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Level 3
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Level 3

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July 27, 2022
1 min read
level-3

Level 3

Solver
s sahuang
Points
50
Flag
CTF{c4pt41N-m0rG4N}

Can you deal with the rocks that appeared in our once so peaceful harbor?

After adding another button to start Level 3, this is the field we start with:

Level 3

They added some rocks to the board, and the ships are now moving at a faster speed. This is unfeasable to complete via multitasking, so we’ll have to come up with a method to keep the ships in place.

Stabilizing the Game State

Here’s the plan: let’s make it so that these ships will constantly rotate at a certain interval——in doing so, they’ll complete a 360° loop within a small area, and we can commandeer them one-at-a-time by disabling the loop for certain ships. Let’s start by adding checkboxes to enable the loop:

index.html
    <p>Steer Ships:</p>
    <div>
        <button id="steer0">Steer 0</button>
        <button id="steer1">Steer 1</button>
        <button id="steer2">Steer 2</button>
    </div>
    <p>Loop Ships:</p>
    <div>
        <input type="checkbox" id="loop0" checked>Loop 0</input>
        <input type="checkbox" id="loop1" checked>Loop 1</input>
        <input type="checkbox" id="loop2" checked>Loop 2</input>
    </div>
</fieldset>

Regarding the JavaScript, I’ll be using performance.now() and checking if the difference between it and window.lastRot is greater than 500ms. This check will happen every tick, and in theory will create a consistently steering ship that doesn’t produce "ILLEGAL_MOVE"s for inputting too quickly:

solve.js
window.lastRot = 0;


// Runs when output from server is received
socket.onmessage = function (event) {
    // Converts server output into object
    let obj = JSON.parse(event.data);
    if (obj.type == "TICK") {
        let ships = [];
        // For each ship in obj.ships, push class object into ships array
        for (const i of obj.ships) {
            ships.push(new Ship(i.id, i.area[0], i.area[1], i.direction));
        }
        // Call the string literal getter
        for (const i of ships) {
            log(i.printState);
        }
    } else {
        log(JSON.stringify(JSON.parse(event.data)));
    }
    // Guard clause for looping ships!
    if (performance.now() - window.lastRot < 500) return;
    window.lastRot = performance.now();
    // Sends steer if checkbox is checked
    findAll("loop").forEach(function (element, index) {
        if (element.checked) {
            socket.send(JSON.stringify({
                type: "SHIP_STEER",
                shipId: `${index}`
            }));
        }
    });
};

Let’s see if it works:

Looping

We’ve managed to stabilize the playing field for a manual solve! Let’s flag the level:

Terminal window
...
ID: 0 | (760, 105) (820, 343) | DIR: UP
ID: 1 | (736, 101) (796, 371) | DIR: UP
ID: 2 | (742, 113) (802, 393) | DIR: UP
{"type":"WIN","flag":"CTF{c4pt41N-m0rG4N}"}