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number-to-word.cs

string ToOnes(long number)
{
    if (number == 0)
    {
        return "";
    }

    var words = new string[] { "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

    return words[number - 1];
}

string ToTeens(long number)
{
    if (number == 0)
    {
        return "";
    }

    var words = new string[] { "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen" };
    return words[number % 10];
}

string ToXty(long number)
{
    if (number == 0)
    {
        return "";
    }

    var words = new string[] { "twenty", "thirty", "fourty", "fifty","sixty", "seventy", "eighty", "ninety" };
    double a = number / 10;
    double b = Math.Floor(a);
    return words[(int)(b - 2)];
}

string ToTens(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 10)
    {
        return ToOnes(number);
    }

    if (number >= 10 && number < 20)
    {
        return ToTeens(number);
    }

    return $"{ToXty(number)} {ToOnes(number % 10)}";
}

string ToHundreds(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 100)
    {
        return ToTens(number);
    }

    var prefix = (long)Math.Floor((double)(number / 100));

    return $"{ToOnes(prefix)} hundred {ToTens(number % 100)}";
}

string ToThousands(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 1000)
    {
        return ToHundreds(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1000));

    return $"{ToOnes(prefix)} thousand {ToHundreds(number % 1000)}";
}

string ToTenThousands(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 10_000)
    {
        return ToThousands(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1000));

    return $"{ToTens(prefix)} thousand {ToHundreds(number % 1000)}";
}

string ToHundredThousands(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 100_000)
    {
        return ToTenThousands(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1000));

    return $"{ToHundreds(prefix)} thousand {ToHundreds(number % 1000)}";
}

string ToMillions(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 1_000_000)
    {
        return ToHundredThousands(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1_000_000));

    return $"{ToTens(prefix)} million {ToHundredThousands(number % 1_000_000)}";
}

string ToTenMillions(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 10_000_000)
    {
        return ToMillions(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1_000_000));

    return $"{ToTens(prefix)} million {ToHundredThousands(number % 1_000_000)}";
}

string ToHundredMillions(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 100_000_000)
    {
        return ToTenMillions(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1_000_000));

    return $"{ToHundreds(prefix)} million {ToHundredThousands(number % 1_000_000)}";
}

string ToBillions(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 1_000_000_000)
    {
        return ToHundredMillions(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1_000_000_000));

    return $"{ToOnes(prefix)} billion {ToHundredMillions(number % 1_000_000_000)}";
}

string ToTenBillions(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 10_000_000_000)
    {
        return ToBillions(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1_000_000_000));

    return $"{ToTeens(prefix)} billion {ToHundredMillions(number % 1_000_000_000)}";
}

string ToHundredBillions(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 100_000_000_000)
    {
        return ToTenBillions(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1_000_000_000));

    return $"{ToHundreds(prefix)} billion {ToHundredMillions(number % 1_000_000_000)}";
}

string ToTrillions(long number)
{
    if (number == 0)
    {
        return "";
    }

    if (number < 1_000_000_000_000)
    {
        return ToHundredBillions(number);
    }

    var prefix = (long)Math.Floor((double)(number / 1_000_000_000_000));

    return $"{ToOnes(prefix)} trillion {ToHundredBillions(number % 1_000_000_000_000)}";
}

while (true)
{
    Console.Write("< ");
    var input = Console.ReadLine();
    try
    {
        var sanitized = input.Replace("_", "");
        var words = ToTrillions(long.Parse(sanitized));
        var filtered = words.Split(' ', StringSplitOptions.RemoveEmptyEntries);
        Console.WriteLine("> " + string.Join(' ', filtered));
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex);
        break;
    }
}