rounding_test.dart

import 'package:flutter_test/flutter_test.dart';

void main() {
  test('1 should round 0.145 to 0.15', () {
    const value = 0.145;
    final rounded = value.toStringAsFixed(2);
    expect(rounded, '0.15'); // actual 0.14
  });

  test('2 should round 0.145 to 0.15', () {
    const value = 0.145;
    final rounded = (value * 100).round() / 100;
    expect(rounded, 0.15); // actual 0.14
  });

  test('3 should round 0.145 to 0.15', () {
    const value = 0.145;
    final rounded = value.toStringWithRoundedDecimals(2);
    expect(rounded, 0.15);
  });

  test('4 should round 1.145 to 0.15', () {
    const value = 1.145;
    final rounded = value.toStringAsFixed(2);
    expect(rounded, '1.15');
  });

  test('5 should round 1.145 to 1.15', () {
    const value = 1.145;
    final rounded = (value * 100).round() / 100;
    expect(rounded, 1.15);
  });

  test('6 should round 1.145 to 0.15', () {
    const value = 1.145;
    final rounded = value.toStringWithRoundedDecimals(2);
    expect(rounded, 1.15);
  });
}

extension DoubleRounding on double {
  /// Rounds this double to [places] decimal places using half-up (>=5) rounding.
  double toStringWithRoundedDecimals(int places) {
    if (places < 0) {
      throw ArgumentError('Decimal places must be non-negative');
    }
    final str = toString();
    if (!str.contains('.')) return this;

    final parts = str.split('.');
    var integerPart = parts[0];
    var fractionPart = parts[1];

    // Handle zero decimal places separately
    if (places == 0) {
      fractionPart = fractionPart.padRight(1, '0');
      final shouldRound = int.parse(fractionPart[0]) >= 5;
      var intPart = int.parse(integerPart);
      intPart += shouldRound ? (this.isNegative ? -1 : 1) : 0;
      return intPart.toDouble();
    }

    // Ensure we have at least places+1 digits to look at
    fractionPart = fractionPart.padRight(places + 1, '0');
    final digitToRound = int.parse(fractionPart[places]);
    var truncated = fractionPart.substring(0, places);

    if (digitToRound >= 5) {
      // Round up the truncated fraction
      final fracInt = BigInt.parse(truncated) + BigInt.one;
      if (fracInt.toString().length > truncated.length) {
        // Overflow into the integer part (e.g. 0.999 -> 1.00)
        final intPart = int.parse(integerPart) + (this.isNegative ? -1 : 1);
        integerPart = intPart.toString();
        truncated = ''.padLeft(places, '0');
      } else {
        truncated = fracInt.toString().padLeft(places, '0');
      }
    }

    final resultStr = '$integerPart.$truncated';
    return double.parse(resultStr);
  }
}