/**
 * 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;
}