import { useState, useCallback, useEffect } from "react";
import type { Conversation } from "../types";
import { readConversations, writeConversations, titleFromMessages } from "../lib/storage";

export interface UseConversationsReturn {
  conversations: Conversation[];
  upsertConversation: (conv: Conversation) => void;
  deleteConversation: (id: string) => void;
  loadConversation: (id: string) => Conversation | undefined;
}

/**
 * Manages conversation persistence in localStorage via storage.ts helpers.
 *
 * - Reads `hmc-conversations` on mount.
 * - Exposes `upsertConversation(conv)` to insert or update a conversation,
 *   automatically deriving the title from the first user message and updating
 *   `last_active_at` to the current time.
 * - Exposes `deleteConversation(id)` to remove a conversation by ID.
 * - Exposes `loadConversation(id)` to retrieve a conversation by ID from
 *   the current in-memory list (no server round-trip).
 *
 * Requirements: 6.1, 6.2, 6.3, 6.5, 6.6, 6.7
 */
export function useConversations(): UseConversationsReturn {
  const [conversations, setConversations] = useState<Conversation[]>(() =>
    readConversations()
  );

  // Sync from localStorage on mount (handles cases where another tab wrote)
  useEffect(() => {
    setConversations(readConversations());
  }, []);

  /**
   * Insert or update a conversation. If a conversation with the same `id`
   * already exists, it is replaced. The title is re-derived from messages
   * and `last_active_at` is set to now.
   *
   * The cap of 20 conversations is enforced by `writeConversations` in storage.ts.
   */
  const upsertConversation = useCallback((conv: Conversation): void => {
    setConversations((prev) => {
      const now = new Date().toISOString();
      const updated: Conversation = {
        ...conv,
        title: titleFromMessages(conv.messages),
        last_active_at: now,
        created_at: conv.created_at || now,
      };

      const existing = prev.findIndex((c) => c.id === conv.id);
      let next: Conversation[];
      if (existing >= 0) {
        next = [...prev];
        next[existing] = updated;
      } else {
        next = [updated, ...prev];
      }

      writeConversations(next);
      return readConversations(); // re-read to get the sorted, capped result
    });
  }, []);

  /**
   * Remove a conversation by ID from localStorage and update state.
   */
  const deleteConversation = useCallback((id: string): void => {
    setConversations((prev) => {
      const next = prev.filter((c) => c.id !== id);
      writeConversations(next);
      return next;
    });
  }, []);

  /**
   * Retrieve a conversation by ID from the current in-memory list.
   * Returns undefined if not found. No server round-trip.
   */
  const loadConversation = useCallback(
    (id: string): Conversation | undefined => {
      return conversations.find((c) => c.id === id);
    },
    [conversations]
  );

  return { conversations, upsertConversation, deleteConversation, loadConversation };
}