JS1k: The JavaScript code golfing competition

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// ==ClosureCompiler==
// @language_out ECMASCRIPT_2017
// @compilation_level ADVANCED_OPTIMIZATIONS
// @js_externs var w = {};
// @js_externs var a = {};
// @js_externs var c = {};
// ==/ClosureCompiler==

/*
    The rules for fishy are:

        Fish have mass.
        Fish gain a small amount of mass over time
        Fish have 0 to 1 energy.
        Fish lose energy over time independent of mass.
        Fish die at 0 energy.
        Fish eat prey and add the prey mass to their own.
        Fish gain energy from eating prey equal to the prey mass / new own mass.

        Much smaller fish are prey.
        Much larger fish are predators.
        The rest of fish are kin.

        The velocity of the fish is determined using a boids-like model. All rules are distance weighted:

            1. Fish tries to move closer to kin.
            2. Fish tries to move in the same direction as kin.
            3. Fish chase prey.
            4. Fish move away from very close kin and nearby predators.
*/

var fishCount = 90;
var maxMass = 2e4;
var padding = 50;
var fish = [...Array(fishCount)].map(() => ({ energy: 0 }));
var lastTime = 0;
var trail = 0;
var weight;
var i;

// Switch on and off trail mode
onmouseup = () => trail = !trail;

// Main loop
var update = time => {
    var elapsedSeconds = (time - lastTime) / 1000;
    lastTime = time;

    var size = [a.width + padding * 2, a.height + padding * 2];

    // Scale the logic based on screen height
    var scale = size[1] / 1000;

    // Clear (blur) the canvas
    c.fillStyle = `rgb(0,40,90,${!trail * .7})`;
    c.fillRect(0, 0, ...size);

    // Update fishes
    fish.map((f, fi) => {
        var r = [[0, 0, 1e-8], [0, 0, 1e-8], [0, 0, 1e-8]];

        // Die if there is no energy
        if (f.energy <= 0) {
            f.p = size.map((s, i) => s * Math.random());
            f.v = [0, 0];
            f.mass = f.energy = 1;
            f.radius = 0;
            return;
        }

        // React to other fishes using a boids model
        fish.map(f2 => {
            // Not itself
            if (f == f2) {
                return;
             }

            // Wrapping delta position and distance
            var dist = 0;
            var d = size.map((s, i) => {
                var d = f2.p[i] - f.p[i];
                if (d < -s / 2) {
                    d += s;
                }
                if (d > s / 2) {
                    d -= s;
                }
                dist += d * d;
                return d;
            });
            dist = Math.sqrt(dist) / scale;

            // Relationship type
            var isPredator = (f.mass - f2.mass) / (f.mass + f2.mass) > .2;
            var isPrey = -(f.mass - f2.mass) / (f.mass + f2.mass) > .2;
            var isKin = !isPredator && !isPrey;

            // Cohesion and alignment
            if (isKin) {
                weight = .1 + (dist < 500);

                i = 0;
                r[0][i] += (d[i] + f2.v[i] - f.v[i]) * weight;
                i++;
                r[0][i] += (d[i] + f2.v[i] - f.v[i]) * weight;
                r[0][2] += weight * 2;
            }

            // Separation and fleeing
            if (isKin && dist < f.radius * 4 || isPrey && dist < 200) {
                weight = Math.max(1, 200 - dist);

                i = 0;
                r[1][i] -= d[i] / dist * weight * 400;
                i++;
                r[1][i] -= d[i] / dist * weight * 400;
                r[1][2] += weight;
            }

            // Chasing
            if (isPredator) {
                weight = Math.max(1, 300 - dist) * f2.mass;

                i = 0;
                r[2][i] += d[i] / dist * weight * 250;
                i++;
                r[2][i] += d[i] / dist * weight * 250;
                r[2][2] += weight;
            }

            // Eat other fish
            if (isPredator && dist < f.radius * 1.1) {
                f.mass = Math.min(maxMass, f.mass + f2.mass);
                f.energy = Math.min(1, f.energy + f2.mass / f.mass);
                f2.energy = 0;
            }
        });

        // Apply boid rules to fish velocity
        r.map(rr => {
            i = 0;
            f.v[i] += (rr[i] / rr[2]) * elapsedSeconds * (110 - f.radius) / 110 * scale;
            i++;
            f.v[i] += (rr[i] / rr[2]) * elapsedSeconds * (110 - f.radius) / 110 * scale;
        });

        // Apply drag
        i = 0;
        f.v[i] *= .55 ** elapsedSeconds;
        i++;
        f.v[i] *= .55 ** elapsedSeconds;

        // Update position
        i = 0;
        f.p[i] = (f.p[i] + f.v[i] * elapsedSeconds + size[i]) % size[i];
        i++;
        f.p[i] = (f.p[i] + f.v[i] * elapsedSeconds + size[i]) % size[i];

        // Grow and consume energy
        f.mass = Math.min(maxMass, f.mass + elapsedSeconds * (fi / 5));
        f.energy -= elapsedSeconds / 50;
        f.radius = Math.sqrt(f.mass / Math.PI);

        // Draw fish
        c.save();
        c.translate(f.p[0] - padding, f.p[1] - padding);
        c.scale(f.radius * scale, f.radius * scale);
        c.rotate(-Math.atan2(...f.v));
        c.beginPath();
        c.moveTo(-1.1, -1.5);
        c.bezierCurveTo(1.5, -.8, 1.5, .8, 0, 1.5);
        c.bezierCurveTo(-1.5, .8, -1.5, -.8, 1.1, -1.5);
        c.fillStyle = `hsl(${80 - f.radius},100%,${f.energy * 50}%)`;
        c.fill();
        c.restore();
    });

    requestAnimationFrame(update);
};
update(0);