Convert C++ function to C#

Question

I am trying to port the following C++ function to C#:

QString Engine::FDigest(const QString & input)
{
    if(input.size() != 32) return "";

    int idx[] = {0xe, 0x3, 0x6, 0x8, 0x2},
        mul[] = {2, 2, 5, 4, 3},
        add[] = {0x0, 0xd, 0x10, 0xb, 0x5},
        a, m, i, t, v;

    QString b;
    char tmp[2] = { 0, 0 };

    for(int j = 0; j <= 4; j++)
    {
        a = add[j];
        m = mul[j];
        i = idx[j];

        tmp[0] = input[i].toAscii();
        t = a + (int)(strtol(tmp, NULL, 16));
        v = (int)(strtol(input.mid(t, 2).toLocal8Bit(), NULL, 16));

        snprintf(tmp, 2, "%x", (v * m) % 0x10);
        b += tmp;
    }

    return b;
}

Some of this code is easy to port however I'm having problems with this part:

tmp[0] = input[i].toAscii();
t = a + (int)(strtol(tmp, NULL, 16));
v = (int)(strtol(input.mid(t, 2).toLocal8Bit(), NULL, 16));

snprintf(tmp, 2, "%x", (v * m) % 0x10);

I have found that (int)strtol(tmp, NULL, 16) equals int.Parse(tmp, "x") in C# and snprintf is String.Format, however I'm not sure about the rest of it.

How can I port this fragment to C#?

Answer 1

Edit I have a suspicion that your code actually does a MD5 digest of the input data. See below for a snippet based on that assumption.

Translation steps

A few hints that should work well¹

Q: tmp[0] = input[i].toAscii();

bytes[] ascii = ASCIIEncoding.GetBytes(input);
tmp[0] = ascii[i];

Q: t = a + (int)(strtol(tmp, NULL, 16));

t = a + int.Parse(string.Format("{0}{1}", tmp[0], tmp[1]),
               System.Globalization.NumberStyles.HexNumber);

Q: v = (int)(strtol(input.mid(t, 2).toLocal8Bit(), NULL, 16));

No clue about the toLocal8bit, would need to read Qt documentation...

Q: snprintf(tmp, 2, "%x", (v * m) % 0x10);

{
    string tmptext = ((v*m % 16)).ToString("X2");
    tmp[0] = tmptext[0];
    tmp[1] = tmptext[1];
}

What if ... it's just MD5?

You could try this directly to see whether it achieves what you need:

using System;

public string FDigest(string input)
{
   MD5 md5 = System.Security.Cryptography.MD5.Create();
   byte[] ascii = System.Text.Encoding.ASCII.GetBytes (input);
   byte[] hash  = md5.ComputeHash (ascii);

   // Convert the byte array to hexadecimal string
   StringBuilder sb = new StringBuilder();
   for (int i = 0; i < hash.Length; i++)
       sb.Append (hash[i].ToString ("X2")); // "x2" for lowercase
   return sb.ToString();
}

¹ explicitly not optimized, intended as quick hints; optimize as necessary