feat: chat

lib/chat-grouping.ts

@@ -0,0 +1,80 @@
+/**
+ * Groups a thread's messages the way Instagram does: consecutive messages from
+ * one sender, sent close together, render as a stack with a single avatar and
+ * one timestamp, and a date separator marks each new day.
+ *
+ * Pure and message-shaped so it can be reasoned about (and checked) without a
+ * DOM.
+ */
+
+import type { ChatMessage } from "@/lib/chat";
+
+/** How long a pause before the same sender starts a new stack. */
+export const GROUP_GAP_MS = 5 * 60 * 1000;
+
+export type ChatRow =
+  | { kind: "date"; key: string; at: string }
+  | {
+      kind: "group";
+      key: string;
+      senderId: string | null;
+      messages: ChatMessage[];
+    };
+
+function dayKey(iso: string): string {
+  const d = new Date(iso);
+  return `${d.getFullYear()}-${d.getMonth()}-${d.getDate()}`;
+}
+
+/**
+ * `messages` must be ascending by seq (oldest first) — the order the thread is
+ * rendered in.
+ */
+export function groupMessages(
+  messages: ChatMessage[],
+  gapMs: number = GROUP_GAP_MS,
+): ChatRow[] {
+  const rows: ChatRow[] = [];
+  let current: Extract<ChatRow, { kind: "group" }> | null = null;
+  let currentDay = "";
+
+  for (const m of messages) {
+    const day = dayKey(m.created_at);
+    if (day !== currentDay) {
+      rows.push({ kind: "date", key: `date-${day}`, at: m.created_at });
+      currentDay = day;
+      current = null; // a new day always starts a new stack
+    }
+
+    const last = current?.messages[current.messages.length - 1];
+    const gap = last
+      ? new Date(m.created_at).getTime() - new Date(last.created_at).getTime()
+      : Infinity;
+
+    // System notices stand alone — they belong to no one and shouldn't be
+    // absorbed into a neighbouring stack.
+    const standalone = m.kind === "system";
+    const continues =
+      current !== null &&
+      !standalone &&
+      current.senderId === (m.sender_id ?? null) &&
+      current.messages[0]?.kind !== "system" &&
+      gap <= gapMs;
+
+    if (continues && current) {
+      current.messages.push(m);
+      continue;
+    }
+
+    current = {
+      kind: "group",
+      key: `group-${m.id}`,
+      senderId: m.sender_id ?? null,
+      messages: [m],
+    };
+    rows.push(current);
+    if (standalone) current = null;
+  }
+
+  return rows;
+}