feat: setup attachment to the freight chat

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

@@ -0,0 +1,54 @@
+import { BadRequestException } from "@nestjs/common";
+
+/**
+ * Keyset cursor for paging a thread backwards from newest.
+ *
+ * The cursor is just a message id. The sort key is the pair `(created_at, id)` —
+ * two messages can share a timestamp, and a cursor on a non-unique key either
+ * re-serves or skips the tied rows — but the *timestamp half is never sent over
+ * the wire*, because it cannot survive the trip.
+ *
+ * `support_messages.created_at` is `timestamptz(6)`; a JS `Date` holds only
+ * milliseconds, so the value TypeORM hands back is already truncated. Encoding
+ * that into the cursor and comparing against it would silently skip every row
+ * sharing the cursor's millisecond but earlier within it (`.254100` is not
+ * `< .254000`) — those rows would never appear on any page. Sending the id alone
+ * and letting Postgres look the real `(created_at, id)` up keeps the comparison
+ * at full precision on the server, where it was never lossy.
+ *
+ * Opaque on purpose (base64): clients must treat it as a token, so the sort key
+ * can change without a contract change.
+ *
+ * The passenger API's twin encodes a timestamp because its column is
+ * `TIMESTAMP(3)` — millisecond, matching JS exactly — so it has no such loss.
+ * The two formats are deliberately NOT interchangeable; each app reads only its
+ * own cursors.
+ */
+export function encodeMessageCursor(id: string): string {
+  return Buffer.from(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): string {
+  let id: string;
+  try {
+    id = Buffer.from(raw, "base64url").toString("utf8");
+  } catch {
+    throw new BadRequestException("Malformed pagination cursor.");
+  }
+
+  // The id goes into a parameterized query, but validate the shape anyway: a
+  // non-uuid can only be a mangled cursor, and failing loudly here beats an
+  // empty page that reads as "start of conversation".
+  if (
+    !/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i.test(id)
+  ) {
+    throw new BadRequestException("Malformed pagination cursor.");
+  }
+
+  return id;
+}