lib/client: add support for AUTHENTICATE chunking

SASL responses need to be split into 400 byte chunks before being
sent to the server.

lib/irc.js

@@ -1,3 +1,5 @@
+import * as base64 from "./base64.js";
+
 // RFC 1459
 export const RPL_WELCOME = "001";
 export const RPL_YOURHOST = "002";
@@ -942,3 +944,19 @@ export class CapRegistry {
 		return { command: "CAP", params: ["REQ", l.join(" ")] };
 	}
 }
+
+const maxSASLLength = 400;
+
+export function generateAuthenticateMessages(payload) {
+	let encoded = base64.encode(payload);
+
+	// <= instead of < because we need to send a final empty response if the
+	// last chunk is exactly 400 bytes long
+	let msgs = [];
+	for (let i = 0; i <= encoded.length; i += maxSASLLength) {
+		let chunk = encoded.substring(i, i + 400);
+		msgs.push({ command: "AUTHENTICATE", params: [chunk || "+"] });
+	}
+
+	return msgs;
+}