Deadlines

Information about how to run the lab remotely or in Windows can be found here.

Lab 1: Trainspotting

In this assignment you will write a program to control two trains. The purpose is to write the program so that the trains move relatively independently of each other, but without colliding. Instead of a real railway, we have a simulator, where one can control the speeds of trains and change rail switches. The simulator gives a signal when a train passes sensors suitably placed on the tracks. It is part of the assignment to decide where sensors should be placed to give sufficient information for your program. The map without sensors looks as follows.

Choice of programming language

In this assignment you must use Java.

Important

In order to make sure you are using the right version of Java and train simulator, you should run

setup_course tda381

on Chalmers GNU/Linux machines.

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Assignment

Your assignment is to write a program that controls the trains. The trains should run simultaneously between the two stations (note that each station has two tracks). It is important that the trains are able to move at the same time, and be as independent from each other as possible, without crashing. At the stations the trains should stop and then wait 1 to 2 seconds. The trains should run forever between the stations.

In the beginning there are no sensors on the tracks. You must find where it is suitable to place sensors so that the control program gets the necessary information.

The program that you deliver should work as follows. Each train should be controlled by an independent train program, which runs in its own Java thread. Train programs cannot communicate with outside world by anything else than acquiring and releasing semaphores. In particular, no shared variables should be read or changed by the train programs. Also, both trains must use the same 'train program', and it is not allowed to make the behaviour of a train depend on its train id. For example, the track that a train chooses should not depend on its train id.

Below is a list of more detailed requirements.

Requirements

• Waiting at stations. The trains must wait 1-2 seconds at each station after coming to a full stop. Avoid randomness. You can use a formula such as: 1000 + (20 * |train_speed|)

• Maximum train speed. You need to find out the maximum train speed maxspeed that is usable with your particular solution. This must be at least 15.

• No minimal speed. There should be no assumption about minimal train speed. All speeds in the range [1,maxspeed] will be tried.

• No excessive semaphores. Solutions that use 10 or more semaphores will be rejected.

• No polling. Your solutions must not use busy-waiting-loops or their close relative, polling.

• Dynamic behaviour. You must not statically encode how the trains choose track at the switches (for example: train 1 always takes the upper path in the middle, while train 2 always takes the lower; same for stations). You must let the trains have one track as default (for example: the track with shortest distance). You must select the other track if the default track is already occupied by the other train. Note that you should not try to find a solution which works for any track layout or any number of trains, as this will be too complicated for all of us! It is enough to solve the problem for the given track layout.

• No map modifications. You may not alter the map in any way, other than by adding sensors.

• No randomization. You may not use randomization in any way that affects the simulation, as this makes testing more difficult.

• Two trains – two threads. When the system is running there should be only two processes (threads): one for each train. This is in addition to the main thread and the thread that handles communication with the simulator. This latter thread is created in the interface code we provide you with.

• Two trains – one implementation. The solution must use the same implementation for both trains. The only thing separating the processes of the different trains is the train id, starting position, and speed. The fact that the two trains start in different positions means that it is permitted (and necessary) to have different initial behaviour for the two processes, but after that the behaviour of the processes must not be dependent on which train it is acting for.

• Use binary semaphores. You must use the provided binary semaphore for mutual exclusion. You will have the opportunity to use other synchronisation mechanisms in the other assignments.

• Trains mind their own business. Trains should operate independently. A train should not make plans/decisions for the other train. Further, a train should not make plans/decisions for itself based on the information (speed, position, direction) of the other one.

• Good train flow. The parallel section in the middle of the map must allow a fast train to overtake a slow train going in the same direction.

• Documentation. In addition to working code you need to provide a convincing argument why your solution is correct. You must document your solution with an ASCII text file. Give a high-level description of your program and then go into detail about all pieces of code that are essential for correctness. The documentation should include everything needed for easy understanding of the solution. In particular we demand that your documentation contains discussion of:

  • Placement of the sensors
  • Choice of critical sections
  • Maximum train speed and the reason for it
  • How you tested your solution

• Code quality. Reasonable code quality is expected; do not complain if your assignment is rejected because we think your program is ugly (magic numbers, poor use of types, cut-and-paste coding style) – even if it works.

• Submission. When you submit your solution, you must provide the following files:

  • Lab1.java (your program to control the trains)
  • Lab1.map (which indicates the placement of your sensors)
  • documentation.txt (ASCII file containing your documentation)

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Train simulator

The simulator is called tsim and with this program you can even modify the train map. You can run tsim as follows:

• tsim
  Make a new, empty map, of default size.

• tsim filename
  Read in an old map from file filename.

• tsim --speed 50 filename
  Read in an old map from file filename, and also set the simulation speed to 50. This value is the number of milliseconds that tsim waits between updates of its state (i.e moving the trains). 100 ms is the default, so 50 makes the simulation go at double speed.

• tsim --help
  See all options of the simulator.

tsim as a map editor

You need to get your own copy of the map in order to be able to add the sensors. The map you will be working on is that shown above.

The map file is included in the archive in the downloads section.

Then you can start to modify the map with tsim Lab1.map. The figure below shows how to use tsim. You should only select the sensor button in the tools window (the small T-like symbol) and then use the mouse to place sensors on the track.

tsim as a simulator

You may start tsim and interact with it using commands on standard input. Try for example the commands

SetSpeed 1 20
SetSwitch 17 7 RightSwitch

and see what happens. (The second command is necessary for the train to survive the first switch which initially is set in the wrong direction). We provide the Java library to handle the communication with tsim, which lets you change the speed of the trains and the state of the switches in a more convenient way (see next section).

Here are some things to note about the simulator:

• When you give a new speed for a train, it takes a while for the train to increase or decrease from its old to its new speed. This means that the train's braking distance puts limits on how fast they could drive.

• The trains can not turn around, but can be given a negative speed to travel backwards.

• You are not able to give a speed which implies that a train changes direction. To change direction the train must first be stopped (by giving it speed 0). There is no signal from the simulator to tell you when the train has stopped, so you will need to wait a suitable time before changing direction.

• You must be careful when you change the switches. If a train comes to a switch which is in the wrong position, then the train will derail. See the figure below.

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Program/Simulator interaction

The simulator and your program run as separate OS processes. They use pipes to communicate. The Main class provided in the skeleton code takes care of joining together the pipes of your solution in Java with the tsim simulator.

All commands between the your program and the simulator are sent line-by-line as text strings, as you saw above. To aid you in your assignment we provide you with a Java library (TSim) that implements the protocol tsim is using. You must use this Java library. The package provides you with an interface to control the behaviour of the trains and switches. You can view the JavaDoc documentation of this library here.

Usage

You cannot create instances of TSimInterface using its constructor, since it is private. Instead you use the static method getInstance() that creates an instance of the class if that has not already been done (singleton pattern).

TSimInterface tsi = TSimInterface.getInstance() ;

Interesting classes

• TSimInterface is the class used to control the trains. It contains methods to control the speed of the train and to switch the positions of the switches. All these methods throw a CommandException if some tsim-related error occurs. In addition there is a method to turn on debugging, which displays the communication between your program and tsim. Read the Javadoc documentation of this class carefully!
• SensorEvent represents the event of a train passing over a sensor. Every time a train passes over a sensor first an ACTIVE event is created followed by an INACTIVE event when the train leaves the sensor. This class contains the status of the sensor, the position of the sensor and the identity of the train causing the event.
• CommandException represents a failure of one of the methods of TSimInterface. Information on the error is stored in the exception and can be acquired by the getMessage method. You should never catch an exception leaving the exception handler empty! For your own and our best always print the error message contained in the exception object. Otherwise, your program will be much harder to debug.

try {
  ...
}
catch (CommandException e) {
  System.err.println(e.getMessage());

  /* If a command failed then something bad has
     happened and we probably don't want to
     continue executing */

  System.exit(1);
}

Downloads

To get started, you need the skeleton code package.

Getting started

You should implement your solution in the Lab1.java file. We provide a Main class which takes care of loading the simulator and connecting it to your solution. You shouldn't need to change this file. This entry point expects at least 3 command line arguments:

1. map file
2. speed of train 1
3. speed of train 2
4. simulator speed [optional]

Using the command line

To build your solution, you can simply run the following command:

make all

from the root directory of the project. This will place the class files in the bin folder.

You can then run your solution with:

java -cp bin Main Lab1.map 5 15

Using Eclipse

You can also open the files as an Eclipse project:

1. Extract the trainspotting folder into your workspace directory.
2. In Eclipse, choose New → Java Project. Give it the exact name trainspotting to use the existing sources.

When running or debugging the Main class in Eclipse, you will need to specify the command-line arguments via the Run Configurations dialog.

Building tsim on your own machine

If you wish to work on this assignment on your own computer, then you will also need to build and run tsim locally. We do not recommend this, since it may not be straightforward, especially if you are not using Linux.

First download the tsim simulator source code. Then build with the following commands:

$ ./configure
$ make
$ make install

The default installation path is /usr/local. If you want to install to some other location instead run ./configure --prefix=/your/location. For more information, see the INSTALL file.

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Tips and Tricks

• Which parts of track are critical resources?

• There are two common ways of writing the train programs. One is that the train remembers in which place on the track it is and performs its actions based on that (stateful solution). The other is that the train decides what to do based on the coordinates of the sensor it steps on (stateless solution). In the stateless solution, you might still have to remember some things, for example which semaphors you are holding.

• Do not try to solve the general problem. Create a solution that works for this particular exercise. For example, it is OK to assume that there will be only 2 trains on the map. If you are interested in solving the general problem, write a short description of how to solve it in the documentation file.

• If you have to lock two critical sections at the same time, this probably indicates that your train flow will be poor. Perhaps you need a little bit more sensors?

• It is not necessary for trains to stop exactly at the end of the track at a station (before changing direction).

• Do not overoptimise modularity. The assignment is simple enough to have the solution in one file.

• For testing, run multiple instances of the simulator (at a high simulation speed), with different train speeds for a long time. e.g. Test your trains with the first going very fast and the second going very slow, and vice versa. Don't set the simulation speed too high (--speed switch should be about 15 or higher), since the trains might not get commands from your program on time.

• Note that get_sensor(TId) blocks until the given train (TId) next passes a sensor.

• In previous courses, the best solutions had usually less than 300 lines of code. If your solution is much bigger than that then you are probably making it too complicated. Try making it simpler. It is very important that your code is understandable.

• There is no need to have more than a single try-catch in the entire program (more information).

Note!

It was stated above but may be stated again: Reasonable code quality is expected. You are programmers – programming should be an art to you. The quality of your code will be vital for its reuse and maintenance.

Common reasons for rejection

Polling/busy waiting

What is polling and what is not is a hard question. We here give some examples in pseudo code. Loops that reduce to something similar to the situation below (where the dots do not include any blocking wait) must be considered as polling.

while (e) { // POLLING!
   ...
   sleep(t);
   ...
}

Using a blocking operation within a loop is not considered as polling.

while (e) { // NO POLLING!
   ...
   wait(o);
   ...
}

If it is not the case that something is regularly waking the process from its wait. Thus, the following example must be considered as an instance of polling:

obj o;

process a {   
  while (true) {
    sleep(t);
    notify(o);
  }
}

process b {  // POLLING!
  while (e) {
    wait(o);
  }
}

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