boids/boids.mjs

import Vector from './vector.mjs';

const meanPos = new Vector(0, 0);
const meanVel = new Vector(0, 0);
const repelForce = new Vector(0, 0);

export const update = (boids, params, width, height, time) =>
{
    const boidsSize = boids.length;

    for (let i = 0; i < boidsSize; ++i)
    {
        const me = boids[i];

        meanPos.reset();
        meanVel.reset();
        repelForce.reset();

        let visibleSize = 0;

        for (let j = 0; j < boidsSize; ++j)
        {
            if (i != j)
            {
                const you = boids[j];
                const dist = Vector.distSquared(me.pos, you.pos);

                if (dist < params.visibleDist * params.visibleDist)
                {
                    meanPos.add(you.pos);
                    meanVel.add(you.vel);
                    visibleSize += 1;

                    if (dist < params.closeDist * params.closeDist)
                    {
                        repelForce.add(me.pos).sub(you.pos);
                    }
                }
            }
        }

        // Attract towards center of visible flock
        if (visibleSize >= 1)
        {
            me.vel.add(meanPos.div(visibleSize)
                              .sub(me.pos)
                              .mul(params.centerAccel));
        }

        // Attract toward center of screen if out of bounds
        if (me.pos.x < width * -.4 || me.pos.x > width * .4 ||
                me.pos.y < height * -.4 || me.pos.y > height * .4)
        {
            me.vel.addMul(me.pos, -params.boundsAccel);
        }

        // Repel away from close boids
        me.vel.add(repelForce.mul(params.repelAccel));

        // Match other boids’ velocity
        if (visibleSize >= 1)
        {
            me.vel.add(meanVel.div(visibleSize)
                              .sub(me.vel)
                              .mul(params.matchAccel));
        }

        // Do not surpass maximum speed
        const speed = me.vel.normSquared();

        if (speed > params.maxSpeed * params.maxSpeed)
        {
            me.vel.div(Math.sqrt(speed)).mul(params.maxSpeed);
        }

        // Integrate speed
        me.pos.addMul(me.vel, time);
    }
};