fix: set In-Reply-To and References on replies #234

lib/email-threading.ts

@@ -0,0 +1,51 @@
+/**
+ * RFC 5322 §3.6.4 reply threading.
+ *
+ * Computes the In-Reply-To and References headers an outgoing reply must
+ * carry so MUAs can stitch the conversation back together.
+ *
+ *   In-Reply-To = parent.Message-ID
+ *   References  = parent.References (if any) + parent.Message-ID
+ *
+ * Bare msg-ids only — angle brackets are stripped because JMAP RFC 8621
+ * §4.1.2.3 stores Message-IDs without them.
+ */
+
+export interface ParentThreadingInfo {
+  // JMAP RFC 8621 §4.1.2.3 specifies messageId as String[]|null, but the
+  // codebase has historically typed it as string. Accept either shape.
+  messageId?: string | string[];
+  references?: string[];
+}
+
+export interface ReplyThreadingHeaders {
+  inReplyTo: string[];
+  references: string[];
+}
+
+export function stripMessageIdBrackets(id: string): string {
+  return id.trim().replace(/^<+/, '').replace(/>+$/, '').trim();
+}
+
+export function computeReplyThreadingHeaders(
+  parent: ParentThreadingInfo | undefined,
+): ReplyThreadingHeaders | null {
+  const rawId = Array.isArray(parent?.messageId) ? parent.messageId[0] : parent?.messageId;
+  const parentId = rawId ? stripMessageIdBrackets(rawId) : '';
+  if (!parentId) return null;
+
+  const ancestors = (parent?.references ?? [])
+    .map(stripMessageIdBrackets)
+    .filter(Boolean);
+
+  // De-dupe while preserving order; the parent's id closes the chain.
+  const seen = new Set<string>();
+  const references: string[] = [];
+  for (const id of [...ancestors, parentId]) {
+    if (seen.has(id)) continue;
+    seen.add(id);
+    references.push(id);
+  }
+
+  return { inReplyTo: [parentId], references };
+}