SlovenianNumberToWords.cs

using System;

namespace Utils;

public static class SlovenianNumberToWords
{
  public enum Gender
  {
    Masculine,
    Feminine,
  }

  public static String Masculine(this Int64 value) => ToWords(value, Gender.Masculine);
  public static String Feminine(this Int64 value) => ToWords(value, Gender.Feminine);

  public static String ToWords(this Int64 value, Gender gender)
  {
    if (value <= - 1_000_000_000_000 || value >= 1_000_000_000_000) throw new ArgumentOutOfRangeException(nameof(value));
    return Convert(value, gender).Trim();
  }

  //

  private static String Convert(Int64 value, Gender gender)
  {
    if (value < 0) return "minus " + Convert(-value, gender);
    if (value > 999_999_999_999) throw new ArgumentOutOfRangeException(nameof(value));

    if (value == 0) return "nič";
    var parts = new List<String>();

    var milliards = value / 1_000_000_000;
    value %= 1_000_000_000;
    if (milliards > 0)
    {
      var n = milliards == 1 ? String.Empty : Convert_999(milliards, Gender.Feminine);
      var m = (milliards % 10) switch
      {
        1 => "milijarda",
        2 => "milijardi",
        3 => "milijarde",
        4 => "milijarde",
        _ => "milijard",
      };
      parts.Add($"{n} {m}");
    }

    var millions = value / 1_000_000;
    value %= 1_000_000;
    if (millions > 0)
    {
      var n = millions == 1 ? "" : Convert_999(millions, parts.Count > 0 ? Gender.Feminine : Gender.Masculine);
      var m = (millions % 10) switch
      {
        1 => "milijon",
        2 => "milijona",
        3 => parts.Count > 0 ? "milijone" : "milijoni",
        4 => "milijoni",
        _ => "milijonov",
      };
      parts.Add($"{n} {m}");
    }

    var thousands = value / 1_000;
    value %= 1_000;
    if (thousands > 0)
    {
      parts.Add(thousands switch
      {
        1 => "tisoč",
        2 => "dva tisoč",
        3 => "tri tisoč",
        4 => "štiri tisoč",
        _ => Convert_999(thousands, thousands % 10 == 3 ? Gender.Feminine : Gender.Masculine) + " tisoč",
      });
    }

    parts.Add(Convert_999(value, gender));
    return String.Join(" ", parts.ToArray());
  }


  private static String Convert_999(Int64 value, Gender gender)
  {
    // 0
    if (value == 0) return "";

    // 1-19
    if (value <= 19)
    {
      return value switch
      {
        01 => gender == Gender.Masculine ? "en" : "ena",
        02 => gender == Gender.Masculine ? "dva" : "dve",
        03 => gender == Gender.Masculine ? "trije" : "tri",
        04 => gender == Gender.Masculine ? "štirje" : "štiri",
        05 => "pet",
        06 => "šest",
        07 => "sedem",
        08 => "osem",
        09 => "devet",
        10 => "deset",
        11 => "enajst",
        12 => "dvanajst",
        _ => Convert_999(value - 10, Gender.Feminine) + "najst",
      };
    }

    // 20-99
    if (value <= 99)
    {
      var tens = value / 10;
      var ones = value % 10;
      var result = tens == 2 ? "dvajset" : Convert(tens, Gender.Feminine) + "deset";
      if (ones == 0) return result;
      if (ones == 1) return $"enain{result}";
      if (ones == 2) return $"dvain{result}";
      return $"{Convert_999(ones, Gender.Feminine)}in{result}";
    }

    // 100-999
    if (value <= 999)
    {
      return ((value / 100) switch
      {
        1 => "sto " + Convert_999(value % 100, gender),
        2 => "dvesto " + Convert_999(value % 100, gender),
        3 => "tristo " + Convert_999(value % 100, gender),
        4 => "štiristo " + Convert_999(value % 100, gender),
        _ => Convert_999(value / 100, Gender.Masculine) + "sto " + Convert_999(value % 100, gender),
      }).TrimEnd();
    }

    throw new ArgumentOutOfRangeException(nameof(value));
  }

}