import 'json_util.dart';

/// 账户信息（对应后端 AccountResponse）
///
/// 语义说明（重要）：
/// - [balance] 是后端计算出来的：`initialBalance + totalIncome - totalExpense`
/// - [initialBalance] 才是可写入数据库的字段（后端 PUT /accounts/balance 改的是它）
/// - [totalIncome] / [totalExpense] 是**全部账单**的汇总，只读
class AccountInfo {
  final int? id;
  final String name;
  final double initialBalance;
  final double balance;
  final double totalIncome;
  final double totalExpense;

  const AccountInfo({
    this.id,
    this.name = '默认账户',
    required this.initialBalance,
    required this.balance,
    required this.totalIncome,
    required this.totalExpense,
  });

  factory AccountInfo.fromJson(Map<String, dynamic> json) {
    final initial = asDouble(json['initialBalance']);
    final income = asDouble(json['totalIncome']);
    final expense = asDouble(json['totalExpense']);
    return AccountInfo(
      id: json['id'] == null ? null : asInt(json['id']),
      name: asNonEmpty(json['name'], '默认账户'),
      initialBalance: initial,
      // 后端没给 balance 时本地按同一公式兜底
      balance: json['balance'] == null
          ? initial + income - expense
          : asDouble(json['balance']),
      totalIncome: income,
      totalExpense: expense,
    );
  }

  static const empty = AccountInfo(
    initialBalance: 0,
    balance: 0,
    totalIncome: 0,
    totalExpense: 0,
  );
}

/// 把"用户想要的目标余额"反算成后端可写入的期初余额。
///
/// 因为 balance = initialBalance + totalIncome - totalExpense，
/// 所以要让余额显示为 target，期初余额必须设为：
/// `target - totalIncome + totalExpense`
double initialBalanceFor({
  required double targetBalance,
  required double totalIncome,
  required double totalExpense,
}) {
  if (!targetBalance.isFinite) return 0;
  return targetBalance - totalIncome + totalExpense;
}