import type TurndownService from "turndown";

/** Module-level Turndown instance — built lazily on first use. */
let turndownPromise: Promise<TurndownService> | undefined;

type TurndownListParent = {
	nodeName: string;
	getAttribute(name: string): string | null;
	children: ArrayLike<unknown>;
};

function getTurndown(): Promise<TurndownService> {
	turndownPromise ||= initTurndown();
	return turndownPromise;
}

async function initTurndown(): Promise<TurndownService> {
	const [{ default: TurndownService }, { gfm }] = await Promise.all([
		import("turndown"),
		import("turndown-plugin-gfm"),
	]);
	const turndown = new TurndownService({
		headingStyle: "atx",
		codeBlockStyle: "fenced",
		bulletListMarker: "-",
	});
	turndown.use(gfm);
	turndown.addRule("strikethrough", {
		filter: ["del", "s", "strike"],
		replacement(content) {
			return `~~${content}~~`;
		},
	});
	turndown.addRule("heading", {
		filter: ["h1", "h2", "h3", "h4", "h5", "h6"],
		replacement(content, node) {
			const level = Number(node.nodeName.charAt(1));
			const prefix = "#".repeat(level);
			const cleaned = content.replace(/\\([.])/g, "$1").trim();
			return `\n\n${prefix} ${cleaned}\n\n`;
		},
	});
	turndown.addRule("listItem", {
		filter: "li",
		replacement(content, node, options) {
			content = content.replace(/^\n+/, "").replace(/\n+$/, "\n").replace(/\n/gm, "\n  ");
			const parent = node.parentNode as unknown as TurndownListParent | null;
			let prefix = `${options.bulletListMarker} `;
			if (parent?.nodeName === "OL") {
				const start = parent.getAttribute("start");
				const index = Array.prototype.indexOf.call(parent.children, node);
				prefix = `${(start ? Number(start) : 1) + index}. `;
			}
			return prefix + content + (node.nextSibling ? "\n" : "");
		},
	});
	return turndown;
}

/**
 * Convert HTML to markdown using Turndown with GFM support.
 * Strips script/style tags before conversion.
 */
export async function htmlToBasicMarkdown(html: string): Promise<string> {
	const cleaned = html.replace(/<script[\s\S]*?<\/script>/gi, "").replace(/<style[\s\S]*?<\/style>/gi, "");
	const turndown = await getTurndown();
	return turndown.turndown(cleaned).trim();
}