Digits.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace Kelly.Digits;

static class Program
{
    static string Divide(long dividend, long divisor, int maxCycles = 128)
    {
        var builder = new StringBuilder();
        var quotient = dividend / divisor;
        builder.Append(quotient);

        if (0 < maxCycles)
        {
            var indexByRemainder = new Dictionary<long, int>();
            var remainder = dividend % divisor;

            if (remainder != 0)
            {
                var remainingCycles = maxCycles;
                var nextDividend = remainder * 10;
                var nextQuotient = nextDividend / divisor;
                var nextRemainder = nextDividend % divisor;
                var index = builder.Length + 1;

                builder.Append('.').Append(nextQuotient);
                indexByRemainder.Add(nextDividend, index);

                while (nextRemainder != 0 && 0 < --remainingCycles)
                {
                    nextDividend = nextRemainder * 10;
                    nextQuotient = nextDividend / divisor;
                    nextRemainder = nextDividend % divisor;

                    if (indexByRemainder.TryAdd(nextDividend, ++index))
                    {
                        builder.Append(nextQuotient);
                    }
                    else
                    {
                        var firstIndex = indexByRemainder[nextDividend];
                        builder.Insert(firstIndex, '[').Append(']');
                        break;
                    }
                }
            }
        }

        builder.Append("...");
        return builder.ToString();
    }

    static void Main(string[] args)
    {
        Console.WriteLine(Divide(30, 5));
        Console.WriteLine();
        Console.WriteLine(Divide(1, 70));
        Console.WriteLine();
        Console.WriteLine(Divide(24, 2300));
        Console.WriteLine();
        Console.WriteLine(Divide(11, 99900));
        Console.WriteLine(Divide(1, 30));
        Console.WriteLine(Divide(1, 9));
        Console.WriteLine();
    }
}