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CircuitSim Autograder Template

Note: A more up-to-date version of this tool is maintained by the Georgia Tech CS 2110 TAs here: If you are a CS 2110 Student or TA, you should consider using that library instead of this template.

Javadoc for API:

This repository aims to bridge the gap between the nitty-gritty details of the CircuitSim API and high-level tests on combinational logic in a CircuitSim file. Until now, we (the CS 2110 staff) handled CircuitSim homeworks with one of the following strategies:

  1. Don't provide a tester because they were scary
  2. Copy and paste some old tests, often missing error handling because the tons of duplicate code and intertwined CircuitSim implementation details made the tests hard to follow

So I have two goals in this template:

  1. Make CircuitSim autograders easy to write, for students too if they want
  2. Insulate tests from the verboseness of the CircuitSim API by wrapping the CircuitSim API, and pass these wrappers to test classes using dependency injection

Getting Started

To make your own CircuitSim autograder:

  1. Clone this repository, probably running rm -r docs/ *.sim and writing a new
  2. Edit the project name in
  3. Write some tests in src/main/java/edu/gatech/cs2110/circuitsim/tests/
  4. Test with ./gradlew run and generate build/libs/hwX-tester.jar with ./gradlew jar

Writing Tests

Testing basic combinational logic

Suppose we had a Toy ALU like the following that we wanted to test:

Screenshot of toy-alu.sim

You can find the full test at src/main/java/edu/gatech/cs2110/circuitsim/tests/, but a brief walkthrough follows if you just want the highlights.

Let's start with the boilerplate test class:

@DisplayName("Toy ALU")
@SubcircuitTest(file="toy-alu.sim", subcircuit="ALU")
public class ToyALUTests {
    private InputPin a;

    private InputPin b;

    private InputPin sel;

    private OutputPin out;

@DisplayName("Toy ALU") lets students will see the name Toy ALU on test failures (if you don't specify it, they will just see the ugly fully-qualified class name instead), and @ExtendWith(CircuitSimExtension.class) tells JUnit to run some custom CircuitSim setup code first which loads up the subcircuit we want and injects pins in it into the test class instance. Indeed, @SubcircuitTest(file="toy-alu.sim", subcircuit="ALU") tells the CircuitSim JUnit extension to load toy-alu.sim and then open its subcircuit matching the name ALU. As described in the API documentation, this subcircuit name (as well as Pin labels later) will be normalized into a lowercase alphanumeric string. That way, 1-bit ALU matches 1bit alu, 1 bit ALU, and so on.

The Pin fields are pretty self-explanatory: the extension looks up Pin components in the subcircuit based on the name of fields annotated with @SubcircuitPin. If you want to match on a different label, say label="something else".

Let's start with testing the last operation, Pass A, since it's the easiest:

    @DisplayName("pass A")
    public void passA() {
        assertEquals(0, out.get(), "out");

        assertEquals(0b1011, out.get(), "out");

Well, that's pretty easy. But what if you need to test more than two cases? That's where JUnit 5 parameterized tests come in:

    @ParameterizedTest(name="a:{0}, b:{1}, sel:00 (a xor b) → out:{2}")
        /*  a      b    |  out */
        "0b1111, 0b0000, 0b1111",
        "0b0000, 0b1111, 0b1111",
        "0b1111, 0b1111, 0b0000",
        "0b1011, 0b0010, 0b1001",
    public void xor(@ConvertWith(BasesConverter.class) int aIn,
                    @ConvertWith(BasesConverter.class) int bIn,
                    @ConvertWith(BasesConverter.class) int outOut) {
        assertEquals(outOut, out.get(), "out");

JUnit will run this test once for every row in the CSV. Unlike a simple for loop in a test, if one fails, it keeps going. The @ConvertWith(BasesConverter.class) parses Strings to ints based on the prefix. So 0x is hex and 0b is binary, and anything else is decimal.

You can also source your parameterized test from a method as follows:

    @ParameterizedTest(name="a:{0}, b:{1}, sel:01 (a + b) → out:{2}")
    public void add(int aIn, int bIn,
                    int outOut) {
        assertEquals(outOut, out.get(), "out");

    public static Stream<Arguments> add() {
        List<Arguments> args = new LinkedList<>();

        for (int a = 0; a < (1 << 4); a++) {
            for (int b = 0; b < (1 << 4); b++) {
                args.add(Arguments.of(a, b, (a + b) % (1 << 4)));


This allows for a much easier way to do an exhaustive test than a CSV! Note that JUnit locates the second (static!) method because it has the same name as the test.

You can see the finished product in src/main/java/edu/gatech/cs2110/circuitsim/tests/

Restrictors (Banning Components)

Suppose in the previous example, we wanted students to build the XOR themselves, without using a XOR gate component. Then we can change the test as follows:

@DisplayName("Toy ALU")
@SubcircuitTest(file="toy-alu.sim", subcircuit="ALU",
public class ToyALUTests {
    public static class BannedGates extends Restrictor {
        public void validate(Subcircuit subcircuit) throws AssertionError {
            blacklistComponents(subcircuit, "XOR");
    // ...

That is, we can provide restrictors to @SubcircuitTest. The fact that it's a class is useful for when you have a bunch of subcircuits with the same restrictions. In such a case, you can reduce code duplication by simply referencing the same Restrictor class in all of the tests.

Consult the Restrictor documentation for more information on restrictions. There is also support for whitelists.

Testing sequential logic

Combinational logic is easy enough, but what about a state machine? Consider the following:


which corresponds to this one-hot circuit:

screenshot of fsm.sim

The register here is kind of a pain, because we want to test the combinational logic on each side of it. Luckily, we can use a feature in the tester which replaces the register with some Pin components before running the tests, (very) roughly like this:

screenshot of fsm.sim after mocking the register

The syntax for this is similar to pins:

@DisplayName("Finite State Machine")
@SubcircuitTest(file="fsm.sim", subcircuit="fsm")
public class FsmTests {
    private InputPin g;

    private InputPin clk;

    private InputPin rst;

    private InputPin en;

    private OutputPin a;

    @SubcircuitRegister(bits=2, onlyRegister=true)
    private MockRegister stateReg;

So we specify the input/output pins we want as usual, but then also require there to be exactly 1 2-bit register in the subcircuit: the state register.

Testing the output a is fairly simple, so let's start with that:

    @DisplayName("output a")
    public void outputA() {
        assertEquals(0, a.get(), "output a in state 00");
        assertEquals(1, a.get(), "output a in state 01");
        assertEquals(0, a.get(), "output a in state 10");

MockRegister.getQ() returns an InputPin that the tester placed where the out port of the register used to be, so we can set it to an arbitrary value just like any other input pin.

MockRegister.getD(), on the other hand, is an OutputPin:

    @ParameterizedTest(name="state:{0}, g:{1} → next state:{2}")
        /* state  g | next state */
        "   0b00, 0,        0b01",
        "   0b00, 1,        0b01",
        "   0b01, 0,        0b01",
        "   0b01, 1,        0b10",
        "   0b10, 0,        0b01",
        "   0b10, 1,        0b10",
    public void transition(@ConvertWith(BasesConverter.class) int stateIn,
                           int gIn,
                           @ConvertWith(BasesConverter.class) int nextStateOut) {
        assertEquals(nextStateOut, stateReg.getD().get(), "next state");

You can see the finished product at src/main/java/edu/gatech/cs2110/circuitsim/tests/


Because CircuitSim is a JavaFX application and this tester simply runs CircuitSim, this tester tries to attach to a display — like an X11 $DISPLAY on GNU/Linux. If you want to run this grader in a headless environment, you can get around this by standing up a dummy X11 server.

For GNU/Linux, zucchini installs xf86-video-dummy and then uses the following

cat >xorg.conf <<'EOF'
# This xorg configuration file is meant to be used by xpra
# to start a dummy X11 server.
# For details, please see:
Section "ServerFlags"
  Option "DontVTSwitch" "true"
  Option "AllowMouseOpenFail" "true"
  Option "PciForceNone" "true"
  Option "AutoEnableDevices" "false"
  Option "AutoAddDevices" "false"
Section "Device"
  Identifier "dummy_videocard"
  Driver "dummy"
  Option "ConstantDPI" "true"
  VideoRam 192000
Section "Monitor"
  Identifier "dummy_monitor"
  HorizSync   5.0 - 1000.0
  VertRefresh 5.0 - 200.0
  Modeline "1024x768" 18.71 1024 1056 1120 1152 768 786 789 807
Section "Screen"
  Identifier "dummy_screen"
  Device "dummy_videocard"
  Monitor "dummy_monitor"
  DefaultDepth 24
  SubSection "Display"
    Viewport 0 0
    Depth 24
    Modes "1024x768"
    Virtual 1024 768
/usr/lib/xorg/Xorg -noreset -logfile ./xorg.log -config ./xorg.conf :69 \
    >/dev/null 2>&1 &
export DISPLAY=:69
kill "$xorg_pid" || {
    printf 'did not kill Xorg!\n' >&2
    exit 1
exit $exitcode

which allows you to say ./ java -jar hwX-tester.jar on a headless system.

Zucchini Support

A zucchini backend exists, CircuitSimGrader, which reads the JSON printed to stdout for

java -jar hwX-tester.jar --zucchini FsmTests

given some test FsmTests. Each subcircuit you want to test (aka each test class) should correspond to a different assignment component, and each test method is a part of that component. Here's a zucchini.yml for the FSM example above:

name: Homework X
author: Austin Adams
- name: Finite State Machine
  weight: 1
  backend: CircuitSimGrader
    grader-jar: hwX-tester.jar
    test-class: FsmTests
  files: [fsm.sim]
  grading-files: [hwX-tester.jar]
  - {test: clockConnected,  weight: 1}
  - {test: resetConnected,  weight: 1}
  - {test: enableConnected, weight: 1}
  - {test: outputA,         weight: 5}
  - {test: transition,      weight: 10}

Note that since hwX-tester.jar is in grading-files, it'll need to be in grading-files/ in the zucchini assignment repository.


a circuitsim autograder template for all of god's children



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