canvas {
  border: 1px solid black;
}
interface Numeric<T> {
    plus(other: T): T
    times(scalar: number): T
}

function runSimulation<T extends Numeric<T>>(
  y0: T,
  f: (t: number, y: T) => T,
  applyConstraints: (y: T) => T,
  iterationsPerFrame: number,
  render: (y: T) => void
) {
    const frameTime = 1 / 60.0
    const h = frameTime / iterationsPerFrame

    function simulationStep(yi: T, ti: number) {
        render(yi)
        requestAnimationFrame(function () {
            for (let i = 0; i < iterationsPerFrame; i++) {
                yi = yi.plus(f(ti, yi).times(h))
                yi = applyConstraints(yi)
                ti = ti + h
            }
            simulationStep(yi, ti)
        })
    }
    simulationStep(y0, 0.0)
}

const canvas = document.createElement("canvas")
canvas.width = 400;
canvas.height = 400;
const ctx = canvas.getContext("2d")!;
document.body.appendChild(canvas);
ctx.fillStyle = "rgba(0, 0, 0, 0, 1)"
ctx.fillRect(0, 0, 400, 400);

const g = -9.8; // m / s^2
const r = 0.2; // m

class Ball implements Numeric<Ball> {
    constructor(readonly x: number, readonly v: number) { }
    plus(other: Ball) { return new Ball(this.x + other.x, this.v + other.v) }
    times(scalar: number) { return new Ball(this.x * scalar, this.v * scalar) }
}

function f(t: number, y: Ball) {
    const { x, v } = y
    return new Ball(v, g)
}

function applyConstraints(y: Ball): Ball {
    const { x, v } = y
    if (x <= 0 && v < 0) {
        return new Ball(x, -v)
    }
    return y
}

const y0 = new Ball(
    /* x */ 0.8,
    /* v */ 0
)

function render(y: Ball) {
    ctx.clearRect(0, 0, 400, 400)
    ctx.fillStyle = '#EB5757'
    ctx.beginPath()
    ctx.ellipse(200, 400 - ((y.x + r) * 300), r * 300, r * 300, 0, 0, 2 * Math.PI)
    ctx.fill()
}

runSimulation(y0, f, applyConstraints, 30, render)
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