class SimplePendulum extends p5.Vector {
    constructor(x, y) {
        super(x, y);
        this.角度 = 0;
        this.角速度 = 0;
        this.摆长 = 100;
        this.重锤 = createVector(this.x, this.y + this.摆长);
    }

    update(damping = 0) {
        if (mouseIsPressed) {
            this.重锤 = createVector(mouseX, mouseY);
            this.摆长 = this.dist(this.重锤);
            this.角速度 = 0;
            const 向下矢量 = createVector(0, 1);
            this.角度 = p5.Vector.sub(this.重锤, this).angleBetween(向下矢量);
        } else {
            // 未考虑时间精确性及重力加速度
            this.角速度 -= sin(this.角度) / this.摆长; // -sin(Θ)g/L
            this.角速度 = lerp(this.角速度, 0, damping); // 模拟阻尼
            this.角度 += this.角速度;
            this.重锤.x = this.x + this.摆长 * sin(this.角度);
            this.重锤.y = this.y + this.摆长 * cos(this.角度);
        }
    }

    draw() {
        push();
        fill(128);
        stroke(255);
        strokeWeight(1);
        line(this.x, this.y, this.重锤.x, this.重锤.y);
        circle(this.重锤.x, this.重锤.y, 20);
        circle(this.x, this.y, 4);
        pop();
    }
}