Scalable solution for converting numbers to English words

number-to-words-inspired-by-chatgpt.cs

using System.Numerics;

string ToWords(BigInteger number)
{
    var ones = new string[] { "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };
    var teens = new string[] { "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen" };
    var xty = new string[] { "twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninety" };
    var suffixes = new string[] {
        "", "thousand", "million", "billion", "trillion", 
        "quadrillion", "quintillion", "sextillion", "septillion", 
        "octillion", "nonillion", "decillion", "undecillion", 
        "duodecillion", "tredecillion", "quattuordecillion", 
        "quindecillion", "sedecillion", "septemdecillion",
        "octodecillion", "novemdecillion", "vigintillion",
        "unvigintillion", "duovigintillion", "tresvigintillion",
        "quattuorvigintillion", "quinvigintillion", "sesvigintillion",
        "septemvigintillion", "octovigintillion", "novemvigintillion",
        "trigintillion", "untrigintillion", "duotrigintillion",
        "trestrigintillion", "quattuor­trigint­illion", "quintrigintillion",
        "sestrigintillion", "septentrigintillion", "octotrigintillion",
        "noventrigintillion", "quadragintillion"
    };

    if (number >= 1 && number <= 9)
    {
        return ones[(int)number - 1];
    }

    if (number >= 10 && number <= 19)
    {
        return teens[(int)number - 10];
    }

    if (number >= 20 && number <= 99)
    {
        var quotient = (int)Math.Floor((double)number / 10);
        var remainder = (int)number % 10;
        return $@"{xty[quotient - 2]} {(remainder > 0 ? ones[remainder - 1] : "")}";
    }

    if (number >= 100 && number <= 999)
    {
        var quotient = (int)number / 100;
        var remainder = (int)number % 100;
        return $@"{ones[quotient - 1]} hundred {(remainder > 0 ? ToWords(remainder) : "")}";
    }

    for(var i = suffixes.Length - 1;i >= 0; --i)
    {
        var magnitude = BigInteger.Pow(10, 3 * i);
        if (number >= magnitude)
        {
            var quotient = number / magnitude;
            var remainder = number % magnitude;
            return $@"{ToWords(quotient)} {suffixes[i]} {(remainder > 0 ? ToWords(remainder) : "")}";
        }
    }

    return "zero";
}

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