Add Tickets domain for the voucher-based ticket shop (#8)

Reviewed-on: #8
Co-authored-by: Patrick Mueller <patrick@mueller-patrick.tech>
Co-committed-by: Patrick Mueller <patrick@mueller-patrick.tech>

src/models/tickets/tickets.codes.ts

@@ -0,0 +1,38 @@
+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');
+};