/// Abstract LLM backend.
///
/// Concrete impls: `GemmaLlmService` (flutter_gemma) and `MockLlmService` (tests).
/// Contract:
/// - After [load], [isLoaded] is true.
/// - [generateStructured] throws [StateError] when not loaded.
/// - [generateStructured] returns a parsed JSON map matching the schema.
///   On schema/parse failure throws [FormatException].
/// - [unload] is idempotent.
abstract class LlmService {
  bool get isLoaded;

  Future<void> load();

  Future<void> unload();

  /// Calls the model with a function-calling [schema] and returns the parsed
  /// JSON arguments map. Caller is responsible for applying `.timeout(...)`.
  Future<Map<String, dynamic>> generateStructured(
    String prompt,
    Map<String, dynamic> schema,
  );
}

/// Programmable stub for tests. Use [enqueueResponse] / [enqueueError].
class MockLlmService implements LlmService {
  final List<_Response> _queue = [];
  bool _loaded = false;
  int callCount = 0;
  String? lastPrompt;
  Map<String, dynamic>? lastSchema;
  Duration responseDelay = Duration.zero;

  @override
  bool get isLoaded => _loaded;

  @override
  Future<void> load() async {
    _loaded = true;
  }

  @override
  Future<void> unload() async {
    _loaded = false;
  }

  void enqueueResponse(Map<String, dynamic> response) {
    _queue.add(_Response.value(response));
  }

  void enqueueError(Object error) {
    _queue.add(_Response.error(error));
  }

  @override
  Future<Map<String, dynamic>> generateStructured(
    String prompt,
    Map<String, dynamic> schema,
  ) async {
    callCount += 1;
    lastPrompt = prompt;
    lastSchema = schema;
    if (!_loaded) {
      throw StateError('LlmService not loaded');
    }
    if (responseDelay > Duration.zero) {
      await Future<void>.delayed(responseDelay);
    }
    if (_queue.isEmpty) {
      throw StateError('MockLlmService: no queued response');
    }
    final r = _queue.removeAt(0);
    if (r.error != null) throw r.error!;
    return r.value!;
  }
}

class _Response {
  final Map<String, dynamic>? value;
  final Object? error;
  const _Response._(this.value, this.error);
  factory _Response.value(Map<String, dynamic> v) => _Response._(v, null);
  factory _Response.error(Object e) => _Response._(null, e);
}