Convert Byte Array into Bitset

Question

I have a byte array generated by a random number generator. I want to put this into the STL bitset.

Unfortunately, it looks like Bitset only supports the following constructors:

1. A string of 1's and 0's like "10101011"
2. An unsigned long. (my byte array will be longer)

The only solution I can think of now is to read the byte array bit by bit and make a string of 1's and 0's. Does anyone have a more efficient solution?

Answer 1

you can initialize the bitset from a stream. I can't remember how to wrangle a byte[] into a stream, but...

from http://www.sgi.com/tech/stl/bitset.html

  bitset<12> x;

  cout << "Enter a 12-bit bitset in binary: " << flush;
  if (cin >> x) {
    cout << "x =        " << x << endl;
    cout << "As ulong:  " << x.to_ulong() << endl;
    cout << "And with mask: " << (x & mask) << endl;
    cout << "Or with mask:  " << (x | mask) << endl;
  }

Answer 2

There's a 3rd constructor for bitset<> - it takes no parameters and sets all the bits to 0. I think you'll need to use that then walk through the array calling set() for each bit in the byte array that's a 1.

A bit brute-force, but it'll work. There will be a bit of complexity to convert the byte-index and bit offset within each byte to a bitset index, but it's nothing a little bit of thought (and maybe a run through under the debugger) won't solve. I think it's most likely simpler and more efficient than trying to run the array through a string conversion or a stream.

Answer 3

Something like this? (Not sure if template magic works here as I'd expect. I'm rusty in C++.)

std::bitset bytesToBitset<int numBytes>(byte *data)
{
    std::bitset<numBytes * CHAR_BIT> b;

    for(int i = 0; i < numBytes; ++i)
    {
        byte cur = data[i];
        int offset = i * CHAR_BIT;

        for(int bit = 0; bit < CHAR_BIT; ++bit)
        {
            b[offset] = cur & 1;
            ++offset;   // Move to next bit in b
            cur >>= 1;  // Move to next bit in array
        }
    }

    return b;
}

Answer 4

Guys, I have spent a lot of time by writing a reverse function (bitset -> byte/char array). There it is:

    bitset<SIZE> data = ...

    // bitset to char array
    char current = 0;
    int offset = 0;
    for (int i = 0; i < SIZE; ++i) {
        if (data[i]) { // if bit is true
            current |= (char)(int)pow(2, i - offset * CHAR_BIT); // set that bit to true in current masked value
        } // otherwise let it to be false
        if ((i + 1) % CHAR_BIT == 0) { // every 8 bits
            buf[offset++] = current; // save masked value to buffer & raise offset of buffer
            current = 0; // clear masked value
        }
    }

    // now we have the result in "buf" (final size of contents in buffer is "offset")