vitaliycto

очень грубо конечно но..:

var banners = [
    {
        url: 'a.jpg',
 
        chance: 100,
        daysWeek: '1,3,4,7'
    },
    {
        url: 'b',
        chance: 50,
        daysWeek: '1,3,4,7'
    },
    {
        url: 'c',
        chance: 30,
        daysWeek: '1,3,4,7'
    }
];
var rotoBann = function (banners, targetId, newoptions) {
    if (typeof banners == 'undefined' || banners.length == 0)
        return false;
    var options = {
        timeout: 1000,
        number: 2, //колличество выбираемых баннеров. 0 - без ограничений,
        random: true
    };
    newoptions = newoptions || {};
    for (var i in options)
        options[i] = newoptions[i] || options[i];
    var currentDayWeek = (new Date()).getDay();
    var biggestChance = 0;
    var filteredByDays = [];
    for (var i in banners) {
        var daysWeek = banners[i].daysWeek.split(',');
        for (var i2 in daysWeek) {
            if (daysWeek[i2] == currentDayWeek) {
                filteredByDays.push(banners[i]);
                break;
            }
        }
    }
    for (var i in filteredByDays) {
        biggestChance = (filteredByDays[i].chance > biggestChance) ? filteredByDays[i].chance : biggestChance;
    }
    filteredByDays.sort(function (a, b) {
        return  b.chance - a.chance
    });
    var selectedBanners = [];
    for (var i in filteredByDays) {
        if (options.number > 0 && selectedBanners.length == options.number)
            break;
        if (options.number == 0 || options.number >= filteredByDays.length || filteredByDays[i].chance == biggestChance) {
            selectedBanners.push(filteredByDays[i].url);
            continue;
        }

        if (Math.round(Math.random()) * 100 < filteredByDays[i].chance / biggestChance)
            selectedBanners.push(filteredByDays[i].url);
    }
    if (options.random)
        selectedBanners.sort(function () {
            return Math.round(Math.random());
        });
    var cS = 0;
    setInterval(function () {
        document.getElementById(targetId).innerHTML = '<img src="'+selectedBanners[cS]+'"/>';
        cS = (cS == selectedBanners.length - 1) ? 0 : cS + 1;
    }, options.timeout);
}
new rotoBann(banners, 'test');

лучше отделить логику выборки от отображения