(function () {
    if (window.__LAZY_LOADER_READY__) return;

    const hasIO = "IntersectionObserver" in window;

    // 统一拿 fallback（支持：data-fallback > data-placeholder > window.__LAZY_PLACEHOLDER__ > 当前 src）
    function getFallback(el) {
        return (
            el.getAttribute("data-fallback") ||
            el.getAttribute("data-placeholder") ||
            window.__LAZY_PLACEHOLDER__ ||
            el.getAttribute("src") ||
            ""
        );
    }

    // 背景图预加载（可选：避免 background-image 失败无感知）
    function preloadImage(url, onOk, onFail) {
        const img = new Image();
        img.onload = function () { onOk && onOk(); };
        img.onerror = function () { onFail && onFail(); };
        img.src = url;
    }

    function applyBg(el) {
        const src = el.getAttribute("data-src");
        if (!src) return;

        // 如果你不需要 bg 失败兜底，可把 preloadImage 这段改成直接 set backgroundImage
        preloadImage(
            src,
            function () {
                el.style.backgroundImage = `url(${src})`;
                el.classList.remove("lazyload-bg");
                el.removeAttribute("data-src");
                el.classList.add("is-bg-loaded");
            },
            function () {
                // bg 回退：如果你不给 data-fallback / window.__LAZY_PLACEHOLDER__，这里就不处理
                const fallback = getFallback(el);
                if (fallback) {
                    el.style.backgroundImage = `url(${fallback})`;
                    el.classList.add("is-bg-fallback");
                }
                el.classList.remove("lazyload-bg");
                el.removeAttribute("data-src");
            }
        );
    }

    function applyImg(img) {
        const src = img.getAttribute("data-src");
        const srcset = img.getAttribute("data-srcset");
        const sizes = img.getAttribute("data-sizes");

        if (!src && !srcset) return;

        const fallback = getFallback(img);
        let done = false;

        // 必须在赋值 src/srcset 之前绑定，才能抓到加载失败
        img.onerror = function () {
            if (done) return;
            done = true;

            // 防止 fallback 也报错导致死循环
            img.onerror = null;
            img.onload = null;

            if (fallback) img.setAttribute("src", fallback);
            img.removeAttribute("srcset");
            img.removeAttribute("sizes");

            img.classList.add("is-img-fallback");

            // 清理 lazy 属性，避免重复触发
            img.classList.remove("lazyload-img");
            img.removeAttribute("data-src");
            img.removeAttribute("data-srcset");
            img.removeAttribute("data-sizes");
        };

        img.onload = function () {
            if (done) return;
            done = true;

            img.onerror = null;
            img.onload = null;

            img.classList.add("is-img-loaded");

            // 清理 lazy 属性
            img.classList.remove("lazyload-img");
            img.removeAttribute("data-src");
            img.removeAttribute("data-srcset");
            img.removeAttribute("data-sizes");
        };

        // 先设置响应式属性
        if (sizes) img.setAttribute("sizes", sizes);
        if (srcset) img.setAttribute("srcset", srcset);

        // 最后触发真实加载
        if (src) img.setAttribute("src", src);

        // 注意：这里不立刻 remove lazy class/attrs，等 onload/onerror 决定
        // 否则失败后你很难判断是否已经处理过
    }

    function observe(elements, callback, options) {
        if (!elements || elements.length === 0) return;

        // 兜底：不支持 IO 直接加载
        if (!hasIO) {
            elements.forEach(callback);
            return;
        }

        const obs = new IntersectionObserver((entries, observer) => {
            entries.forEach(entry => {
                if (!entry.isIntersecting) return;
                callback(entry.target);
                observer.unobserve(entry.target);
            });
        }, options);

        elements.forEach(el => obs.observe(el));
    }

    function scan(root) {
        const ctx = root || document;

        observe(
            ctx.querySelectorAll(".lazyload-bg"),
            applyBg,
            { rootMargin: "50px 0px", threshold: 0 }
        );

        observe(
            ctx.querySelectorAll(".lazyload-img"),
            applyImg,
            { rootMargin: "0px 0px", threshold: 0 }
        );
    }

    document.addEventListener("DOMContentLoaded", function () {
        scan(document);
    });

    // 给“加载更多/分页追加 DOM”使用
    window.lazyScan = scan;

    window.__LAZY_LOADER_READY__ = true;
})();