window.addEventListener('load', function(event) {
  var anchors = document.getElementsByTagName("a");
  for (var i = 0, max = anchors.length; i < max; i++) {
    var anchor = anchors[i];
    var href = anchor.getAttribute("href");
    if (href === null || !href.startsWith("#")) {
      continue;
    }
    anchor.addEventListener("click", function(e) {
      e.preventDefault();
      var href = this.getAttribute("href");
      var name = href.substring(1);
      var target = document.getElementById(name);
      if (!target) {
        target = document.getElementsByTagName("body")[0];
      }
      if (!target) {
        return;
      }

      var targetTop = target.getBoundingClientRect().top;
      var currentTop = document.documentElement.scrollTop;
      var diff = Math.abs(targetTop - currentTop);
      scrollToElem(target, 100 + diff /4);
    });
  }
});

// based on https://gist.github.com/andjosh/6764939
function scrollToElem(target, duration) {
  var start = window.pageYOffset;
  var to = target.getBoundingClientRect().top;
  var change = to - start;
  var currentTime = 0;
  var increment = 20;

  function animateScroll() {
    currentTime += increment;
    var val = easeInOutQuad(currentTime, start, change, duration);
    window.scrollTo(window.pageXOffset, val);
    if (currentTime < duration) {
        setTimeout(animateScroll, increment);
    }
  };
  animateScroll();

  //t = current time
  //b = start value
  //c = change in value
  //d = duration
  function easeInOutQuad(t, b, c, d) {
    t /= d/2;
    if (t < 1) return c/2*t*t + b;
    t--;
    return -c/2 * (t*(t-2) - 1) + b;
  };
}