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

    const hasIO = "IntersectionObserver" in window;

    function applyBg(el) {
        const src = el.getAttribute("data-src");
        if (src) el.style.backgroundImage = `url(${src})`;
        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 (sizes) img.setAttribute("sizes", sizes);
        if (srcset) img.setAttribute("srcset", srcset);
        if (src) img.setAttribute("src", src);

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

    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: "400px 0px", threshold: 0.01 }
        );

        observe(
            ctx.querySelectorAll(".lazyload-img"),
            applyImg,
            { rootMargin: "200px 0px", threshold: 0.01 }
        );
    }

    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;
})();