tds-code1.cs

private static char[] digits = new[] 
  {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9' };
 
public static char ToChar(int digit)
{
    return digits[digit];
}

tds-div10.cs

public static int div10(int n)
{
    int q, r;
    q = (n >> 1) + (n >> 2);
    q = q + (q >> 4);
    q = q + (q >> 8);
    q = q + (q >> 16);
    q = q >> 3;
    r = n - q * 10;
    return q + ((r + 6) >> 4);
}

tds-sample1.cs

var digitCount = (int)Math.Ceiling(Math.Log10(value));

tds-v1.cs

public class V1 
{
  public static string ToDecimalString(int value)
  {
    if (value == 0)
      return "0";
    var list = new List<char>(1);
    while (value != 0)
    {
      var digitAsInt = value%10;
      var digitAsChar = (char) ('0' + digitAsInt);
      list.Add(digitAsChar);
      value /= 10;
    }
    list.Reverse();
       
    return new string(list.ToArray());
  }
}

tds-v2.cs

public static string ToDecimalString(int value)
{
  if (value == 0)
    return "0";
  var log10 = Math.Log10(value);
  var digitCount = (int) Math.Ceiling(log10);
  //fix if needed for the values of 10^n (10, 100, etc.)
  if (digitCount == (int)log10)
    ++digitCount;
  var chars = new char[digitCount];
  for (var k = chars.Length - 1; k >= 0 ; --k)
  {
    var mod = value%10;
    chars[k] = (char) ('0' + mod);
    value /= 10;
  }
  return new string(chars);
}

tds-v3.cs

private static int[] powersOf10 = new int[]
                                        {
                                            1, 10, 100, 1000, 10000, 100000, 1000000,
                                            10000000, 100000000, 1000000000
                                        };
public static string ToDecimalString(int value)
{
    if (value == 0)
        return "0";
    var index = Array.BinarySearch(powersOf10, value);
    if (index < 0)
        index = ~index; // not found, get closest index
    else
        ++index;
 
 
    var digitCount = index;
    var chars = new char[digitCount];
    for (var k = chars.Length - 1; k >= 0; --k)
    {
        var mod = value % 10;
        chars[k] = (char) ('0' + mod);
        value /= 10;
    }
    return new string(chars);
}

tds-v4.cs

public class V4
{
  private static readonly int[] powersOf10 = new int[]
                      {
                        1, 10, 100, 1000, 10000, 100000, 1000000,
                        10000000, 100000000, 1000000000
                      };
 
  private static readonly int[] powersOf2 = new int[]
                      {
                        1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 
                        2048, 4096, 8192, 16384, 32768, 65536, 131072, 
                        262144, 524288, 1048576, 2097152, 4194304,
                        8388608, 16777216, 33554432, 67108864,
                        134217728, 268435456, 536870912, 1073741824
                      };
 
 
  private static int[] decimalDigitsByMsb;
 
  static V4()
  {
    decimalDigitsByMsb = new int[powersOf2.Length];
    for (int k = 1; k < powersOf2.Length; k++)
    {
      decimalDigitsByMsb[k] = powersOf2[k - 1].ToString().Length;
    }
  }
 
  public static string ToDecimalString(int value)
  {
    var mostSignificantBit = 0;
    for (int k = powersOf2.Length - 1; k >= 0; k--)
    {
      var bit = (value & powersOf2[k]) != 0;
      if (bit)
      {
        mostSignificantBit = k + 1;
        break;
      }
    }
    if (mostSignificantBit == 0)
      return "0";
 
    int index;
    if (mostSignificantBit >= decimalDigitsByMsb.Length)
      index = decimalDigitsByMsb[decimalDigitsByMsb.Length - 1];
    else
      index = decimalDigitsByMsb[mostSignificantBit];
    if (value >= powersOf10[index])
      index++;
    var digitCount = index;
    var chars = new char[digitCount];
    for (var k = chars.Length - 1; k >= 0; --k)
    {
      var mod = value%10;
      chars[k] = (char) ('0' + mod);
      value /= 10;
    }
    return new string(chars);
  }
}

tds-v5.cs

public static string ToDecimalString(int value)
{
  // ...
  // old code remains the same
  // ...
  var chars = new char[digitCount];
  for (var k = chars.Length - 1; k != (-1); --k)
  {
    // inline the div10 operation
    int q, r;
    q = (value >> 1) + (value >> 2);
    q = q + (q >> 4);
    q = q + (q >> 8);
    q = q + (q >> 16);
    q = q >> 3;
    r = value - q * 10;
    var valueDiv10 = q + ((r + 6) >> 4);
    var mod = value - (valueDiv10 * 10);
    chars[k] = digits[mod];
    value = valueDiv10;
  }
  return new string(chars);
}