import type { APIRoute } from "astro";
import { getCollection, getEntry } from "astro:content";
import config from "virtual:starlight/user-config";

export const prerender = true;

type SidebarItem = NonNullable<typeof config.sidebar>[number];
type DocsEntry = Awaited<ReturnType<typeof getCollection<"docs">>>[number];
type Link = { label: string; href: string };

const TRIM_SLASHES = /^\/|\/$/g;
const ABSOLUTE_URL = /^https?:\/\//;

export const GET: APIRoute = async ({ site }) => {
	// Pre-load the docs collection so autogenerated groups can resolve
	// without one `getEntry` call per page.
	const docs = await getCollection("docs");

	const lang = config.defaultLocale.lang;
	const title = (lang ? config.title[lang] : undefined) ?? Object.values(config.title)[0] ?? "";
	const lines: string[] = [`# ${title}`];
	if (config.tagline) {
		lines.push("", `> ${config.tagline}`);
	}

	for (const item of config.sidebar ?? []) {
		const links = await resolveItem(item, docs);
		if (links.length === 0) continue;

		const groupLabel = typeof item !== "string" && "label" in item ? item.label : undefined;
		if (groupLabel && typeof item !== "string" && ("items" in item || "autogenerate" in item)) {
			lines.push("", `## ${groupLabel}`, "");
		} else {
			lines.push("");
		}

		for (const link of links) {
			lines.push(`- [${link.label}](${absUrl(link.href, site)})`);
		}
	}

	return new Response(lines.join("\n") + "\n", {
		headers: { "Content-Type": "text/plain; charset=utf-8" },
	});
};

async function resolveItem(item: SidebarItem, docs: DocsEntry[]): Promise<Link[]> {
	// Shorthand: a string is a slug reference.
	if (typeof item === "string") {
		const entry = getEntry("docs", item);
		return entry ? [entryLink(entry)] : [];
	}

	if ("link" in item && item.link) {
		return [{ label: item.label, href: item.link }];
	}

	if ("slug" in item && item.slug) {
		const entry = await getEntry("docs", item.slug);
		return entry ? [entryLink(entry, item.label)] : [];
	}

	if ("items" in item && item.items) {
		const out: Link[] = [];
		for (const child of item.items) {
			out.push(...(await resolveItem(child, docs)));
		}
		return out;
	}

	if ("autogenerate" in item && item.autogenerate) {
		const prefix = item.autogenerate.directory.replace(TRIM_SLASHES, "") + "/";
		return docs
			.filter((entry) => entry.id.startsWith(prefix))
			.toSorted((a, b) => a.id.localeCompare(b.id))
			.map((entry) => entryLink(entry));
	}

	return [];
}

function entryLink(entry: DocsEntry, override?: string): Link {
	const slug = entry.id === "index" ? "" : entry.id;
	const href = "/" + slug + (slug ? "/" : "");
	const label = override ?? entry.data.sidebar?.label ?? entry.data.title;
	return { label, href };
}

function absUrl(href: string, site: URL | undefined): string {
	if (ABSOLUTE_URL.test(href)) return href;
	if (!site) return href;
	return new URL(href, site).toString();
}