/* eslint-env browser */

/**
 * Infinite Scroll
 * Used on all pages where there is a list of posts (homepage, tag index, etc).
 *
 * When the page is scrolled to 300px from the bottom, the next page of posts
 * is fetched by following the the <link rel="next" href="..."> that is output
 * by {{ghost_head}}.
 *
 * The individual post items are extracted from the fetched pages by looking for
 * a wrapper element with the class "post-card". Any found elements are appended
 * to the element with the class "post-feed" in the currently viewed page.
 */

(function (window, document) {
    // next link element
    var nextElement = document.querySelector('link[rel=next]');
    if (!nextElement) {
        return;
    }

    // post feed element
    var feedElement = document.querySelector('.post-feed');
    if (!feedElement) {
        return;
    }

    // disable infinite scroll on home page
    var homePage = document.querySelector('body.home-template')
    if (!!homePage) {
        return;
    }

    var buffer = 300;

    var ticking = false;
    var loading = false;

    var lastScrollY = window.scrollY;
    var lastWindowHeight = window.innerHeight;
    var lastDocumentHeight = document.documentElement.scrollHeight;

    function onPageLoad() {
        if (this.status === 404) {
            window.removeEventListener('scroll', onScroll);
            window.removeEventListener('resize', onResize);
            return;
        }

        // append contents
        var postElements = this.response.querySelectorAll('article.post-card');
        postElements.forEach(function (item) {
            // document.importNode is important, without it the item's owner
            // document will be different which can break resizing of
            // `object-fit: cover` images in Safari
            feedElement.appendChild(document.importNode(item, true));
        });

        // set next link
        var resNextElement = this.response.querySelector('link[rel=next]');
        if (resNextElement) {
            nextElement.href = resNextElement.href;
        } else {
            window.removeEventListener('scroll', onScroll);
            window.removeEventListener('resize', onResize);
        }

        // sync status
        lastDocumentHeight = document.documentElement.scrollHeight;
        ticking = false;
        loading = false;
    }

    function onUpdate() {
        // return if already loading
        if (loading) {
            return;
        }

        // return if not scroll to the bottom
        if (lastScrollY + lastWindowHeight <= lastDocumentHeight - buffer) {
            ticking = false;
            return;
        }

        loading = true;

        var xhr = new window.XMLHttpRequest();
        xhr.responseType = 'document';

        xhr.addEventListener('load', onPageLoad);

        xhr.open('GET', nextElement.href);
        xhr.send(null);
    }

    function requestTick() {
        ticking || window.requestAnimationFrame(onUpdate);
        ticking = true;
    }

    function onScroll() {
        lastScrollY = window.scrollY;
        requestTick();
    }

    function onResize() {
        lastWindowHeight = window.innerHeight;
        lastDocumentHeight = document.documentElement.scrollHeight;
        requestTick();
    }

    window.addEventListener('scroll', onScroll, {passive: true});
    window.addEventListener('resize', onResize);

    requestTick();
})(window, document);