d3-geo-polygon/src/adder.js

// Adds floating point numbers with twice the normal precision.
// Reference: J. R. Shewchuk, Adaptive Precision Floating-Point Arithmetic and
// Fast Robust Geometric Predicates, Discrete & Computational Geometry 18(3)
// 305–363 (1997).
// Code adapted from GeographicLib by Charles F. F. Karney,
// http://geographiclib.sourceforge.net/

export default function() {
  return new Adder;
}

function Adder() {
  this.reset();
}

Adder.prototype = {
  constructor: Adder,
  reset: function() {
    this.s = // rounded value
    this.t = 0; // exact error
  },
  add: function(y) {
    add(temp, y, this.t);
    add(this, temp.s, this.s);
    if (this.s) this.t += temp.t;
    else this.s = temp.t;
  },
  valueOf: function() {
    return this.s;
  }
};

var temp = new Adder;

function add(adder, a, b) {
  var x = adder.s = a + b,
      bv = x - a,
      av = x - bv;
  adder.t = (a - av) + (b - bv);
}