Eric Lippert's challenge “comma-quibbling”, best answer?

Question

I wanted to bring this challenge to the attention of the stackoverflow community. The original problem and answers are here. BTW, if you did not follow it before, you should

Answer 1

This isn't brilliantly readable, but it scales well up to tens of millions of strings. I'm developing on an old Pentium 4 workstation and it does 1,000,000 strings of average length 8 in about 350ms.

public static string CreateLippertString(IEnumerable strings)
{
    char[] combinedString;
    char[] commaSeparator = new char[] { ',', ' ' };
    char[] andSeparator = new char[] { ' ', 'A', 'N', 'D', ' ' };

    int totalLength = 2;  //'{' and '}'
    int numEntries = 0;
    int currentEntry = 0;
    int currentPosition = 0;
    int secondToLast;
    int last;
    int commaLength= commaSeparator.Length;
    int andLength = andSeparator.Length;
    int cbComma = commaLength * sizeof(char);
    int cbAnd = andLength * sizeof(char);

    //calculate the sum of the lengths of the strings
    foreach (string s in strings)
    {
        totalLength += s.Length;
        ++numEntries;
    }

    //add to the total length the length of the constant characters
    if (numEntries >= 2)
        totalLength += 5;  // " AND "

    if (numEntries > 2)
        totalLength += (2 * (numEntries - 2)); // ", " between items

    //setup some meta-variables to help later
    secondToLast = numEntries - 2;
    last = numEntries - 1;

    //allocate the memory for the combined string
    combinedString = new char[totalLength];
    //set the first character to {
    combinedString[0] = '{';
    currentPosition = 1;

    if (numEntries > 0)
    {
        //now copy each string into its place
        foreach (string s in strings)
        {
            Buffer.BlockCopy(s.ToCharArray(), 0, combinedString, currentPosition * sizeof(char), s.Length * sizeof(char));
            currentPosition += s.Length;

            if (currentEntry == secondToLast)
            {
                Buffer.BlockCopy(andSeparator, 0, combinedString, currentPosition * sizeof(char), cbAnd);
                currentPosition += andLength;
            }
            else if (currentEntry == last)
            {
                combinedString[currentPosition] = '}'; //set the last character to '}'
                break;  //don't bother making that last call to the enumerator
            }
            else if (currentEntry < secondToLast)
            {
                Buffer.BlockCopy(commaSeparator, 0, combinedString, currentPosition * sizeof(char), cbComma);
                currentPosition += commaLength;
            }

            ++currentEntry;
        }
    }
    else
    {
        //set the last character to '}'
        combinedString[1] = '}';
    }

    return new string(combinedString);
}