import 'dart:io';

import 'package:flutter_riverpod/flutter_riverpod.dart';

import '../data/ai/llm_service.dart';
import '../data/ai/model_lifecycle.dart';
import 'ai_providers.dart';

/// State machine for ChatScreen LLM warm-up (#311).
///
/// See `docs/design/311-llm-warmup/README.md` §6 / fn-chat_warmup_controller.md.
sealed class ChatWarmupState {
  const ChatWarmupState();
}

final class ChatWarmupIdle extends ChatWarmupState {
  const ChatWarmupIdle();
}

final class ChatWarmupLoading extends ChatWarmupState {
  const ChatWarmupLoading();
}

final class ChatWarmupReady extends ChatWarmupState {
  const ChatWarmupReady();
}

/// Warm-up was not attempted because [ModelLifecycle.quickCheck] returned
/// something other than `ready` (download incomplete, opt-out, corrupt).
/// UI behaves as if warm-up didn't exist; the first user send falls back to
/// the existing lazy `userTurn` path.
final class ChatWarmupUnavailable extends ChatWarmupState {
  const ChatWarmupUnavailable();
}

/// `kind` discriminates the retry copy: `fileMissing` is a settings-level
/// recovery; `runtime` is a transient retry.
enum ChatWarmupFailureKind { fileMissing, runtime }

final class ChatWarmupFailed extends ChatWarmupState {
  final String message;
  final ChatWarmupFailureKind kind;
  const ChatWarmupFailed(this.message, this.kind);
}

/// Drives `LlmService.load()` on ChatScreen mount so the first user send
/// doesn't pay native-init latency. AC1-AC12 (12개) 모두 본 controller 가
/// 흡수한다 (UI binding 은 chat_screen.dart 가 본 state 를 watch).
class ChatWarmupController extends StateNotifier<ChatWarmupState> {
  ChatWarmupController({
    required this.llm,
    required this.lifecycle,
  }) : super(const ChatWarmupIdle());

  final LlmService llm;
  final ModelLifecycle lifecycle;
  bool _disposed = false;

  /// Idempotent. Re-entrant guard via the Loading state — duplicate `start`
  /// calls during an in-flight load do nothing (the running future will set
  /// the final state). External callers use [retry] instead.
  Future<void> start() async {
    if (state is ChatWarmupLoading) return;

    // AC11 / UX R4: fast path. Skip Loading entirely if the underlying
    // service is already loaded — prevents 1-frame label flicker on
    // ChatScreen re-entry.
    if (llm.isLoaded) {
      _safeSet(const ChatWarmupReady());
      return;
    }

    final availability = await lifecycle.quickCheck();
    if (_disposed) return;
    if (availability != ModelAvailability.ready) {
      _safeSet(const ChatWarmupUnavailable());
      return;
    }

    _safeSet(const ChatWarmupLoading());
    try {
      await llm.load();
    } catch (e) {
      if (_disposed) return;
      final kind = e is FileSystemException
          ? ChatWarmupFailureKind.fileMissing
          : ChatWarmupFailureKind.runtime;
      _safeSet(ChatWarmupFailed(_messageFor(kind), kind));
      return;
    }
    if (_disposed) return;
    _safeSet(const ChatWarmupReady());
  }

  Future<void> retry() async {
    if (_disposed) return;
    _safeSet(const ChatWarmupIdle());
    await start();
  }

  /// AC6 / AC12: state는 disposed 인스턴스에는 더 이상 쓰지 않는다.
  /// StateNotifier 의 setter 는 disposed 시 throw 하므로 가드 필수.
  void _safeSet(ChatWarmupState s) {
    if (_disposed) return;
    state = s;
  }

  /// UX R5 / AC12: 메시지는 **상태**만 기술. "다시 시도해주세요" 같은
  /// 명령형은 [다시 시도] 버튼이 담당하므로 본 문안에 넣지 않는다.
  String _messageFor(ChatWarmupFailureKind kind) {
    switch (kind) {
      case ChatWarmupFailureKind.fileMissing:
        return 'AI 모델 파일을 찾을 수 없어요.';
      case ChatWarmupFailureKind.runtime:
        return 'AI 를 시작하지 못했어요.';
    }
  }

  @override
  void dispose() {
    _disposed = true;
    super.dispose();
  }
}

/// autoDispose: ChatScreen 이 pop 되면 controller 도 dispose → mount race 안전.
final chatWarmupProvider =
    StateNotifierProvider.autoDispose<ChatWarmupController, ChatWarmupState>(
  (ref) {
    return ChatWarmupController(
      llm: ref.watch(llmServiceProvider),
      lifecycle: ref.watch(modelLifecycleProvider),
    );
  },
);