"use client";

import { useEffect, useRef, useState } from "react";
import { scrollBehavior } from "@/lib/motion";
import { flashElement } from "@/lib/flash";

export interface TocItem {
  level: 2 | 3;
  text: string;
  id: string;
}

/**
 * 文章目录树：右栏 flex 限高列中的可收缩模块。
 * - 标题常驻，目录列表在超高时于面板内部滚动（细滚动条，不顶出侧栏）；
 * - IntersectionObserver 高亮当前小节；
 * - 当前项被滚动到目录可视区外时，列表自动最小化跟随（首屏不滚动、尊重 reduced-motion）。
 */
export default function PostToc({
  items,
  title = "目录",
  bare = false,
}: {
  items: TocItem[];
  title?: string;
  bare?: boolean;
}) {
  const [active, setActive] = useState<string>(items[0]?.id ?? "");
  const navRef = useRef<HTMLElement>(null);
  // 首次挂载时 active 取首项，不触发跟随滚动，避免进页跳动
  const skipFollow = useRef(true);

  useEffect(() => {
    const heads = items
      .map((i) => document.getElementById(i.id))
      .filter((el): el is HTMLElement => Boolean(el));
    if (heads.length === 0) return;
    const obs = new IntersectionObserver(
      (entries) => {
        for (const e of entries) if (e.isIntersecting) setActive(e.target.id);
      },
      // 顶部让出固定头部高度，底部提前切换到下一节
      { rootMargin: "-80px 0px -70% 0px", threshold: 0 }
    );
    heads.forEach((h) => obs.observe(h));
    return () => obs.disconnect();
  }, [items]);

  // active 变化时把当前目录项滚入列表可视区（仅在列表容器内最小滚动）。
  // 不用 scrollIntoView：移动端目录下沉到文末（普通文档流、容器本身不可滚），
  // scrollIntoView(nearest) 会连带滚动窗口，把正在阅读的页面拽到底部目录，
  // 表现为"进页自动下滚 / 无法向上滑动"。
  useEffect(() => {
    if (skipFollow.current) {
      skipFollow.current = false;
      return;
    }
    const list = navRef.current?.querySelector<HTMLElement>(".j13-rail-scroll");
    const el = list?.querySelector<HTMLElement>(`[data-toc-id="${active}"]`);
    if (!list || !el) return;
    const listRect = list.getBoundingClientRect();
    const elRect = el.getBoundingClientRect();
    if (elRect.top < listRect.top) {
      list.scrollBy({ top: elRect.top - listRect.top, behavior: scrollBehavior() });
    } else if (elRect.bottom > listRect.bottom) {
      list.scrollBy({ top: elRect.bottom - listRect.bottom, behavior: scrollBehavior() });
    }
  }, [active]);

  const jump = (id: string) => (e: React.MouseEvent) => {
    e.preventDefault();
    const target = document.getElementById(id);
    if (!target) return;
    target.scrollIntoView({ behavior: scrollBehavior(), block: "start" });
    flashElement(target); // 主题色浅底高亮渐隐，标记落点
    setActive(id);
  };

  return (
    <nav
      ref={navRef}
      aria-label="文章目录"
      className={`${bare ? "py-4" : "panel p-4"} flex min-h-0 flex-col`}
      style={
        bare
          ? {
              background: "var(--panel)",
              borderTop: "1px solid var(--line)",
              borderBottom: "1px solid var(--line)",
            }
          : undefined
      }
    >
      <p
        className="text-[11.5px] font-bold tracking-[0.08em] uppercase mb-2.5 px-1 shrink-0"
        style={{ color: "var(--ink-3)" }}
      >
        {title}
      </p>
      <ul
        className="j13-rail-scroll min-h-0 flex-1 space-y-0.5 pr-0.5"
        style={bare ? undefined : { borderLeft: "1px solid var(--line)" }}
      >
        {items.map((it) => {
          const isActive = it.id === active;
          return (
            <li key={it.id}>
              <a
                data-toc-id={it.id}
                href={`#${it.id}`}
                onClick={jump(it.id)}
                className={`j13-toc-link${it.level === 3 ? " is-h3" : ""}${isActive ? " is-active" : ""}`}
              >
                {it.text}
              </a>
            </li>
          );
        })}
      </ul>
    </nav>
  );
}