Add session, client and server with the promise result API

src/udh.ts

@@ -0,0 +1,42 @@
+export type ConcatInfo = {
+	part: number;
+	reference: number;
+	total: number;
+};
+
+/**
+ * Finds the concatenation information element in a User Data Header, walking the elements rather
+ * than assuming the header holds nothing else — real SMSCs put ports, language indicators and more
+ * alongside it.
+ *
+ * `message` is a short_message whose first octet is the UDH length.
+ */
+export function concatInfo(message: Buffer): ConcatInfo | undefined {
+	const udhLength = message[0];
+
+	if (udhLength === undefined) return undefined;
+
+	const end = Math.min(udhLength + 1, message.length);
+	let offset = 1;
+
+	while (offset + 2 <= end) {
+		const iei = message.readUInt8(offset);
+		const ieLength = message.readUInt8(offset + 1);
+		const data = message.subarray(offset + 2, offset + 2 + ieLength);
+
+		if (offset + 2 + ieLength > end) return undefined;
+
+		// 0x00 is an 8-bit concatenation reference, 0x08 a 16-bit one.
+		if (iei === 0x00 && ieLength === 3) {
+			return { part: data.readUInt8(2), reference: data.readUInt8(0), total: data.readUInt8(1) };
+		}
+
+		if (iei === 0x08 && ieLength === 4) {
+			return { part: data.readUInt8(3), reference: data.readUInt16BE(0), total: data.readUInt8(2) };
+		}
+
+		offset += 2 + ieLength;
+	}
+
+	return undefined;
+}