feat(invoice): add money and currency types

lib/src/invoice/money.dart

@@ -0,0 +1,115 @@
+import 'dart:math' as math;
+
+class Currency {
+  final String code;
+  final String symbol;
+  final int decimals;
+  final bool symbolBefore;
+
+  const Currency(
+    this.code, {
+    this.symbol = '',
+    this.decimals = 2,
+    this.symbolBefore = false,
+  });
+
+  int get scale => math.pow(10, decimals).toInt();
+
+  static const pln = Currency('PLN', symbol: 'zl', decimals: 2);
+  static const eur = Currency('EUR', symbol: 'EUR', decimals: 2);
+  static const usd = Currency('USD', symbol: r'$', decimals: 2, symbolBefore: true);
+  static const gbp = Currency('GBP', symbol: 'GBP', decimals: 2, symbolBefore: true);
+
+  static const _known = {'PLN': pln, 'EUR': eur, 'USD': usd, 'GBP': gbp};
+
+  static Currency byCode(String code) =>
+      _known[code.toUpperCase()] ?? Currency(code.toUpperCase());
+
+  @override
+  bool operator ==(Object other) => other is Currency && other.code == code;
+
+  @override
+  int get hashCode => code.hashCode;
+}
+
+class Money {
+  final int minor;
+  final Currency currency;
+
+  const Money(this.minor, this.currency);
+
+  factory Money.of(num amount, Currency currency) =>
+      Money(_roundHalfUp(amount * currency.scale), currency);
+
+  factory Money.parse(String text, Currency currency) {
+    final parsed = double.parse(text.trim());
+    return Money.of(parsed, currency);
+  }
+
+  static Money zero(Currency currency) => Money(0, currency);
+
+  double get amount => minor / currency.scale;
+
+  Money operator +(Money other) {
+    _sameCurrency(other);
+    return Money(minor + other.minor, currency);
+  }
+
+  Money operator -(Money other) {
+    _sameCurrency(other);
+    return Money(minor - other.minor, currency);
+  }
+
+  Money scaled(num factor) => Money(_roundHalfUp(minor * factor), currency);
+
+  Money convertedTo(Currency target, double rate) =>
+      Money.of(amount * rate, target);
+
+  bool get isNegative => minor < 0;
+
+  void _sameCurrency(Money other) {
+    if (other.currency != currency) {
+      throw ArgumentError('Currency mismatch: ${currency.code} vs ${other.currency.code}');
+    }
+  }
+
+  String formatPlain() {
+    final negative = minor < 0;
+    final digits = minor.abs().toString().padLeft(currency.decimals + 1, '0');
+    final splitAt = digits.length - currency.decimals;
+    final intPart = _group(digits.substring(0, splitAt));
+    final fracPart = currency.decimals == 0 ? '' : '.${digits.substring(splitAt)}';
+    return '${negative ? '-' : ''}$intPart$fracPart';
+  }
+
+  String format() {
+    final body = formatPlain();
+    if (currency.symbol.isEmpty) return '$body ${currency.code}';
+    return currency.symbolBefore ? '${currency.symbol}$body' : '$body ${currency.symbol}';
+  }
+
+  @override
+  bool operator ==(Object other) =>
+      other is Money && other.minor == minor && other.currency == currency;
+
+  @override
+  int get hashCode => Object.hash(minor, currency);
+
+  @override
+  String toString() => format();
+}
+
+int _roundHalfUp(num value) {
+  if (value < 0) return -_roundHalfUp(-value);
+  return (value + 0.5).floor();
+}
+
+String _group(String digits) {
+  final buffer = StringBuffer();
+  final length = digits.length;
+  for (var i = 0; i < length; i++) {
+    if (i > 0 && (length - i) % 3 == 0) buffer.write(' ');
+    buffer.write(digits[i]);
+  }
+  return buffer.toString();
+}