/*jslint browser: true, indent: 4 */
(function (g) {
    "use strict";
    var ScrollGuard,
        started = false,
        lock = false,
        timer,
        breakpoints = [],
        getDocHeight,
        getScrollTop;

    // found at http://james.padolsey.com/javascript/
    // get-document-height-cross-browser/
    getDocHeight = function () {
        return Math.max(
            Math.max(document.body.scrollHeight,
                document.documentElement.scrollHeight),
            Math.max(document.body.offsetHeight,
                document.documentElement.offsetHeight),
            Math.max(document.body.clientHeight,
                document.documentElement.clientHeight)
        );
    };

    // found at http://stackoverflow.com/questions/871399/
    // cross-browser-method-for-detecting-the-scrolltop-of-the-browser-g
    getScrollTop = function () {
        var b, d;
        if (g.pageYOffset !== undefined) {
            return g.pageYOffset;
        }

        b = document.body;
        d = document.documentElement;

        d = (d.clientHeight) ? d : b;

        return d.scrollTop;
    };

    ScrollGuard = {
        // from: a distance from top, given in pixels (required)
        // to: a distance from top, given in pixels (optional)
        // 
        // special cases:
        // from = -1 will instead look if bottom of page 
        // is visible.
        add: function (callback, from, to) {
            var range;

            if (from === -1) {
                range = function (topPos) {
                    return getDocHeight() - topPos <= g.innerHeight;
                };
            } else {
                if (to === undefined) {
                    range = function (topPos) {
                        return topPos >= from;
                    };
                } else {
                    range = function (topPos) {
                        return topPos >= from && topPos <= to;
                    };
                }
            }

            breakpoints.push({
                within: range,
                callback: callback
            });
        },
        fire: function (pos) {
            var max = breakpoints.length,
                i,
                bp;

            lock = true;

            for (i = 0; i < max; i += 1) {
                bp = breakpoints[i];

                if (bp.within(pos)) {
                    bp.callback();
                }
            }

            lock = false;
        },
        start: function () {
            if (started) { return; }

            var callback = function () {
                if (lock === false) {
                    clearTimeout(timer);

                    timer = setTimeout(function () {
                        ScrollGuard.fire(getScrollTop());
                    }, 100);
                }
            };

            if (g.attachEvent) {
                g.attachEvent('onscroll', callback);
            } else {
                g.addEventListener('scroll', callback, false);
            }

            started = true;

            // initial call to catch initial scroll positions
            callback();
        }
    };

    g.ScrollGuard = ScrollGuard;
}(this));