var initPhotoSwipeFromDOM = function(gallerySelector) {
    // parse slide data (url, title, size ...) from DOM elements
    // (children of gallerySelector)
    var parseThumbnailElements = function(el) {
        var thumbElements = el.childNodes, numNodes = thumbElements.length, items = [], figureEl, linkEl, size, item;
        for (var i = 0; i < numNodes; i++) {
            figureEl = thumbElements[ i ]; // <figure> element
            // include only element nodes
            if (figureEl.nodeType !== 1) {
                continue;
            }
            linkEl = figureEl.children[ 0 ]; // <a> element
            size = linkEl.getAttribute('data-size')
                         .split('x');
            // create slide object
            item = {
                src: linkEl.getAttribute('href'),
                w: parseInt(size[ 0 ], 10),
                h: parseInt(size[ 1 ], 10)
            };
            if (figureEl.children.length > 1) {
                // <figcaption> content
                item.title = figureEl.children[ 1 ].innerHTML;
            }
            if (linkEl.children.length > 0) {
                // <img> thumbnail element, retrieving thumbnail url
                item.msrc = linkEl.children[ 0 ].getAttribute('src');
            }
            item.el = figureEl; // save link to element for getThumbBoundsFn
            items.push(item);
        }
        return items;
    };
    // find nearest parent element
    var closest = function closest(el, fn) {
        return el && ( fn(el) ? el : closest(el.parentNode, fn) );
    };
    // triggers when user clicks on thumbnail
    var onThumbnailsClick = function(e) {
        e = e || window.event;
        e.preventDefault ? e.preventDefault() : e.returnValue = false;
        var eTarget = e.target || e.srcElement;
        // find root element of slide
        var clickedListItem = closest(
            eTarget, function(el) {
                return (el.tagName && el.tagName.toUpperCase() === 'FIGURE');
            }
        );
        if (!clickedListItem) {
            return;
        }
        // find index of clicked item by looping through all child nodes
        // alternatively, you may define index via data- attribute
        var clickedGallery = clickedListItem.parentNode, childNodes = clickedListItem.parentNode.childNodes, numChildNodes = childNodes.length, nodeIndex = 0, index;
        for (var i = 0; i < numChildNodes; i++) {
            if (childNodes[ i ].nodeType !== 1) {
                continue;
            }
            if (childNodes[ i ] === clickedListItem) {
                index = nodeIndex;
                break;
            }
            nodeIndex++;
        }
        if (index >= 0) {
            // open PhotoSwipe if valid index found
            openPhotoSwipe(index, clickedGallery);
        }
        return false;
    };
    // parse picture index and gallery index from URL (#&pid=1&gid=2)
    var photoswipeParseHash = function() {
        var hash = window.location.hash.substring(1), params = {};
        if (hash.length < 5) {
            return params;
        }
        var vars = hash.split('&');
        for (var i = 0; i < vars.length; i++) {
            if (!vars[ i ]) {
                continue;
            }
            var pair = vars[ i ].split('=');
            if (pair.length < 2) {
                continue;
            }
            params[ pair[ 0 ] ] = pair[ 1 ];
        }
        if (params.gid) {
            params.gid = parseInt(params.gid, 10);
        }
        return params;
    };
    var openPhotoSwipe = function(index, galleryElement, disableAnimation, fromURL) {
        var pswpElement = document.querySelectorAll('.pswp')[ 0 ], gallery, options, items;
        items = parseThumbnailElements(galleryElement);
        // define options (if needed)
        options = {
            // define gallery index (for URL)
            galleryUID: galleryElement.getAttribute('data-pswp-uid'),
            getThumbBoundsFn: function(index) {
                // See Options -> getThumbBoundsFn section of documentation for more info
                var thumbnail = items[ index ].el.getElementsByTagName('img')[ 0 ], // find thumbnail
                    pageYScroll = window.pageYOffset || document.documentElement.scrollTop, rect = thumbnail.getBoundingClientRect();
                return {
                    x: rect.left,
                    y: rect.top + pageYScroll,
                    w: rect.width
                };
            }
        };
        // PhotoSwipe opened from URL
        if (fromURL) {
            if (options.galleryPIDs) {
                // parse real index when custom PIDs are used
                // http://photoswipe.com/documentation/faq.html#custom-pid-in-url
                for (var j = 0; j < items.length; j++) {
                    if (items[ j ].pid == index) {
                        options.index = j;
                        break;
                    }
                }
            } else {
                // in URL indexes start from 1
                options.index = parseInt(index, 10) - 1;
            }
        } else {
            options.index = parseInt(index, 10);
        }
        // exit if index not found
        if (isNaN(options.index)) {
            return;
        }
        if (disableAnimation) {
            options.showAnimationDuration = 0;
        }
        // Pass data to PhotoSwipe and initialize it
        gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, options);
        gallery.init();
    };
    // loop through all gallery elements and bind events
    var galleryElements = document.querySelectorAll(gallerySelector);
    for (var i = 0, l = galleryElements.length; i < l; i++) {
        galleryElements[ i ].setAttribute('data-pswp-uid', i + 1);
        galleryElements[ i ].onclick = onThumbnailsClick;
    }
    // Parse URL and open gallery if it contains #&pid=3&gid=1
    var hashData = photoswipeParseHash();
    if (hashData.pid && hashData.gid) {
        openPhotoSwipe(hashData.pid, galleryElements[ hashData.gid - 1 ], true, true);
    }
};
// execute above function
initPhotoSwipeFromDOM('.my-gallery');
