import 'dart:async';
import 'dart:io';

import 'package:crypto/crypto.dart';
import 'package:http/http.dart' as http;
import 'package:path/path.dart' as p;
import 'package:path_provider/path_provider.dart';

import '../db/daos/meta_dao.dart';

/// meta_kv keys (#215 README §6).
class AiMetaKeys {
  static const optIn = 'ai_opt_in';
  static const modelPath = 'ai_model_path';
  static const modelSha = 'ai_model_sha256';
  static const downloadState = 'ai_download_state';
  static const downloadBytes = 'ai_download_bytes';

  static const all = <String>[
    optIn,
    modelPath,
    modelSha,
    downloadState,
    downloadBytes,
  ];
}

enum ModelAvailability { ready, missing, corrupt, downloading }

enum DownloadState { idle, downloading, paused, completed, failed }

class DownloadProgress {
  final int bytesReceived;
  final int totalBytes; // -1 if unknown
  final DownloadState state;
  final String? errorMessage;
  const DownloadProgress({
    required this.bytesReceived,
    required this.totalBytes,
    required this.state,
    this.errorMessage,
  });
}

/// File-system / HTTP abstraction so tests can inject a fake.
abstract class StorageAdapter {
  Future<Directory> supportDir();
  Future<http.StreamedResponse> rangeGet(Uri url, int from);
}

class _ProdStorage implements StorageAdapter {
  final http.Client client;
  _ProdStorage(this.client);
  @override
  Future<Directory> supportDir() => getApplicationSupportDirectory();
  @override
  Future<http.StreamedResponse> rangeGet(Uri url, int from) async {
    final req = http.Request('GET', url);
    if (from > 0) req.headers['Range'] = 'bytes=$from-';
    return client.send(req);
  }
}

/// Default config for the Gemma 4 E2B Q4_0 model. OQ-1: real URL + SHA
/// to be pinned in Developer phase after `flutter_gemma` docs review.
class ModelConfig {
  final Uri url;
  final String expectedSha256;
  final String filename;
  const ModelConfig({
    required this.url,
    required this.expectedSha256,
    this.filename = 'gemma4_e2b_q4.bin',
  });
}

/// Owns the model file: download (resumable) → SHA-256 verify → availability
/// query → purge. All paths graceful: on failure surfaces as `corrupt` /
/// `failed` rather than throwing through the call chain.
class ModelLifecycle {
  final MetaDao meta;
  final ModelConfig config;
  final StorageAdapter _storage;

  ModelLifecycle({
    required this.meta,
    required this.config,
    StorageAdapter? storage,
    http.Client? httpClient,
  }) : _storage = storage ?? _ProdStorage(httpClient ?? http.Client());

  Future<String> _modelPath() async {
    final dir = await _storage.supportDir();
    return p.join(dir.path, config.filename);
  }

  Future<ModelAvailability> checkAvailability() async {
    try {
      final optIn = await meta.find(AiMetaKeys.optIn);
      if (optIn != 'true') return ModelAvailability.missing;

      final state = await meta.find(AiMetaKeys.downloadState);
      if (state == 'downloading' || state == 'paused') {
        return ModelAvailability.downloading;
      }

      final pathStr = await meta.find(AiMetaKeys.modelPath);
      if (pathStr == null) return ModelAvailability.missing;

      final file = File(pathStr);
      if (!file.existsSync()) return ModelAvailability.missing;

      final expected = await meta.find(AiMetaKeys.modelSha);
      if (expected == null) return ModelAvailability.corrupt;

      final actual = await _hashFile(file);
      if (actual != expected) return ModelAvailability.corrupt;

      return ModelAvailability.ready;
    } catch (_) {
      return ModelAvailability.corrupt;
    }
  }

  Future<String> _hashFile(File file) async {
    final digest = await sha256.bind(file.openRead()).first;
    return digest.toString();
  }

  /// Streams resumable download progress. Mutates `meta_kv` as it goes.
  /// Errors are emitted as `DownloadProgress(state: failed)` rather than
  /// thrown — UI listens to the stream.
  Stream<DownloadProgress> download() async* {
    final path = await _modelPath();
    final tempPath = '$path.tmp';
    final tempFile = File(tempPath);
    int existing =
        tempFile.existsSync() ? await tempFile.length() : 0;

    await meta.put(AiMetaKeys.downloadState, 'downloading');
    await meta.put(AiMetaKeys.downloadBytes, existing.toString());
    yield DownloadProgress(
      bytesReceived: existing,
      totalBytes: -1,
      state: DownloadState.downloading,
    );

    http.StreamedResponse response;
    try {
      response = await _storage.rangeGet(config.url, existing);
    } catch (e) {
      await meta.put(AiMetaKeys.downloadState, 'paused');
      yield DownloadProgress(
        bytesReceived: existing,
        totalBytes: -1,
        state: DownloadState.failed,
        errorMessage: 'network: $e',
      );
      return;
    }

    final status = response.statusCode;
    if (status != 200 && status != 206) {
      // 416 etc. — restart from 0.
      if (tempFile.existsSync()) await tempFile.delete();
      existing = 0;
      await meta.put(AiMetaKeys.downloadBytes, '0');
      yield DownloadProgress(
        bytesReceived: 0,
        totalBytes: -1,
        state: DownloadState.failed,
        errorMessage: 'http $status',
      );
      return;
    }

    final contentLength = response.contentLength ?? 0;
    final total = status == 206 ? existing + contentLength : contentLength;
    final sink = tempFile.openWrite(mode: FileMode.append);
    int received = existing;

    try {
      await for (final chunk in response.stream) {
        sink.add(chunk);
        received += chunk.length;
        await meta.put(AiMetaKeys.downloadBytes, received.toString());
        yield DownloadProgress(
          bytesReceived: received,
          totalBytes: total,
          state: DownloadState.downloading,
        );
      }
      await sink.flush();
      await sink.close();
    } catch (e) {
      await sink.close();
      await meta.put(AiMetaKeys.downloadState, 'paused');
      yield DownloadProgress(
        bytesReceived: received,
        totalBytes: total,
        state: DownloadState.failed,
        errorMessage: 'stream: $e',
      );
      return;
    }

    // Verify SHA-256.
    final finalFile = File(path);
    await tempFile.rename(path);
    final sha = await _hashFile(finalFile);
    if (sha != config.expectedSha256) {
      await finalFile.delete();
      await meta.put(AiMetaKeys.downloadState, 'failed');
      yield DownloadProgress(
        bytesReceived: received,
        totalBytes: total,
        state: DownloadState.failed,
        errorMessage: 'sha mismatch',
      );
      return;
    }

    await meta.put(AiMetaKeys.modelPath, path);
    await meta.put(AiMetaKeys.modelSha, sha);
    await meta.put(AiMetaKeys.downloadState, 'completed');
    yield DownloadProgress(
      bytesReceived: received,
      totalBytes: total,
      state: DownloadState.completed,
    );
  }

  /// opt-out: delete model file + clear all ai_* meta keys (except opt_in
  /// which the caller toggles). Returns freed bytes (0 if nothing existed).
  /// Idempotent.
  Future<int> purge() async {
    int freed = 0;
    final pathStr = await meta.find(AiMetaKeys.modelPath);
    if (pathStr != null) {
      final f = File(pathStr);
      if (f.existsSync()) {
        freed += await f.length();
        await f.delete();
      }
    }
    final tempPath = '${await _modelPath()}.tmp';
    final temp = File(tempPath);
    if (temp.existsSync()) {
      freed += await temp.length();
      await temp.delete();
    }
    for (final k in [
      AiMetaKeys.modelPath,
      AiMetaKeys.modelSha,
      AiMetaKeys.downloadState,
      AiMetaKeys.downloadBytes,
    ]) {
      await meta.remove(k);
    }
    return freed;
  }
}