/// A single line item in the balance computation.
class BalanceEntry {
  final String label;
  final int amountCents;
  final int runningBalanceCents;

  const BalanceEntry({
    required this.label,
    required this.amountCents,
    required this.runningBalanceCents,
  });
}

/// Snapshot of a pay period item (gas, groceries, other, etc.)
class ItemSnapshot {
  final String label;
  final int amountCents; // always positive

  const ItemSnapshot({required this.label, required this.amountCents});
}

/// Snapshot of a paycheck with its associated period items.
class PaycheckSnapshot {
  final String label;
  final int amountCents;
  final List<ItemSnapshot> items;

  const PaycheckSnapshot({
    required this.label,
    required this.amountCents,
    required this.items,
  });
}

/// Snapshot of a bill for balance computation.
class BillSnapshot {
  final int id;
  final String label;
  final int defaultAmountCents;
  final int? amountOverrideCents;
  final bool isFreeform;

  const BillSnapshot({
    required this.id,
    required this.label,
    required this.defaultAmountCents,
    this.amountOverrideCents,
    this.isFreeform = false,
  });

  int get effectiveAmountCents => amountOverrideCents ?? defaultAmountCents;
}

/// All data needed to compute a month's rolling balance.
class MonthSnapshot {
  final int openingBalanceCents;
  final List<PaycheckSnapshot> paychecks;
  final List<ItemSnapshot> priorityPayouts;
  final List<BillSnapshot> bills;
  final List<ItemSnapshot> notablePayouts;

  const MonthSnapshot({
    required this.openingBalanceCents,
    this.paychecks = const [],
    this.priorityPayouts = const [],
    this.bills = const [],
    this.notablePayouts = const [],
  });
}

/// Result of the balance computation.
class BalanceResult {
  final List<BalanceEntry> entries;
  final int remainingCents;
  final int balanceBeforeCcCents;

  const BalanceResult({
    required this.entries,
    required this.remainingCents,
    required this.balanceBeforeCcCents,
  });
}

/// Computes the rolling balance for a month. Pure function.
BalanceResult computeBalance({
  required MonthSnapshot month,
  Map<int, int> ccPayments = const {},
}) {
  final entries = <BalanceEntry>[];
  var balance = month.openingBalanceCents;

  void add(String label, int amount) {
    balance += amount;
    entries.add(BalanceEntry(
      label: label,
      amountCents: amount,
      runningBalanceCents: balance,
    ));
  }

  entries.add(BalanceEntry(
    label: 'Opening Balance',
    amountCents: 0,
    runningBalanceCents: balance,
  ));

  for (final paycheck in month.paychecks) {
    add(paycheck.label, paycheck.amountCents);
    for (final item in paycheck.items) {
      add(item.label, -item.amountCents);
    }
  }

  for (final payout in month.priorityPayouts) {
    add(payout.label, -payout.amountCents);
  }

  // All bills (rule-computed amounts are already baked into defaultAmountCents by assembler)
  for (final bill in month.bills) {
    add(bill.label, -bill.effectiveAmountCents);
  }

  final balanceBeforeCc = balance;

  for (final payout in month.notablePayouts) {
    add(payout.label, -payout.amountCents);
  }

  return BalanceResult(
    entries: entries,
    remainingCents: balance,
    balanceBeforeCcCents: balanceBeforeCc,
  );
}