feat: setup the attachment to the passenger api

apps/edr-passenger-api/src/modules/support/message-cursor.ts

@@ -0,0 +1,52 @@
+import { BadRequestException } from '@nestjs/common';
+
+/**
+ * Keyset cursor for paging a thread backwards from newest.
+ *
+ * The sort key is the pair `(createdAt, id)`, not `createdAt` alone: two
+ * messages can share a millisecond, and a cursor on a non-unique key either
+ * re-serves or skips the tied rows depending which side of the boundary they
+ * land on. The id breaks ties with a stable total order.
+ *
+ * Deliberately a twin of the freight API's `message-cursor.ts`, not a shared
+ * import: the two APIs share no runtime package, and @edr/types is Nest-free by
+ * design (this throws Nest exceptions). The wire format matches so a client can
+ * treat both chats identically — keep them in step if either changes.
+ */
+export interface MessageCursor {
+  createdAt: Date;
+  id: string;
+}
+
+export function encodeMessageCursor(cursor: MessageCursor): string {
+  return Buffer.from(`${cursor.createdAt.toISOString()}|${cursor.id}`, 'utf8').toString(
+    'base64url',
+  );
+}
+
+/**
+ * Parse a client-supplied cursor. Rejects anything malformed rather than
+ * silently falling back to "first page" — a corrupted cursor that degrades to
+ * page 1 makes an infinite scroll loop forever over the same rows.
+ */
+export function decodeMessageCursor(raw: string): MessageCursor {
+  let decoded: string;
+  try {
+    decoded = Buffer.from(raw, 'base64url').toString('utf8');
+  } catch {
+    throw new BadRequestException('Malformed pagination cursor.');
+  }
+
+  const separator = decoded.lastIndexOf('|');
+  if (separator === -1) {
+    throw new BadRequestException('Malformed pagination cursor.');
+  }
+
+  const createdAt = new Date(decoded.slice(0, separator));
+  const id = decoded.slice(separator + 1);
+  if (Number.isNaN(createdAt.getTime()) || !id) {
+    throw new BadRequestException('Malformed pagination cursor.');
+  }
+
+  return { createdAt, id };
+}