import * as crypto from 'crypto';

// Excludes 0/O, 1/I/L to avoid look-alike confusion when a code is
// hand-written, read aloud, or typed from a printed fallback under a QR
// code. 31 symbols * 8 chars ≈ 39.6 bits of entropy - effectively
// unguessable combined with rate-limiting on the redeem endpoint.
const CODE_ALPHABET = 'ABCDEFGHJKMNPQRSTUVWXYZ23456789';
const CODE_LENGTH = 8;

/**
 * Generates a single random code. Uses crypto.randomInt (uniform, no
 * modulo bias) rather than Math.random() since these gate a real-world
 * concert invitation.
 */
export const generateCode = (): string => {
	let code = '';
	for (let i = 0; i < CODE_LENGTH; i++) {
		code += CODE_ALPHABET[crypto.randomInt(CODE_ALPHABET.length)];
	}
	return code;
};

/**
 * Generates a code guaranteed not to collide with any row `existingCodes`
 * already contains, tries up to `maxAttempts` times before giving up.
 * Collisions are astronomically unlikely at this entropy - this exists as
 * a correctness backstop, not because collisions are expected.
 */
export const generateUniqueCode = (existingCodes: Set<string>, maxAttempts = 20): string => {
	for (let i = 0; i < maxAttempts; i++) {
		const candidate = generateCode();
		if (!existingCodes.has(candidate)) {
			existingCodes.add(candidate);
			return candidate;
		}
	}
	throw new Error('Could not generate a unique voucher code after ' + maxAttempts + ' attempts');
};