Calculate pi

pi.js

// See "Unbounded Spigot Algorithms for the Digits of Pi," by Jeremy Gibbons,
// Math. Monthly, April 2006, pages 318-328.
// Adapted from https://gustavus.edu/mcs/max/pi/

let app;

let q = BigInt(1);
let r = BigInt(0);
let t = BigInt(1);
let k = BigInt(1);
let n = BigInt(3);

function piDigit() {
  while (true) {
    if (q * BigInt(4) + r - t < n * t) {
      app.textContent += n;

      if (k === BigInt(2)) {
        app.textContent += ".";
      }

      const oldQ = q;
      const oldR = r;
      q *= BigInt(10);
      r = BigInt(10) * (r - n * t);
      n = (BigInt(10) * (BigInt(3) * oldQ + oldR)) / t - BigInt(10) * n;
      break;
    } else {
      const l = BigInt(2) * k + BigInt(1);
      const oldQ = q;
      const oldR = r;
      const oldT = t;
      const oldK = k;
      q *= k;
      r = (BigInt(2) * oldQ + r) * l;
      t *= l;
      k += BigInt(1);
      n = (oldQ * (BigInt(7) * oldK + BigInt(2)) + oldR * l) / (oldT * l);
    }
  }

  requestAnimationFrame(piDigit);
}

function startPi() {
  app = document.createElement("div");
  app.setAttribute("style", "word-wrap: break-word;");
  document.body.appendChild(app);

  requestAnimationFrame(piDigit);
}

startPi();