/* globals */
var today = new Date();
var thisYear = today.getFullYear();
var logoDir = '';
var logoExt = '.gif';

/* getting Easter date */
function getEaster(Y) {
    var C = Math.floor(Y/100);
    var N = Y - 19*Math.floor(Y/19);
    var K = Math.floor((C - 17)/25);
    var I = C - Math.floor(C/4) - Math.floor((C - K)/3) + 19*N + 15;
    I = I - 30*Math.floor((I/30));
    I = I - Math.floor(I/28)*(1 - Math.floor(I/28)*Math.floor(29/(I + 1))*Math.floor((21 - N)/11));
    var J = Y + Math.floor(Y/4) + I + 2 - C + Math.floor(C/4);
    J = J - 7*Math.floor(J/7);
    var L = I - J;
    var M = 3 + Math.floor((L + 40)/44);
    var D = L + 28 - 31*Math.floor(M/4);

    return new Date( Y, (M-1), D );
}

/* dates */
var newYear = new Date(thisYear, 0, 1);
var stValentine = new Date(thisYear, 1, 14);
var easter = getEaster(thisYear);
var christmas = new Date(thisYear, 11, 25);

var holidays = [
    {
        name: 'New Year',
        fname: 'newyear',
        date: newYear,
        before: 3,
        after: 3
    },
    {
        name: 'St. Valentine\'s Day',
        fname: 'valentine',
        date: stValentine,
        before: 13,
        after: 1
    },
    {
        name: 'Easter Sunday',
        fname: 'easter',
        date: easter,
        before: 13,
        after: 1
    },
    {
        name: 'Christmas',
        fname: 'christmas',
        date: christmas,
        before: 24,
        after: 4
    }
]

function switchLogo() {
    var logo = document.getElementById('logo');
    var day = 24 * 60 * 60 * 1000;
    var now = today.getTime();

    for (var i in holidays) {
        var min = holidays[i].date.getTime() - holidays[i].before*day;
        var max = holidays[i].date.getTime() + holidays[i].after*day;

        if (holidays[i].date != newYear) {
            if ( (min < now) && (now < max) ) {
                logo.src = logoDir +'logo_'+ holidays[i].fname + logoExt;
            }
        } else {
            min = ( new Date(min) ).setFullYear(thisYear);
            if ( ( (newYear.getTime() < now) && (now < max) ) || (now > min) ) {
                logo.src = logoDir +'logo_'+ holidays[i].fname + logoExt;
            }
        }
    }
}