dgate 2.1.0

export default function(polygon) {
  var i = -1,
      n = polygon.length,
      b = polygon[n - 1],
      xa,
      ya,
      xb = b[0],
      yb = b[1],
      perimeter = 0;

  while (++i < n) {
    xa = xb;
    ya = yb;
    b = polygon[i];
    xb = b[0];
    yb = b[1];
    xa -= xb;
    ya -= yb;
    perimeter += Math.hypot(xa, ya);
  }

  return perimeter;
}