import 'dart:convert';

/// Tool execution result. See design fn-tool_dispatcher.md §4.
///
/// Always JSON-serialisable so the model can consume it. `toJson()` returns a
/// shape with a stable `status` discriminator — easier for the LLM to parse
/// than relying on key presence.
sealed class ToolResult {
  const ToolResult();
  Map<String, dynamic> toJson();
}

final class ToolOk extends ToolResult {
  final Map<String, dynamic> data;
  const ToolOk(this.data);

  @override
  Map<String, dynamic> toJson() => {'status': 'ok', 'data': data};
}

final class ToolErr extends ToolResult {
  final String code;
  final String reason;
  const ToolErr(this.code, this.reason);

  @override
  Map<String, dynamic> toJson() =>
      {'status': 'error', 'code': code, 'reason': reason};
}

final class ToolCancelled extends ToolResult {
  const ToolCancelled();

  @override
  Map<String, dynamic> toJson() =>
      {'status': 'cancelled', 'reason': 'user did not confirm'};
}

/// Encode a [ToolResult] to a JSON string ≤ [maxBytes].
///
/// Tool result token budget (ADR-0005 / OQ-2): keep model context bounded.
/// If serialised payload exceeds [maxBytes], we replace the tail of the data
/// field with a truncation hint instead of letting the LLM blow its window.
String encodeToolResult(ToolResult result, {int maxBytes = 2048}) {
  final encoded = jsonEncode(result.toJson());
  if (encoded.length <= maxBytes) return encoded;

  // Truncate strategy: only ToolOk has unbounded payload. Replace data with
  // a hint pointing at follow-up tools. Errors/cancellations are always small.
  if (result is ToolOk) {
    final hint = {
      'status': 'ok',
      'data': {
        '_truncated': true,
        '_hint': '결과가 ${encoded.length} 바이트로 잘렸습니다. '
            '구체 ID 가 필요하면 query_protocol 같은 단건 조회 도구를 사용하세요.',
      },
    };
    return jsonEncode(hint);
  }
  // Defensive: if some future ToolResult adds bulk, fall back to hard cut.
  return encoded.substring(0, maxBytes);
}