Converts a GUID to a decimal number string

GuidToDecimalString.cs

/*
Written in 2013 by Peter O.

Any copyright is dedicated to the Public Domain.
http://creativecommons.org/publicdomain/zero/1.0/
If you like this, you should donate to Peter O.
at: http://upokecenter.com/d/
 */

    public static string GuidToDecimalString(Guid guid) {
      byte[] bytes = guid.ToByteArray();
      int[] tempReg = new int[8];
      // Arrange the GUID to 16-bit integers in least-significant-integer first,
      // to conform to how they would appear if the result
      // guid.ToString(), with the hyphens removed, were converted
      // to a BigDecimal
      unchecked {
        tempReg[7] = ((((int)bytes[3]) & 0xFF) << 8) | (((int)bytes[2]) & 0xFF);
        tempReg[6] = ((((int)bytes[1]) & 0xFF) << 8) | (((int)bytes[0]) & 0xFF);
        tempReg[5] = ((((int)bytes[5]) & 0xFF) << 8) | (((int)bytes[4]) & 0xFF);
        tempReg[4] = ((((int)bytes[7]) & 0xFF) << 8) | (((int)bytes[6]) & 0xFF);
        tempReg[3] = ((((int)bytes[8]) & 0xFF) << 8) | (((int)bytes[9]) & 0xFF);
        tempReg[2] = ((((int)bytes[10]) & 0xFF) << 8) | (((int)bytes[11]) & 0xFF);
        tempReg[1] = ((((int)bytes[12]) & 0xFF) << 8) | (((int)bytes[13]) & 0xFF);
        tempReg[0] = ((((int)bytes[14]) & 0xFF) << 8) | (((int)bytes[15]) & 0xFF);
      }
      int wordCount = tempReg.Length;
      while (wordCount != 0 && tempReg[wordCount - 1] == 0) {
        --wordCount;
      }
      int i = 0;
      // Set "characters" to be big enough for any 128-bit
      // integer (2^128-1 uses 39 decimal digits)
      char[] characters = new char[40];
      // Extract each digit of the 128-bit integer
      while (wordCount != 0) {
        if (wordCount == 1 && tempReg[0] > 0 && tempReg[0] <= 0x7fff) {
          int rest = tempReg[0];
          while (rest != 0) {
            // accurate approximation to rest/10 up to 43698,
            // and rest can go up to 32767
            int newrest = (rest * 26215) >> 18;
            characters[i++] = (char)(0x30 + rest - (newrest * 10));
            rest = newrest;
          }
          break;
        } else if (wordCount == 2 && tempReg[1] > 0 && tempReg[1] <= 0x7fff) {
          int rest = ((int)tempReg[0]) & 0xffff;
          rest |= (((int)tempReg[1]) & 0xffff) << 16;
          while (rest != 0) {
            int newrest = rest / 10;
            characters[i++] = (char)(0x30 + rest - (newrest * 10));
            rest = newrest;
          }
          break;
        } else {
          int wci = wordCount;
          int remainderShort = 0;
          int quo, rem;
          // Divide by 10000
          while ((wci--) > 0) {
            int currentDividend = unchecked((int)((((int)tempReg[wci]) & 0xffff) |
                                                  ((int)remainderShort << 16)));
            quo = currentDividend / 10000;
            tempReg[wci] = (quo & 0xFFFF);
            rem = currentDividend - (10000 * quo);
            remainderShort = rem;
          }
          int remainderSmall = remainderShort;
          // Recalculate word count
          while (wordCount != 0 && tempReg[wordCount - 1] == 0) {
            --wordCount;
          }
          // accurate approximation to rest/10 up to 16388,
          // and rest can go up to 9999
          int newrest = (remainderSmall * 3277) >> 15;
          characters[i++] = (char)(0x30 + remainderSmall - (newrest * 10));
          remainderSmall = newrest;
          newrest = (remainderSmall * 3277) >> 15;
          characters[i++] = (char)(0x30 + remainderSmall - (newrest * 10));
          remainderSmall = newrest;
          newrest = (remainderSmall * 3277) >> 15;
          characters[i++] = (char)(0x30 + remainderSmall - (newrest * 10));
          remainderSmall = newrest;
          characters[i++] = (char)(0x30 + remainderSmall);
        }
      }
      // Reverse the characters, since they were
      // written least-significant digit first
      int half = i >> 1;
      int right = i - 1;
      for (int j = 0; j < half; j++, right--) {
        char value = characters[j];
        characters[j] = characters[right];
        characters[right] = value;
      }
      // Return the generated string
      return new String(characters, 0, i);
    }