import { parseDocument } from "yaml";

import { assertPageIcon, type PageIcon } from "../notion-as-code/page-icon.js";

export type AppPageContent = {
	content?: string;
	customBlockKey?: string;
	icon?: PageIcon;
};

/** Read APP.md metadata without changing the Markdown body. */
export function parseAppPage(markdown: string): AppPageContent {
	const opening = /^(?:\uFEFF)?---[ \t]*\r?\n/u.exec(markdown);
	if (!opening) {
		return { content: markdown };
	}
	const remainder = markdown.slice(opening[0].length);
	const closing = /^(?:---|\.\.\.)[ \t]*(?:\r?\n|$)/mu.exec(remainder);
	if (!closing) {
		throw new Error("APP.md frontmatter must end with --- or ... on its own line.");
	}

	const content = remainder.slice(closing.index + closing[0].length);
	const metadata = readFrontmatterMetadata(remainder.slice(0, closing.index));
	if (metadata.customBlockKey !== undefined) {
		if (content.trim()) {
			throw new Error("APP.md cannot have body content when custom_block is set.");
		}
		return metadata;
	}
	return { content, ...metadata };
}

function readFrontmatterMetadata(source: string): Omit<AppPageContent, "content"> {
	try {
		const document = parseDocument(source, { schema: "core" });
		const error = document.errors[0] ?? document.warnings[0];
		if (error) {
			throw error;
		}
		// Frontmatter needs no aliases; reject them to prevent expansion of untrusted YAML.
		const metadata: unknown = document.toJS({ maxAliasCount: 0 });
		if (metadata === null) {
			return {};
		}
		if (typeof metadata !== "object" || Array.isArray(metadata)) {
			throw new Error("Metadata must be a YAML mapping.");
		}
		const result: Omit<AppPageContent, "content"> = {};
		if ("custom_block" in metadata) {
			if (typeof metadata.custom_block !== "string" || !metadata.custom_block.trim()) {
				throw new Error("custom_block must be a non-empty string.");
			}
			result.customBlockKey = metadata.custom_block;
		}
		if (!("icon" in metadata)) {
			return result;
		}

		const icon =
			typeof metadata.icon === "string"
				? { type: "emoji", emoji: metadata.icon }
				: metadata.icon;
		assertPageIcon(icon);
		return { ...result, icon };
	} catch (error) {
		throw new Error(
			`Invalid APP.md frontmatter: ${error instanceof Error ? error.message : String(error)}`,
			{ cause: error },
		);
	}
}