Added Google Drive storage location

lib/services/drive_service.dart

@@ -0,0 +1,229 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:googleapis/drive/v3.dart' as drive;
+import 'package:http/http.dart' as http;
+
+const appFolderName = 'MO-Fuel-Tax-Back';
+const receiptsFolderName = 'receipts';
+const dataFileName = 'fuel_tax_data.json';
+
+const _folderMimeType = 'application/vnd.google-apps.folder';
+
+class DriveFolder {
+  final String id;
+  final String name;
+
+  DriveFolder({required this.id, required this.name});
+}
+
+class DriveLockFile {
+  final String id;
+  final String username;
+  final DateTime createdAtUtc;
+
+  DriveLockFile({required this.id, required this.username, required this.createdAtUtc});
+}
+
+/// Raw Google Drive API operations, built on top of an already-authenticated
+/// [http.Client] (see [DriveAuthService.authenticatedHttpClient]). Callers
+/// own the client's lifecycle (create it, use a [DriveService] instance for
+/// the duration of one sync, then close it).
+class DriveService {
+  final drive.DriveApi _api;
+
+  DriveService(http.Client authenticatedClient) : _api = drive.DriveApi(authenticatedClient);
+
+  /// Lists folders under [parentId], or (if [sharedWithMe] is true) the
+  /// top-level folders that other users have shared with the signed-in
+  /// account, regardless of parent.
+  Future<List<DriveFolder>> listFolders({String? parentId, bool sharedWithMe = false}) async {
+    final query = sharedWithMe
+        ? "sharedWithMe=true and mimeType='$_folderMimeType' and trashed=false"
+        : "'$parentId' in parents and mimeType='$_folderMimeType' and trashed=false";
+
+    final result = await _api.files.list(
+      q: query,
+      orderBy: 'name',
+      $fields: 'files(id,name)',
+      spaces: 'drive',
+    );
+
+    return (result.files ?? [])
+        .where((f) => f.id != null && f.name != null)
+        .map((f) => DriveFolder(id: f.id!, name: f.name!))
+        .toList();
+  }
+
+  /// Finds a folder named [appFolderName] under [parentId], or creates one
+  /// if none exists. Multiple devices pointed at the same shared parent
+  /// converge on the same folder this way. If duplicates exist (Drive
+  /// allows same-named folders), the earliest-created one wins.
+  Future<String> findOrCreateAppFolder(String parentId) {
+    return _findOrCreateFolder(name: appFolderName, parentId: parentId);
+  }
+
+  Future<String> findOrCreateReceiptsFolder(String appFolderId) {
+    return _findOrCreateFolder(name: receiptsFolderName, parentId: appFolderId);
+  }
+
+  Future<String> _findOrCreateFolder({required String name, required String parentId}) async {
+    final existing = await _api.files.list(
+      q: "'$parentId' in parents and mimeType='$_folderMimeType' and trashed=false and name='$name'",
+      orderBy: 'createdTime',
+      $fields: 'files(id,name)',
+      spaces: 'drive',
+    );
+
+    final firstMatch = (existing.files ?? []).firstOrNullWithId();
+    if (firstMatch != null) return firstMatch;
+
+    final created = await _api.files.create(
+      drive.File()
+        ..name = name
+        ..mimeType = _folderMimeType
+        ..parents = [parentId],
+    );
+    return created.id!;
+  }
+
+  Future<String?> findDataFileId(String appFolderId) async {
+    final result = await _api.files.list(
+      q: "'$appFolderId' in parents and trashed=false and name='$dataFileName'",
+      orderBy: 'modifiedTime desc',
+      $fields: 'files(id,name)',
+      spaces: 'drive',
+    );
+    return (result.files ?? []).firstOrNullWithId();
+  }
+
+  Future<String> downloadTextFile(String fileId) async {
+    final media = await _api.files.get(
+      fileId,
+      downloadOptions: drive.DownloadOptions.fullMedia,
+    ) as drive.Media;
+    final bytes = await _collectBytes(media.stream);
+    return utf8.decode(bytes);
+  }
+
+  Future<List<int>> downloadFileBytes(String fileId) async {
+    final media = await _api.files.get(
+      fileId,
+      downloadOptions: drive.DownloadOptions.fullMedia,
+    ) as drive.Media;
+    return _collectBytes(media.stream);
+  }
+
+  /// Creates the data file if [existingFileId] is null, otherwise
+  /// overwrites its content. Returns the (possibly new) file ID.
+  Future<String> uploadDataFile({
+    required String appFolderId,
+    required String? existingFileId,
+    required String jsonContent,
+  }) async {
+    final bytes = utf8.encode(jsonContent);
+    final media = drive.Media(
+      Stream.value(bytes),
+      bytes.length,
+      contentType: 'application/json',
+    );
+
+    if (existingFileId != null) {
+      final updated = await _api.files.update(
+        drive.File(),
+        existingFileId,
+        uploadMedia: media,
+      );
+      return updated.id ?? existingFileId;
+    }
+
+    final created = await _api.files.create(
+      drive.File()
+        ..name = dataFileName
+        ..parents = [appFolderId],
+      uploadMedia: media,
+    );
+    return created.id!;
+  }
+
+  Future<String> uploadReceiptImage({
+    required String receiptsFolderId,
+    required String fileName,
+    required File imageFile,
+  }) async {
+    final length = await imageFile.length();
+    final media = drive.Media(
+      imageFile.openRead(),
+      length,
+      contentType: 'image/jpeg',
+    );
+    final created = await _api.files.create(
+      drive.File()
+        ..name = fileName
+        ..parents = [receiptsFolderId],
+      uploadMedia: media,
+    );
+    return created.id!;
+  }
+
+  Future<void> deleteFile(String fileId) async {
+    try {
+      await _api.files.delete(fileId);
+    } on drive.DetailedApiRequestError catch (e) {
+      // Already gone (e.g. deleted by another device) — not an error for
+      // our purposes.
+      if (e.status != 404) rethrow;
+    }
+  }
+
+  Future<String> createLockFile({required String appFolderId, required String name}) async {
+    final created = await _api.files.create(
+      drive.File()
+        ..name = name
+        ..parents = [appFolderId],
+      uploadMedia: drive.Media(Stream.value(const []), 0, contentType: 'text/plain'),
+    );
+    return created.id!;
+  }
+
+  Future<List<DriveLockFile>> listLockFiles(String appFolderId) async {
+    final result = await _api.files.list(
+      q: "'$appFolderId' in parents and trashed=false and name contains '.lock'",
+      $fields: 'files(id,name)',
+      spaces: 'drive',
+    );
+
+    final locks = <DriveLockFile>[];
+    for (final f in result.files ?? <drive.File>[]) {
+      final parsed = _parseLockFileName(f.name);
+      if (f.id != null && parsed != null) {
+        locks.add(DriveLockFile(id: f.id!, username: parsed.$1, createdAtUtc: parsed.$2));
+      }
+    }
+    return locks;
+  }
+
+  static (String, DateTime)? _parseLockFileName(String? name) {
+    if (name == null || !name.endsWith('.lock')) return null;
+    final withoutExt = name.substring(0, name.length - '.lock'.length);
+    final lastDash = withoutExt.lastIndexOf('-');
+    if (lastDash == -1) return null;
+    final username = withoutExt.substring(0, lastDash);
+    final epochStr = withoutExt.substring(lastDash + 1);
+    final epoch = int.tryParse(epochStr);
+    if (epoch == null) return null;
+    return (username, DateTime.fromMillisecondsSinceEpoch(epoch, isUtc: true));
+  }
+
+  Future<List<int>> _collectBytes(Stream<List<int>> stream) async {
+    final bytes = <int>[];
+    await for (final chunk in stream) {
+      bytes.addAll(chunk);
+    }
+    return bytes;
+  }
+}
+
+extension _FirstMatchExtension on List<drive.File> {
+  String? firstOrNullWithId() => isEmpty ? null : first.id;
+}