shuttles between the two airports, which take off at # and land at *, or vice versa.

You should not allow an aeroplane to leave your airspace if it is intending to land at your airports. Neither should you land a plane which is destined to leave your air-space. An aeroplane wanting to land must not do so at the wrong airport.

If you direct an aeroplane to reduce its altitude to zero anywhere other than at an airport, it will of course crash. And if two aeroplanes are made simultaneously to occupy the same map location and have the same altitude, there will be a collision in mid-air.

Finally, the number of aeroplanes awaiting take-off at any one time should not exceed five.

Any of these undesirable events will signal the end of the game. These are the rules — now read on to discover how to play by them...

5 Controlling the Aeroplanes

All commands in ATC are two or three characters long. The first character specifies the plane for which the command is intended, which is a letter of the alphabet between A and Z. The second indicates the action being requested. The third, when a numerical argument is appropriate to the command, is a number between 0 and 9.

The command being composed at any time is shown in the top left corner of the screen after the exclamation mark. Typing <DEL> at any time will remove any partly written command that is there so that you can start a new command afresh.

As an example of giving a command, suppose you want to direct aeroplane F to turn left through 180 degrees. First you press F to specify the plane, and the top line will display:

Then press L to indicate that the command is for a turn to the left — the top line of the screen now looks like:

No command has yet been executed because turning left requires a numerical argument to say how much to turn. In this case you would finally press 4. The command is then transmitted to the aeroplane which will begin to respond in its next moves, and the command line is blanked out ready for the next directive. Were you to have changed your mind halfway through the command, and you had decided that the turn should be to the right rather than to the left, the whole sequence of key presses would have been F L <DEL> F R 4.

The following subsections describe in more detail ATC’s rather limited (but sufficient) command set. The available action letters are: A, to change altitude, L, to turn left, R, to turn right, and ?, to give information about the aeroplane’s provenance and destination.

5.1 Changing Altitude

There are four reasons why you may need to change an aeroplane’s altitude: to land, to take off, to achieve the required altitude 5 in open air-space, and to avoid a mid-air collision.

The numerical argument of the A command is a target altitude: after a directive is given the aeroplane will require one move to prepare for the change, and will then change its altitude by +1 or -1 every move until the target altitude is reached.

When a plane is landing, the giving of the command to change altitude to zero must be carefully timed so that zero altitude is reached precisely as the aeroplane moves into the airport. If the timing is wrong by even one move, the plane will crash-land.

To make a plane take off, instruct it to increase its altitude to 1. Once this command has been given, others may follow, for example to make a turn or to climb to higher altitude; before this command is given, the aeroplane can do nothing.

5.2 Turning Left

To instruct an aeroplane to turn left, type the plane’s letter, then L, and then a number between 0 and 9. An aeroplane can change its direction by 45 degrees every time it moves, and the numeric argument that you type represents the number of 45 degree turns you want it to execute. Aeroplanes require a move to prepare for the turn they are about to make. Here is an example of turning left, showing how an aeroplane moves if it is initially heading North at location 0 when the command xL5 (where x is the letter of the aeroplane) is given:

. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 3 2 . . . . . . . . . .
. . . . . . . . 4 . . 1 . . . . . . . . .
. . . . . . . . 5 . . 0 . . . . . . . . .
. . . . . . . . . 6 . . . . . . . . . . .
. . . . . . . . . . 7 . . . . . . . . . .
. . . . . . . . . . . 8 . . . . . . . . .
. . . . . . . . . . . . 9 . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .

If a turning command is given while a previous one is still being executed, the previous one is interrupted and replaced immediately by the new command. Thus the command xL0 can be used to break off a turn half-way through. For example, if such a command had been given when the aeroplane above was in position 4, the plane would have moved to 5 and then continued towards the South.

5.3 Turning Right

To instruct an aeroplane to turn right, type the plane’s letter, then R, and then a number between 0 and 9. An aeroplane can change its direction by 45 degrees every time it moves, and the numeric argument that you type represents the number of 45 degree turns you want it to execute. Aeroplanes require a move to prepare for the turn they are about to make.

Turning right works in exactly the same way as turning left. @xref{Turning Left}, for a detailed example.

5.4 Requesting Information

To find out where an aeroplane has come from and where it wants to go, type the plane’s letter and ?. A message will appear in the minibuffer of the form:

Aeroplane A: from area 8/9 to airport #.

This is intended to be self-explanatory. If it isn’t, please send me a bug report. Information may be requested when an aeroplane is currently moving on the screen or when it is in the list awaiting take off.

6 Options for Customisation

You can make ATC more difficult or more easy by changing the values of the following Emacs Lisp variables.

The procedure for changing one of these variable’s values (say atc-acceleration) is:

M-x set-variable <RET> atc-acceleration <RET> 4 <RET>

Changes are not possible while the game is active. You can make a change before starting a game of ATC (but after the Lisp has been initially loaded), or you can suspend ATC (see section Suspending ATC), change one of the variables, and then resume play.

About This Document

Button	Name	Go to	From 1.2.3 go to
[ << ]	FastBack	Beginning of this chapter or previous chapter	1
[ < ]	Back	Previous section in reading order	1.2.2
[ Up ]	Up	Up section	1.2
[ > ]	Forward	Next section in reading order	1.2.4
[ >> ]	FastForward	Next chapter	2
[Top]	Top	Cover (top) of document
[Contents]	Contents	Table of contents
[Index]	Index	Index
[ ? ]	About	About (help)

where the Example assumes that the current position is at Subsubsection One-Two-Three of a document of the following structure:

• General Public License		How you may copy and distribute this game.
1 History		The pedigree of the game.
2 Installing ATC		Setting up the game on your system.
3 Firing Up and Suspending ATC		To start playing ATC.
4 Rules of the Game		The rules of the game.
5 Controlling the Aeroplanes		Commands available for controlling planes.
6 Options for Customisation		Bits you can tweak to make it easier or harder.

5.1 Changing Altitude		Change altitude.
5.2 Turning Left		Turn to the left.
5.3 Turning Right		Turn to the right.
5.4 Requesting Information		Give information about the plane’s itinerary.