for http://habrahabr.ru/post/193642/

pi.js.js

var pi = (function () {
  var NTP = 25;
  var tp = [];

  tp[0] = 1.;
  for (var i = 1; i < NTP; i++) tp[i] = 2. * tp[i-1];

  function series(m, id) {
    var s = 0;
    var eps = 0.00000000000000001;

    /*  Sum the series up to id. */
    for (var k = 0; k < id; k++){
      var ak = 8 * k + m;
      var p = id - k;
      var t = expm(p, ak);
      s = s + t / ak;
      s = s - Math.floor(s);
    }

    /*  Compute a few terms where k >= id. */
    for (var k = id; k <= id + 100; k++){
      var ak = 8 * k + m;
      var t = Math.pow (16, id - k) / ak;
      if (t < eps) break;
      s = s + t;
      s = s - Math.floor(s);
    }

    return s;
  }

  function expm (p, ak) {
    if (ak == 1.) return 0.;

    /*  Find the greatest power of two less than or equal to p. */
    for (var i = 0; i < tp.length; i++) {
      if (tp[i] > p) break;
    }

    var pt = tp[i-1];
    var p1 = p;
    var r = 1.;

    /*  Perform binary exponentiation algorithm modulo ak. */
    for (var j = 1; j <= i; j++){
      if (p1 >= pt){
        r = 16. * r;
        r = r - Math.floor(r / ak) * ak;
        p1 = p1 - pt;
      }
      pt = 0.5 * pt;
      if (pt >= 1.){
        r = r * r;
        r = r - Math.floor(r / ak) * ak;
      }
    }

    return r;
  }

  function ihex(x, nhx, chx) {
    var i, y, hx = "0123456789ABCDEF";

    y = Math.abs(x);
    for (i = 0; i < nhx; i++) {
      y = 16. * (y - (y | 0));
      chx[i] = hx[y | 0];
    }
  }

  function pi(id) {
    var pid, s1, s2, s3, s4
      , hex = [];
    s1 = series(1, id);
    s2 = series(4, id);
    s3 = series(5, id);
    s4 = series(6, id);
    pid = 4 * s1 - 2 * s2 - s3 - s4;
    pid = pid - (pid | 0) + 1;
    ihex(pid, 16, hex);

    return {
      hex:     hex.join('').substr(0, 10),
      fraction:pid
    };
  }

  return pi;
})();