Read a delivery receipt's body as the octets that arrived (#81)

* Regression tests for a receipt body read under a data_coding it is not written in

* Read a delivery receipt's body as the octets that arrived, not by its data_coding

* Assert SMPPSim's UCS2 receipt body parses like every other segment's

* Record the receipt-body defect as fixed

* Retain a multipart segment's octets once, not twice

* Sort the new type import into its file's order

src/pdu-refusal.ts

@@ -0,0 +1,57 @@
+import type { CommandName } from './defs/commands.ts';
+import type { ErrorName } from './defs/errors.ts';
+import { respNameFor } from './defs/commands.ts';
+
+/** A hostile peer must not be able to make us allocate arbitrarily. */
+export const maxPduLength = 1024 * 1024;
+
+/** The 16 octets a framed PDU always has, whatever its body turns out to be. */
+export type PduHeader = {
+	cmdId: number;
+	cmdLength: number;
+	cmdName: CommandName | undefined;
+	cmdStatusId: number;
+	seqNr: number;
+};
+
+/** Which part of a PDU the codec could not read. */
+export type PduRefusalReason = 'body' | 'command' | 'tlvs';
+
+/** A PDU refused with the stream still in sync, so only this one PDU is lost. */
+export class PduRefusedError extends Error {
+	readonly header: PduHeader;
+	readonly reason: PduRefusalReason;
+
+	constructor(header: PduHeader, reason: PduRefusalReason, cause: Error) {
+		const named = header.cmdName ?? `command id ${String(header.cmdId)}`;
+
+		super(`Refused ${named} with seqNr ${String(header.seqNr)}: ${cause.message}`, { cause });
+		this.header = header;
+		this.name = 'PduRefusedError';
+		this.reason = reason;
+	}
+}
+
+// ESME_RINVTLVSTREAM is SMPP 5.0's name for 0xC0, which SMPP 3.4 spells ESME_RINVOPTPARSTREAM.
+const refusalStatus = {
+	body: 'ESME_RINVCMDLEN',
+	command: 'ESME_RINVCMDID',
+	tlvs: 'ESME_RINVTLVSTREAM',
+} as const satisfies Record<PduRefusalReason, ErrorName>;
+
+/** Why a command_length cannot frame a stream: past it nothing can say where the next PDU starts. */
+export function framingRefusal(cmdLength: number): Error | undefined {
+	if (cmdLength < 16 || cmdLength > maxPduLength) {
+		return new Error(`Refusing a cmd_length of ${String(cmdLength)}`);
+	}
+
+	return undefined;
+}
+
+/** SMPP 3.4 4.3: a PDU whose command has no response of its own is refused with generic_nack. */
+export function refusalAnswer(refused: PduRefusedError): { cmdName: CommandName; cmdStatus: ErrorName } {
+	return {
+		cmdName: respNameFor(refused.header.cmdName) ?? 'generic_nack',
+		cmdStatus: refusalStatus[refused.reason],
+	};
+}