nav.js

var amountVisible = function(element) {
  var scroll_top = $(window).scrollTop();
  var scroll_bottom = scroll_top + $(window).height();
  var element_top = element.offset().top;
  var element_bottom = element_top + element.height();
  if (element_bottom > scroll_top && element_bottom < scroll_bottom) {
    return element_bottom - scroll_top;
  } else {
    return scroll_bottom - element_top;
  }
}

var fetchMatchingNavItem = function(section) {
  return $('#' + $(section).attr('id') + '-link').parent();
}

var setCurrentClass = function(item) {
  item.addClass("current");
  item.siblings("#nav li").removeClass("current");
}

var determineCurrentNodeAndSetNav = function() {
  var node = null;
  var elements_in_view = $('.scroll-item:in-viewport');
  if (elements_in_view.get(0).id == 'header') {
    node = elements_in_view.get(0);
  } else {
    elements_in_view.each(function(i){
      var current_element = $(this);
      if (node == null || amountVisible(current_element) >= amountVisible(node)) {
        node = current_element;
      }
    });
  }
  current_index = $('.scroll-item').index(node);
  if (node && node.id == 'header') {
    $('#crushlovely #nav ol li').removeClass('current');
  } else {
    setCurrentClass(fetchMatchingNavItem(node));
  }
}

$(document).ready(function() {
  // Original concept borrowed from Pictory: http://www.pictorymag.com
  var sections = $('.scroll-item');
  var current_index = 0;
  var node;

  $(window).scroll(function () { 
    determineCurrentNodeAndSetNav();
  });

  $('#crushlovely #nav #logo, #coda #pop-back-up').click(function(e){
    $.scrollTo('#header', 800);
    node = $('#header');
    current_index = sections.index(node);
    e.preventDefault();
  });
  
  $('#crushlovely #nav ol li').click(function(e){
    var target = $(this).children('a').get(0).href.split('#')[1];
    $.scrollTo('#' + target, 800);
    node = $('#' + target);
    current_index = sections.index(node);
    e.preventDefault();
  });

  $('#crushlovely .scroll-link').click(function(e){
    var anchor = $(this);
    var target = anchor.attr('href').split('#')[1];
    var nav_item = $($('#' + target + '-link').parent())
    $.scrollTo('#' + target, 800);
    node = $('#' + target);
    current_index = sections.index(node);
    e.preventDefault();
  });

  $(document).keydown(function(e){
    switch(e.keyCode){
      case 39:
        node = sections[++current_index];
        if (node) {
          $.scrollTo(node,800);
        } else {
          current_index = sections.length-1;
        }
        break;
      case 37:
        node = sections[--current_index];
        if (node) {
          $.scrollTo(node,800);
        } else {
          current_index = 0;
        }
        break;
    }
  });
});