HALF_EVEN of Dart

main.dart

void main() {
  print(roundHalfEven(4.5)); // 4
  print(roundHalfEven(3.5)); // 4
  print(roundHalfEven(4.500001)); // 5
  print(roundHalfEven(3.500001)); // 4
  print('-------');
  print(roundHalfEven(-4.5)); //- 4
  print(roundHalfEven(-3.5)); //- 4
  print(roundHalfEven(-4.500001)); //- 5
  print(roundHalfEven(-3.500001)); //- 4
}

// Ref: https://cplusplus.com/articles/1UCRko23/
double roundHalfEven(double value, {double epsilon = 0.0000001}) {
  if (value < 0.0) return -roundHalfEven(-value, epsilon: epsilon);
  double ipart = value.toInt().toDouble();

  // If 'value' is exctly halfway between two integers
  if ((value - (ipart + 0.5)).abs() < epsilon) {
    // If 'ipart' is even then return 'ipart'
    if (ipart % 2.0 < epsilon) {
      return ipart;
    }

    // Else return the nearest even integer
    return (ipart + 0.5).ceilToDouble();
  }

  // Otherwise use the usual round to closest
  // (Either symmetric half-up or half-down will do0
  return value.roundToDouble();
}

I use the kotlin to verify it.

import java.math.*

fun main() {
    println(BigDecimal(4.5).setScale(0, RoundingMode.HALF_EVEN)) // 4
    println(BigDecimal(3.5).setScale(0, RoundingMode.HALF_EVEN)) // 4
    println(BigDecimal(4.500001).setScale(0, RoundingMode.HALF_EVEN)) // 5
    println(BigDecimal(3.500001).setScale(0, RoundingMode.HALF_EVEN)) // 4
    println("--------")
    println(BigDecimal(-4.5).setScale(0, RoundingMode.HALF_EVEN)) // -4
    println(BigDecimal(-3.5).setScale(0, RoundingMode.HALF_EVEN)) // -4
    println(BigDecimal(-4.500001).setScale(0, RoundingMode.HALF_EVEN)) // -5
    println(BigDecimal(-3.500001).setScale(0, RoundingMode.HALF_EVEN)) // -4
}

https://dartpad.dev/?id=1300963a6be50844e332c059ef27c2c1