PhoneNumberFormatter.cs (Fixed)

PhoneNumberFormatter.cs

using System.Text.RegularExpressions;
using System.Linq;
using System.Text;

namespace Sample
{
    public class PhoneNumberFormatter
    {
        private readonly string number;

        public PhoneNumberFormatter(string number)
        {
            this.number = number;
        }

        public string Format()
        {
            var cleanNumber = GetCleanedNumber(number);
            
            if (!string.IsNullOrWhiteSpace(number))
            {
                var reversed = GetReversedNumber(cleanNumber);
                var formatted = getReversedFormatted(reversed);
                var ordered = GetReversedNumber(formatted);
                return WrapValue(ordered);
            }
            else
                return string.Empty;

        }

        private static string WrapValue(char[] ordered)
        {
            return new string(ordered).Trim(new[] { '-' });
        }

        private static string getReversedFormatted(char[] reversedNumber)
        {
            var output = new StringBuilder();
            for (int i = 0; i < reversedNumber.Count(); i++)
            {
                output.Append(reversedNumber[i]);
                if (ShouldBreakGroup(i)) output.Append("-");
            }

            return output.ToString();
        }

        private static bool ShouldBreakGroup(int i)
        {
            // breakes the group every 4 numbers
            return (i + 1) % 4 == 0;
        }

        private static char[] GetReversedNumber(string number)
        {
            return number
                .ToCharArray()
                .Reverse()
                .ToArray();
        }

        private static string GetCleanedNumber(string number) {
            if (number != null)
                number = Regex.Replace(number, "[^0-9]", string.Empty);
            
            return number.Trim();
        }
    }
}