"use client";

import { useEffect, useMemo, useState } from "react";
import Link from "next/link";
import { ChevronDown, List } from "lucide-react";
import {
  Collapsible,
  CollapsibleContent,
  CollapsibleTrigger,
} from "@/components/ui/collapsible";
import { ScrollArea } from "@/components/ui/scroll-area";
import { cn } from "@/lib/utils";
import type { TocItem } from "@/lib/blog-utils";

interface TocGroup {
  heading: TocItem;
  children: TocItem[];
}

function groupByLevel2(items: TocItem[]): TocGroup[] {
  const groups: TocGroup[] = [];
  let current: TocGroup | null = null;
  for (const item of items) {
    if (item.level === 2) {
      current = { heading: item, children: [] };
      groups.push(current);
    } else if (current) {
      current.children.push(item);
    }
  }
  return groups;
}

interface BlogTocProps {
  items: TocItem[];
  className?: string;
}

const tocLinkClass = (
  level: number,
  isActive: boolean
) =>
  cn(
    "block rounded-md border-r-2 rounded-s-none border-transparent py-1.5 text-sm transition-colors hover:text-foreground hover:bg-muted/60",
    level === 2 && "pr-3",
    level === 3 && "pr-5",
    level === 4 && "pr-7",
    isActive
      ? "font-medium text-foreground border-primary bg-muted/50"
      : "text-muted-foreground"
  );

export function BlogToc({ items, className }: BlogTocProps) {
  const [activeId, setActiveId] = useState<string | null>(items[0]?.id ?? null);
  const [mobileOpen, setMobileOpen] = useState(false);
  const [openSections, setOpenSections] = useState<Set<string>>(() => new Set());

  const groups = useMemo(() => groupByLevel2(items), [items]);

  useEffect(() => {
    setOpenSections(new Set(groups.map((g) => g.heading.id)));
  }, [groups]);

  useEffect(() => {
    if (items.length === 0) return;

    const observer = new IntersectionObserver(
      (entries) => {
        for (const entry of entries) {
          if (entry.isIntersecting) {
            const id = entry.target.getAttribute("id");
            if (id) setActiveId(id);
          }
        }
      },
      { rootMargin: "-80px 0% -70% 0%", threshold: 0 }
    );

    items.forEach(({ id }) => {
      const el = document.getElementById(id);
      if (el) observer.observe(el);
    });

    return () => observer.disconnect();
  }, [items]);

  useEffect(() => {
    if (activeId && groups.length > 0) {
      const group = groups.find(
        (g) => g.heading.id === activeId || g.children.some((c) => c.id === activeId)
      );
      if (group) {
        setOpenSections((prev) => new Set(prev).add(group.heading.id));
      }
    }
  }, [activeId, groups]);

  if (items.length === 0) return null;

  const renderTocLink = (item: TocItem, onNav?: () => void) => (
    <Link
      key={item.id}
      href={`#${item.id}`}
      onClick={() => {
        setMobileOpen(false);
        onNav?.();
      }}
      className={tocLinkClass(item.level, activeId === item.id)}
    >
      <span className="line-clamp-2">{item.text}</span>
    </Link>
  );

  const tocList = (
    <nav aria-label="فهرست مطالب" className="space-y-1">
      {groups.map(({ heading, children }) => {
        const hasChildren = children.length > 0;
        const isOpen = openSections.has(heading.id);

        if (!hasChildren) {
          return renderTocLink(heading);
        }

        return (
          <Collapsible
            key={heading.id}
            open={isOpen}
            onOpenChange={(open) => {
              setOpenSections((prev) => {
                const next = new Set(prev);
                if (open) next.add(heading.id);
                else next.delete(heading.id);
                return next;
              });
            }}
          >
            <div
              className={cn(
                "flex items-center gap-0.5 rounded-md border-r-2 rounded-s-none py-1.5 pr-3",
                activeId === heading.id ? "border-primary bg-muted/50" : "border-transparent"
              )}
            >
              <Link
                href={`#${heading.id}`}
                onClick={() => setMobileOpen(false)}
                className={cn(
                  "min-w-0 flex-1 text-sm transition-colors hover:text-foreground hover:bg-muted/60 rounded-md py-0.5 -my-0.5",
                  activeId === heading.id ? "font-medium text-foreground" : "text-muted-foreground"
                )}
              >
                <span className="line-clamp-2">{heading.text}</span>
              </Link>
              <CollapsibleTrigger
                className="shrink-0 rounded p-1 text-muted-foreground hover:bg-muted/60 hover:text-foreground transition-colors"
                aria-label={isOpen ? "بستن" : "باز کردن"}
              >
                <ChevronDown
                  className={cn("size-4 transition-transform", isOpen && "rotate-180")}
                />
              </CollapsibleTrigger>
            </div>
            <CollapsibleContent>
              <div className="space-y-1 pt-0.5 pr-2 border-r border-border/50">
                {children.map((item) => renderTocLink(item))}
              </div>
            </CollapsibleContent>
          </Collapsible>
        );
      })}
    </nav>
  );

  return (
    <>
      {/* Desktop: sticky sidebar left */}
      <aside
        className={cn(
          "hidden lg:block shrink-0",
          "sticky top-24 self-start max-h-[calc(100vh-6rem)]",
          className
        )}
      >
        <p className="mb-3 text-xs font-semibold uppercase tracking-wider text-muted-foreground">
          فهرست مطالب
        </p>
        <ScrollArea className="h-full pb-8" dir="rtl">
          {tocList}
        </ScrollArea>
      </aside>

      {/* Mobile: collapsible, sticky when scrolling */}
      <Collapsible
        open={mobileOpen}
        onOpenChange={setMobileOpen}
        className={cn(
          "lg:hidden w-full sticky top-20 z-10 bg-background",
          className
        )}
      >
        <CollapsibleTrigger
          className={cn(
            "flex w-full items-center justify-between gap-2 rounded-lg border border-border bg-muted/30 px-4 py-3 text-sm font-medium transition-colors hover:bg-muted/50",
            mobileOpen && "rounded-b-none border-b-0"
          )}
        >
          <span className="flex items-center gap-2">
            <List className="size-4 text-muted-foreground" />
            فهرست مطالب
          </span>
          <ChevronDown
            className={cn("size-4 text-muted-foreground transition-transform", mobileOpen && "rotate-180")}
          />
        </CollapsibleTrigger>
        <CollapsibleContent>
          <div className="max-h-64 overflow-y-auto rounded-b-lg border border-t-0 border-border bg-muted/20 p-3">
            {tocList}
          </div>
        </CollapsibleContent>
      </Collapsible>
    </>
  );
}