class Particle {
    constructor(x, y) {
        this.position = createVector(x, y);
        this.velocity = createVector(0, 0);
        this.gravity = createVector(0, 10); // G = 9.81
        this.acceleration = createVector(0, 0);
        this.duration = 0;
        this.life = -1;
        this.mass = 1;
    }

    get 高度() {
        return height - this.position.y;
    }

    get 势能() {
        return this.mass * this.gravity.y * this.高度;
    }

    get 速率() {
        return this.velocity.mag();
    }

    get 动能() {
        return 0.5 * this.mass * this.速率 ** 2;
    }

    get 机械能() {
        return this.势能 + this.动能;
    }

    get 运动方向() {
        return this.velocity.heading();
    }

    get finished() {
        return this.life > 0 && this.duration >= this.life;
    }

    applyForce(force) {
        this.acceleration.add(force.div(this.mass));
    }

    // 反弹算法未完成
    move(bounce = false) {
        // 重力加速度
        this.acceleration.add(this.gravity); // G = 9.81
        // 计算时间差
        const duration = deltaTime / 1000; // 秒
        this.duration += duration;
        // 计算速度差及位移
        const deltaVelocity = this.acceleration.mult(duration);
        const averageVelocity = deltaVelocity.copy().div(2).add(this.velocity);
        const displacement = averageVelocity.mult(duration);
        // 更新速度及位置
        this.velocity.add(deltaVelocity);
        this.position.add(displacement);
    }

    // 未完成
    bounce(rate) {
        if (this.position.x <= 0) {
            this.position.x = 0;
            this.velocity.x *= -1;
            if (rate > 0) this.velocity.mult(rate);
        } else if (this.position.x > width) {
            this.position.x = width;
            this.velocity.x *= -1;
            if (rate > 0) this.velocity.mult(rate);
        }
        if (this.position.y <= 0) {
            this.position.y = 0;
            this.velocity.y *= -1;
            if (rate > 0) this.velocity.mult(rate);
        } else if (this.position.y >= height) {
            this.position.y = height;
            this.velocity.y *= -1;
            if (rate > 0) this.velocity.mult(rate);
        }
    }
}