Reassemble a concatenated message segmented with the sar_* TLVs (#91)

* Regression tests for inbound sar_* concatenation

* Reassemble a concatenated message segmented with the sar_* TLVs

* Record the sar_* fix in the jasmin and Java-client findings

* Name the sar_* TLVs in the refusal, and separate group keys the addresses cannot forge

* Name the field a refused segment got wrong, and key groups on a separator no address holds

* Bound the new session tests, and derive Concat from the UDH fields

src/concat.ts

@@ -0,0 +1,40 @@
+import type { ConcatInfo } from './udh.ts';
+import type { PduObject } from './pdu.ts';
+import { concatInfo } from './udh.ts';
+import { hasUdh } from './defs/constants.ts';
+import { messageOctets } from './message-body.ts';
+import { paramNumber } from './defs/types.ts';
+
+/** Where a segment sits in its message, and what ties it to the rest of that message. */
+export type Concat = ConcatInfo & {
+	/** Which of the two carried the numbering: their references are counters of their own. */
+	spelling: 'sar' | 'udh';
+};
+
+function sarConcat(pduObj: PduObject): Concat | undefined {
+	const reference = pduObj.tlvs.sar_msg_ref_num?.tagValue;
+	const part = pduObj.tlvs.sar_segment_seqnum?.tagValue;
+	const total = pduObj.tlvs.sar_total_segments?.tagValue;
+
+	if (typeof reference !== 'number' || typeof part !== 'number' || typeof total !== 'number') {
+		return undefined;
+	}
+
+	return { part, reference, spelling: 'sar', total };
+}
+
+/**
+ * How a PDU says it is one segment of a longer message: the UDH its body starts with, or the
+ * sar_* TLVs SMPP 3.4 5.3.2.31-5.3.2.33 define in its place. A UDH that names the concatenation
+ * wins; one carrying only a port or a language indicator leaves the TLVs to say.
+ */
+export function concatOf(pduObj: PduObject): Concat | undefined {
+	const body = messageOctets(pduObj);
+	const udh = body !== undefined && hasUdh(paramNumber(pduObj.params.esm_class, 0))
+		? concatInfo(body)
+		: undefined;
+
+	if (udh) return { ...udh, spelling: 'udh' };
+
+	return sarConcat(pduObj);
+}