lobster

Interactive Program Visualization Tools

This repository is a complete mess. I'll clean it up someday. Feel free to look around.

There's a discord now: https://discord.gg/rHvsm3eCu2

Setup

These instructions cover everything you'll need if you want to do development work for Lobster's frontend, C++ compiler/interpreter, visualization tools, analysis framework, or regression tests.

This is also the place to start if you'd like to create exercises with custom checkpoints and heuristics.

Start by cloning this repository:

git clone https://github.com/jamesjuett/lobster.git

Ensure you have node and npm installed:

sudo apt update
sudo apt install nodejs

Install Lobster's dependencies using npm:

npm install

That's it!

I recommend using VS Code, which has built-in Typescript support, but you can use any editor you like.

Compilation

Lobster is written in Typescript. That means you need to recompile after any changes to .ts files. To compile and bundle the code, use:

npm run build

If you make any changes to grammar.txt, run this to regenerate the parser module (and then recompile):

./node_modules/pegjs/bin/pegjs --plugin ./node_modules/ts-pegjs/src/tspegjs --allowed-start-rules start,declaration,declarator,function_definition -o src/js/parse/cpp_parser.ts other/grammar.txt

Local Preview

After compiling, just open up public/index.html. You won't have any saved files or anything, but you can test out any changes to the compiler, visualizer, or general frontend UI here.

Creating Embeddable Exercises

Lobster exercises can be embedded by:

1. Make sure your page includes these 3rd part libraries used by Lobster:

<!-- jquery and bootstrap -->
<script src="https://code.jquery.com/jquery-3.5.1.min.js" integrity="sha256-9/aliU8dGd2tb6OSsuzixeV4y/faTqgFtohetphbbj0=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
<script src="https://stackpath.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>

<!-- bootstrap icons -->
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/font/bootstrap-icons.css">

2. Copy some files out of this repository and ensure they are included in your page. If you make changes or recompile, you'll need to re-copy.

• public/css/main.css
• public/css/code.css
• public/css/frontend.css
• public/css/exercises.css
• public/js/embedded_exercises.js

3. The embedded_exercises.js script will look for html elements with the lobster-ex class and initialize them as Lobster exercises. Here's an example that just creates a simple Lobster example with given starter code. (Note that you need to use html entites for special characters like <, >, &, etc. which is kind of annoying.)

4. Alternatively, you may specify an exercise key to match up against starter code, checkpoints, heuristics, etc. This is probably how you'll want to handle anything but the very simplest embeds.

<div class="lobster-ex" style="width: 800px; margin-left: auto; margin-right: auto">
    <div class="lobster-ex-key">ch13_03_ex</div>
</div>

To define exercises by key, find EXERCISE_SPECIFICATIONS in src/exercises.ts. Write typescript code to add entries for each new exercise. For example, here's one entry:

"ch13_03_ex": {
        starterCode: `#include <iostream>
using namespace std;

int main() {

  int x = 0;

  while (x < 4) {
    cout << x << endl;
    x = x + 1;
  }

  while (x >= 0) {
    cout << x << endl;
    x = x - 1;
  }

  cout << "done!" << endl;
}`,
        completionCriteria: COMPLETION_ALL_CHECKPOINTS,
        checkpoints: [
            new StaticAnalysisCheckpoint("Use ++", (program: Program) => {
                return !!findConstructs(program, Predicates.byKinds(["prefix_increment_expression", "postfix_increment_expression"])).find(
                    construct => construct.operator === "++"
                );
            }),
            new StaticAnalysisCheckpoint("Use --", (program: Program) => {
                return !!findConstructs(program, Predicates.byKinds(["prefix_increment_expression", "postfix_increment_expression"])).find(
                    construct => construct.operator === "--"
                );
            }),
            new OutputCheckpoint("Correct Output", (output: string) => {
                return output === "0\n1\n2\n3\n4\n3\n2\n1\n0\ndone!\n";
            })
        ],
        completionMessage: 'Nice work! The secret word is "bubble".'
    },

Exercise Specifications

Completion Criteria

An exercise may specify completion criteria. Two predefined criteria are available:

• COMPLETION_ALL_CHECKPOINTS
• COMPLETION_LAST_CHECKPOINT

You can also provide a custom predicate that takes an Exercise as an argument and returns true/false.

Checkpoints

An exercise specification may includes a list of checkpoints. There are several different kinds of checkpoints. Here are a few characteristic examples:

A checkpoint for compiling without errors:

new IsCompiledCheckpoint("Compiles"),

Checkpoints based on static analysis of a student's program:

new StaticAnalysisCheckpoint("Use ++", (program: Program) => {
    return !!findConstructs(program, Predicates.byKinds(["prefix_increment_expression", "postfix_increment_expression"])).find(
        construct => construct.operator === "++"
    );
}),

Checkpoints to run the program and check its output to cout:

new OutputCheckpoint("Correct Output", (output: string) => {
    // Output must match exactly, including whitespace
    return output === "0\n1\n2\n3\n4\n3\n2\n1\n0\ndone!\n";
})

new OutputCheckpoint("Output Correct Result", (output: string) => {
    // 2623.4 may appear in larger string
    return output.indexOf("2623.4") !== -1;
})

A checkpoint to check the program state (e.g. values of variables) at the end of the main() function

// Verifies that the array variable arr contains the values [16, 25, 4]
new EndOfMainStateCheckpoint("arr modified to {16, 25, 4}", (sim: Simulation) => {
    
    let main = sim.program.mainFunction;
    let arrEntity = main.context.functionLocals.localObjects.find(local => local.name === "arr");

    if (!arrEntity) {
        return false;
    }

    let mainFrame = sim.memory.stack.topFrame()!;
    let arr = mainFrame.localObjectLookup(arrEntity);
    if (!arr.isTyped(isBoundedArrayOfType(isType(Int)))) {
        return false;
    }
    let elts = arr.rawValue();
    return isEqual(elts, [16, 25, 4]);
})

Helpful Links

An easy-to-navigate copy of the C++ language standard
https://timsong-cpp.github.io/cppwp