import { timingSafeEqual } from 'node:crypto';
import type { NextFunction, Request, Response } from 'express';

/**
 * Bearer-token gate for the public endpoint.
 *
 * This server is reachable from the internet by construction — Claude's
 * connectors call it from Anthropic's cloud, not from the user's machine — so
 * the token is the only thing between a stranger and the account's data.
 * Comparison is constant-time, and a miss returns a bare 401 with a
 * `WWW-Authenticate` challenge and no detail about why.
 */
export function bearerAuth(expected: string) {
	const expectedBytes = Buffer.from(expected, 'utf8');

	return function authenticate(req: Request, res: Response, next: NextFunction): void {
		const presented = extractToken(req.get('authorization'), req.get('x-api-key'));
		if (presented === undefined || !constantTimeEquals(Buffer.from(presented, 'utf8'), expectedBytes)) {
			res.setHeader('WWW-Authenticate', 'Bearer realm="schulcloud-mcp"');
			res.status(401).json({
				jsonrpc: '2.0',
				error: { code: -32001, message: 'Unauthorized' },
				id: null,
			});
			return;
		}
		next();
	};
}

function extractToken(authorization: string | undefined, apiKey: string | undefined): string | undefined {
	if (authorization) {
		const match = /^Bearer\s+(.+)$/i.exec(authorization.trim());
		if (match?.[1]) return match[1].trim();
	}
	// Some connector UIs only offer a custom header rather than Authorization.
	return apiKey?.trim() || undefined;
}

function constantTimeEquals(a: Buffer, b: Buffer): boolean {
	// timingSafeEqual throws on length mismatch, which would itself leak length.
	// Hash-free equalisation: compare against a padded copy of the same size.
	if (a.length !== b.length) {
		const padded = Buffer.alloc(b.length);
		a.copy(padded, 0, 0, Math.min(a.length, b.length));
		timingSafeEqual(padded, b);
		return false;
	}
	return timingSafeEqual(a, b);
}