class Particle {
    constructor(x, y) {
        this.mass = 1;
        this.position = createVector(x, y);
        this.velocity = createVector(0, 0);
        this.gravity = createVector(0, 10); // G = 9.81
        this.hue = 0;
        this.saturation = 0;
        this.brightness = 100;
        this.alpha = 1;
        this.size = 2;
        this.lifetime = 0;
        this.endtime = 5;
    }

    lifeover() {
        return this.lifetime >= this.endtime;
    }

    update(force) {
        if (this.lifeover()) return;
        // 计算加速度
        const acceleration = createVector(0, 0);
        acceleration.add(this.gravity);
        if (force) acceleration.add(force.div(this.mass));
        // 计算时间差
        const duration = deltaTime / 1000;
        this.lifetime += duration;
        // 计算速度差及位移
        const deltaVelocity = acceleration.mult(duration);
        const averageVelocity = deltaVelocity.copy().div(2).add(this.velocity);
        const movement = averageVelocity.mult(duration);
        // 更新速度及位置
        this.velocity.add(deltaVelocity);
        this.position.add(movement);
    }

    show() {
        if (this.lifeover()) return;
        noStroke();
        fill(this.hue, this.saturation, this.brightness, this.alpha);
        circle(this.position.x, this.position.y, this.size);
    }
}

class Particles {
    constructor(x, y, n) {
        this.particles = [];
        for (let i = 0; i < n; i++) {
            const p = new Particle(x, y);
            this.particles.push(p);
        }
    }

    lifeover() {
        return this.particles.length === 0;
    }

    update() {
        this.particles.forEach((p, i) => {
            if (p.lifeover()) {
                this.particles.splice(i, 1);
            } else {
                p.update();
            }
        });
    }
    show() {
        this.particles.forEach((p) => {
            p.show();
        });
    }
}