spsays-training/app/books/kb/[volume]/[slug]/page.tsx

import Link from "next/link";
import { notFound } from "next/navigation";
import { getProseChapter, getProseChapters } from "@/lib/api";
import { PageHeader } from "@/components/page-header";
import { Card, CardContent } from "@/components/ui/card";
import { buttonVariants } from "@/components/ui/button";
import { ChevronLeft, ChevronRight } from "lucide-react";
import { cn } from "@/lib/utils";

export default async function KbChapterPage({
  params,
}: {
  params: Promise<{ volume: string; slug: string }>;
}) {
  const { volume, slug } = await params;
  const vol = parseInt(volume);
  if (isNaN(vol) || vol < 1 || vol > 2) notFound();

  const chapter = await getProseChapter("kb", vol, slug);
  if (!chapter) notFound();

  // Fetch all chapters/segments in this volume for sidebar + prev/next
  const allChapters = await getProseChapters("kb", vol);
  const currentIdx = allChapters.findIndex((ch) => ch.slug === slug);

  const prev = currentIdx > 0 ? allChapters[currentIdx - 1] : null;
  const next =
    currentIdx < allChapters.length - 1 ? allChapters[currentIdx + 1] : null;

  const chapterLabel =
    chapter.type === "chapter" && chapter.chapter != null
      ? `Chapter ${chapter.chapter}`
      : null;

  return (
    <div className="min-h-screen bg-background">
      <PageHeader sticky wide>
        <Link
          href={`/books/kb/${vol}`}
          className="text-sm text-muted-foreground hover:text-foreground transition-colors"
        >
          &larr; Volume {vol}
        </Link>
        <h1 className="mt-3 text-2xl font-bold text-foreground">
          {chapterLabel && (
            <span className="text-primary">{chapterLabel}: </span>
          )}
          {chapter.title}
        </h1>
      </PageHeader>

      <div className="w-full md:max-w-6xl mx-auto flex">
        {/* Left sidebar — chapter index */}
        <ChapterSidebar
          chapters={allChapters}
          currentSlug={slug}
          volume={vol}
        />

        {/* Main content */}
        <main className="flex-1 min-w-0 px-6 py-8 space-y-8">
          {/* Top navigation */}
          <ChapterNav prev={prev} next={next} volume={vol} />

          {/* Chapter content */}
          <Card>
            <CardContent className="py-8 px-6 sm:px-8">
              <div className="space-y-4">
                {chapter.paragraphs.map((paragraph, i) => (
                  <div
                    key={i}
                    className="text-foreground leading-relaxed"
                    dangerouslySetInnerHTML={{ __html: paragraph }}
                  />
                ))}
              </div>
            </CardContent>
          </Card>

          {/* Bottom navigation */}
          <ChapterNav prev={prev} next={next} volume={vol} />
        </main>
      </div>
    </div>
  );
}

// ---------------------------------------------------------------------------
// Sidebar — chapter index
// ---------------------------------------------------------------------------

type SidebarChapter = {
  slug: string;
  title: string;
  type: string;
  chapter: number | null;
};

function ChapterSidebar({
  chapters,
  currentSlug,
  volume,
}: {
  chapters: SidebarChapter[];
  currentSlug: string;
  volume: number;
}) {
  return (
    <aside className="hidden lg:block w-56 shrink-0 sticky top-[89px] self-start max-h-[calc(100vh-89px)] overflow-y-auto py-8 pr-4">
      <h2 className="text-xs font-bold uppercase tracking-wider text-muted-foreground mb-3 px-2">
        Chapters
      </h2>
      <nav className="flex flex-col gap-0.5">
        {chapters.map((ch) => {
          const isActive = ch.slug === currentSlug;
          const label =
            ch.type === "chapter" && ch.chapter != null
              ? `${ch.chapter}. ${ch.title}`
              : ch.title;

          return (
            <Link
              key={ch.slug}
              href={`/books/kb/${volume}/${ch.slug}`}
              className={cn(
                "px-2 py-1.5 rounded-md text-sm transition-colors truncate",
                isActive
                  ? "bg-primary/10 text-primary font-medium"
                  : "text-muted-foreground hover:bg-muted hover:text-foreground",
              )}
              title={label}
            >
              {label}
            </Link>
          );
        })}
      </nav>
    </aside>
  );
}

// ---------------------------------------------------------------------------
// Prev / Next navigation
// ---------------------------------------------------------------------------

type NavChapter = {
  slug: string;
  title: string;
  type: string;
  chapter: number | null;
};

function ChapterNav({
  prev,
  next,
  volume,
}: {
  prev: NavChapter | null;
  next: NavChapter | null;
  volume: number;
}) {
  return (
    <div className="flex items-center justify-between gap-3">
      {prev ? (
        <Link
          href={`/books/kb/${volume}/${prev.slug}`}
          className={cn(
            buttonVariants({ variant: "outline", size: "sm" }),
            "gap-1.5",
          )}
        >
          <ChevronLeft className="size-4" />
          <span className="hidden sm:inline truncate max-w-[180px]">
            {prev.type === "chapter" && prev.chapter != null
              ? `Ch. ${prev.chapter}`
              : prev.title}
          </span>
          <span className="sm:hidden">Previous</span>
        </Link>
      ) : (
        <div />
      )}

      {next ? (
        <Link
          href={`/books/kb/${volume}/${next.slug}`}
          className={cn(
            buttonVariants({ variant: "outline", size: "sm" }),
            "gap-1.5",
          )}
        >
          <span className="hidden sm:inline truncate max-w-[180px]">
            {next.type === "chapter" && next.chapter != null
              ? `Ch. ${next.chapter}`
              : next.title}
          </span>
          <span className="sm:hidden">Next</span>
          <ChevronRight className="size-4" />
        </Link>
      ) : (
        <div />
      )}
    </div>
  );
}