function process_google_sheet(sheet) {
    //processes the data brought in by the google sheet jsonp
    var entries = sheet['feed']['entry'];
    var events = [];
    var years = [];
    for(var i=0; i<entries.length; i++) {
        var entry_date = new Date(entries[i]['gsx$date']['$t']).getTime();
        var now = new Date().getTime();
        //the difference between the event's date and today tells us if it is a future or past event,
        // as well as how far from right now the event begins
        var diff = entry_date - now + (24 * 60 * 60 * 1000);
        var upcoming = diff > 0 ? true : false;
        var diff = Math.abs(diff); //convert that difference to an absolute value. will momentarily sort on this basis
        var entry = entries[i];
        entry['diff'] = diff;
        entry['upcoming'] = upcoming;
        events.push(entry);
    }
    //sorts items based on how far (either into the past or future) from today the event begins.
    events.sort(function(a, b) {return a['diff'] - b['diff']});
    //why? so that past events are sorted most recent first, and upcoming are soonest first
    var html = "";
    //iterate through the events. append them to different sections if they are future or past events
    for (var i=0; i<events.length; i++){
        if (events[i]['upcoming']) {
            html = gen_upcoming_event(events[i]);
            document.getElementById("upcoming-events").innerHTML += html;
        } else {
            html = gen_past_event(events[i]);
            document.getElementById("past-events").innerHTML += html;
            var year = new Date(events[i]['gsx$date']['$t']).getFullYear();
            if($("option#past-event-option-"+year).length < 1) {
                $("select.past-events-dropdown").append("<option value=\""+year+"\" id=\"past-event-option-"+year+"\">"+year+"</option>");
                years.push(year);
            }
        }
    }
    select_event_year({value:(years.length>0?years[0]:new Date().getFullYear())});
}

function gen_upcoming_event(event) {
    //generates the contents of upcoming event items
    var headline = event['gsx$headline']['$t'];
    var href = event['gsx$url']['$t'];
    var image = event['gsx$image']['$t'];
    var description = event['gsx$description']['$t'];
    var date = event['gsx$date']['$t'];
    var text_date = event['gsx$textdate']['$t'];
    var time = event['gsx$time']['$t'];
    var location1 = event['gsx$location1']['$t'];
    var location2 = event['gsx$location2']['$t'];
    var happening = gen_happening_blurb(event['gsx$date']['$t']);

    var html = "<div class=\"event-background-wrapper\"";
    if(image.length>0) {
        html += " style=\"background:url('"+image+"') no-repeat scroll center center #000000;"
            +"background-size: 100% auto;"
            +"filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src=/concepts/images/ct1.png,sizingMethod=scale);"
            +"-ms-filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src=/concepts/images/ct1.png,sizingMethod=scale);\"";
    }
    html += ">";
    html += "<div class=\"event-main-wrapper\">"
        +"<div class=\"event-main\">"
        +"<div class=\"event-date\">"+happening+"</div>"
    +"<h2><a href=\""+href+"\">"+headline+"</a></h2>"
    +"<p class=\"event-blurb\">"+description+"</p>"
    +"</div>"
    +"</div>"
    +"<div class=\"event-detail\">"
        +"<div class=\"event-detail-inner\">"
        +"<p>"+text_date+"</p>"
    +"<div class=\"line\"></div>"
        +"<p>"+time+"</p>"
    +"<div class=\"line\"></div>"
        +"<p>"+location1+"</p>"
    +"<div class=\"line\"></div>"
        +"<p>"+location2+"</p>"
    +"</div>"
    +"</div>"
    +"<div class=\"event-detail-small\">"
        +"<p class=\"small-date\"><a href=\""+href+"\">"+text_date+" / "+time+"</a></p>"
    +"<p class=\"small-location\"><a href=\""+href+"\">"+location1+", "+location2+"</a></p>"
    +"</div>"
    +"</div>";

    return html;
}

function gen_past_event(event) {
    //generates the contents of past event items
    var headline = event['gsx$headline']['$t'];
    var href = event['gsx$url']['$t'];
    var image = event['gsx$image']['$t'];
    var description = event['gsx$description']['$t'];
    var date = event['gsx$date']['$t'];
    var year = new Date(date).getFullYear();
    var text_date = event['gsx$textdate']['$t'];
    var time = event['gsx$time']['$t'];
    var location1 = event['gsx$location1']['$t'];
    var location2 = event['gsx$location2']['$t']; 

    var html = "<div class=\"past-event-background-wrapper "+year+"\" style=\"background-size: auto 100%; background: linear-gradient( rgba(255, 255, 255, 0.7), rgba(255, 255, 255, 0.7) ),url("+image+");\">"
        +"<div class=\"past-event-wrapper\">" 
        +"<h3><a href=\""+href+"\">"+headline+"</a></h3>" 
        +"<p class=\"past-event-info\">"+text_date+" | "+location2+"</p>"
        +"<p class=\"past-event-blurb\">"+description+"</p>"
    +"<div class=\"past-event-info-small\">"
        +"<p>"+text_date+" / "+time+"</p>"
    +"<p>"+location1+", "+location2+"</p>"
    +"</div>"
    +"</div>"
    +"</div>";

    return html;
}

function gen_happening_blurb(date) {
    //date will come as the text of the Date field from the google sheet
    var month_names = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];
    var weekday_names = ['Sunday', 'Monday' , 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
    var return_text = "";
    var date_obj = new Date(date);
    var diff = date_obj.getTime()-(new Date().getTime());
    if (diff>(7*24*60*60*1000)) {
        return_text = "Happening "+month_names[date_obj.getMonth()]+" "+date_obj.getDate();
    } else if (diff<=(7*24*60*60*1000) && diff>(24*60*60*1000)) {
        return_text = "Happening "+weekday_names[date_obj.getDay()];
    } else if (diff<=(24*60*60*1000) && diff>0) {
        return_text = "Happening tomorrow";
    } else if (diff<=0) {
        return_text = "Happening today";
    }

    return return_text;
}

function select_event_year(element) {
    var year = element.value;
    $("div.past-event-background-wrapper").css({display:'none'});
    $("div.past-event-background-wrapper."+year).css({display:'block'});
}