Question

I’ve seen the following code, taken from the libb64 project.
I’m trying to understand what is the purpose of the while loop within the switch block –

switch (state_in->step)
    {
        while (1)
        {
    case step_a:
            do {
                if (codechar == code_in+length_in)
                {
                    state_in->step = step_a;
                    state_in->plainchar = *plainchar;
                    return plainchar - plaintext_out;
                }
                fragment = (char)base64_decode_value(*codechar++);
            } while (fragment < 0);
            *plainchar    = (fragment & 0x03f) << 2;
    case step_b:
            do {
                if (codechar == code_in+length_in)
                {
                    state_in->step = step_b;
                    state_in->plainchar = *plainchar;
                    return plainchar - plaintext_out;
                }
                fragment = (char)base64_decode_value(*codechar++);
            } while (fragment < 0);
            *plainchar++ |= (fragment & 0x030) >> 4;
            *plainchar    = (fragment & 0x00f) << 4;
    case step_c:
            do {
                if (codechar == code_in+length_in)
                {
                    state_in->step = step_c;
                    state_in->plainchar = *plainchar;
                    return plainchar - plaintext_out;
                }
                fragment = (char)base64_decode_value(*codechar++);
            } while (fragment < 0);
            *plainchar++ |= (fragment & 0x03c) >> 2;
            *plainchar    = (fragment & 0x003) << 6;
    case step_d:
            do {
                if (codechar == code_in+length_in)
                {
                    state_in->step = step_d;
                    state_in->plainchar = *plainchar;
                    return plainchar - plaintext_out;
                }
                fragment = (char)base64_decode_value(*codechar++);
            } while (fragment < 0);
            *plainchar++   |= (fragment & 0x03f);
        }
    }

What can give the while? It seems that anyway, always the switch will perform only one of the cases. Did I miss something?

Thanks.

Answer 1

Although this is Duff’s Device, this version is not about implementing a loop-unrolling optimisation but rather to implement an iterator over a Base64 encoded stream. So you can do something like this:

Base64Stream stream; // the base64 data

char c;

while ((c == stream->NextChar ()) != 0)
{
  // do something with c
}

In the given code, the first switch us used to jump back to where the previous return exited, and then the while (1) allows the iteration loop to continue indefinately. However, there’s no protection against a buffer overrun in this function.

In C#, there is a neater solution to this, the yield statement.