using System;

using System.Collections.Generic;

using System.Linq;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        int space = 7;

        double d = 0.0021989234;

        //var str = fmtx(6, d, 4);

        var str = FitNumberToSize(d, space);

        Console.WriteLine($"[       ] <--- needs to fit in here ({space} characters)");

        write(FitNumberToSize(-0.00000000000123456, space));

        write(FitNumberToSize(0.0021989234, space));

        write(FitNumberToSize(0.0021999999, space));

    }

    static void write(string str)

    {

        Console.Write("[");

        Console.Write(str);

        Console.WriteLine("]");

    }

    public static string FitNumberToSize(double inputNum, int displaySize)

    {

​

        if (displaySize < 1) return "";

​

        double absNum = Math.Abs(inputNum);

​

        //Stop .Net from auto converting to Exp notation when converting to string

        string numStr = inputNum.ToString("0." + new String('#', 339));  

        string absNumStr = absNum.ToString("0." + new String('#', 339));

​

        bool isNegative = inputNum < 0;

        bool hasDecimal = absNumStr.IndexOf('.') != -1;

​

        string fittedNumStr = numStr;

​

        int absNumLen = absNumStr.Length;

​

        //Available space for display in normal format.. accounting for sign and always required decimal point

        int avlSize = isNegative ? displaySize - 2 : displaySize - 1;

​

        //return all # for numbers that are too big for the available space

        int numLenBeforeDec = hasDecimal ? absNumStr.IndexOf('.') : absNumLen;

        if (numLenBeforeDec > avlSize) return new string('#', displaySize);

​

        //add a decimal at the end if not already present

        if (!hasDecimal) fittedNumStr = fittedNumStr + '.';

​

        //add trailing 0 after decimal if space is available

        if (!hasDecimal && fittedNumStr.Length < displaySize) fittedNumStr = fittedNumStr + '0';

​

        //return scientific notation for tiny numbers

        if (absNum < Math.Pow(10, -(avlSize-1)))

        {

            //Available space for exponential notation.. accounting for sign (decimal not required)

            int avlSizeExpNot = isNegative ? displaySize - 1 : displaySize;

​

            //Check if enough space is available to fit exponential notation.. if not, return all #

            char firstNon0char = absNumStr.First<char>(c => c != '.' && c > '0');

            bool twoDigitExp = absNumStr.IndexOf(firstNon0char)-1 > 9;

            int minSizeNeededExpNot = twoDigitExp ? 5 : 4;   //extra space needed for two digit exponent

            if (avlSizeExpNot < minSizeNeededExpNot) return new string('#', displaySize);

​

            //How many numbers can we squeeze in after the decimal?

            int avlDecimalSpaces = avlSizeExpNot - minSizeNeededExpNot - 1;

            avlDecimalSpaces = avlDecimalSpaces < 0 ? 0 : avlDecimalSpaces;

​

            fittedNumStr = inputNum.ToString("0." + new string('#', avlDecimalSpaces) + "E-0", CultureInfo.InvariantCulture);

        }

        else if (absNum < 1)

        {

            fittedNumStr = inputNum.ToString("0." + new string('#', avlSize - 1), CultureInfo.InvariantCulture);

        }

​

​

        //Pad spaces to left if not in Exp notation

        if (!fittedNumStr.ToUpper().Contains("E"))

        {

            if (displaySize > fittedNumStr.Length)

                fittedNumStr = fittedNumStr.PadLeft(fittedNumStr.Length + (displaySize - fittedNumStr.Length));

        }

​

        return fittedNumStr;

​

    }

​

}