var currentSlide = 0;

function doSlideshow() {
    slideshow(1);
    setTimeout(doSlideshow, 5000);
}

function slideshow(advance) {
    currentSlide += advance;

    if (typeof(slides) != "undefined") {

        if (currentSlide < 0) currentSlide = slides.length - 1;
        else if (currentSlide >= slides.length) currentSlide = 0;
        var img = document.getElementById(currentSlideId);
        img.src = slides[currentSlide];
    }
    preloadImage(currentSlide);
}

function preloadImage(current) {
    var img = new Image();
    var next = currentSlide + 1;
    var prev = currentSlide - 1;

    if (typeof (slides) != "undefined") {
        if (next >= slides.length) next = 0;
        if (prev < 0) prev = slides.length - 1;

        img.src = slides[next];
        img.src = slides[prev];
    }
}