CODEWITHSUNDEEP
c++cbytesequencebit
Smaug
Smaug

Reputation: 307

How to put bit sequence into bytes (C/C++)

I have a couple of integers, for example (in binary represetation):

00001000, 01111111, 10000000, 00000001

and I need to put them in sequence to array of bytes(chars), without the leading zeros, like so:

10001111 11110000 0001000

I understand that it is must be done by bit shifting with <<,>> and using binary or |. But I can't find the correct algorithm, can you suggest the best approach?

The integers I need to put there are unsigned long long ints, so the length of one can be anywhere from 1 bit to 8 bytes (64 bits).

Upvotes: 5

Views: 4563

Answers (4)

6502
6502

Reputation: 114569

A simple approach would be having the "current byte" (acc in the following), the associated number of used bits in it (bitcount) and a vector of fully processed bytes (output):

int acc = 0;
int bitcount = 0;
std::vector<unsigned char> output;

void writeBits(int size, unsigned long long x)
{
    while (size > 0)
    {
        // sz = How many bit we're about to copy
        int sz = size;

        // max avail space in acc
        if (sz > 8 - bitcount) sz = 8 - bitcount;

        // get the bits
        acc |= ((x >> (size - sz)) << (8 - bitcount - sz));

        // zero them off in x
        x &= (1 << (size - sz)) - 1;

        // acc got bigger and x got smaller
        bitcount += sz;
        size -= sz;

        if (bitcount == 8)
        {
            // got a full byte!
            output.push_back(acc);
            acc = bitcount = 0;
        }
    }
}

void writeNumber(unsigned long long x)
{
    // How big is it?
    int size = 0;
    while (size < 64 && x >= (1ULL << size))
        size++;
    writeBits(size, x);
}

Note that at the end of the processing you should check if there is any bit still in the accumulator (bitcount > 0) and you should flush them in that case by doing a output.push_back(acc);.

Note also that if speed is an issue then probably using a bigger accumulator is a good idea (however the output will depend on machine endianness) and also that discovering how many bits are used in a number can be made much faster than a linear search in C++ (for example x86 has a special machine language instruction BSR dedicated to this).

Upvotes: 0

Chad
Chad

Reputation: 19052

Convert them to strings, then erase all leading zeros:

#include <iostream>
#include <sstream>
#include <string>
#include <cstdint>

std::string to_bin(uint64_t v)
{
   std::stringstream ss;

   for(size_t x = 0; x < 64; ++x)
   {
       if(v & 0x8000000000000000)
          ss << "1";
       else
          ss << "0";

        v <<= 1;
   }

   return ss.str();
}

void trim_right(std::string& in)
{
   size_t non_zero = in.find_first_not_of("0");

   if(std::string::npos != non_zero)
      in.erase(in.begin(), in.begin() + non_zero);
   else
   {
       // no 1 in data set, what to do?
       in = "<no data>";
   }
}

int main()
{
  uint64_t v1 = 437148234;
  uint64_t v2 = 1;
  uint64_t v3 = 0;

  std::string v1s = to_bin(v1);
  std::string v2s = to_bin(v2);
  std::string v3s = to_bin(v3);

  trim_right(v1s);
  trim_right(v2s);
  trim_right(v3s);

  std::cout << v1s << "\n"
            << v2s << "\n"
            << v3s << "\n";

  return 0;
}

Upvotes: 0

strcat
strcat

Reputation: 5622

You could use a std::bitset:

#include <bitset>
#include <iostream>

int main() {
    unsigned i = 242122534;
    std::bitset<sizeof(i) * 8> bits;
    bits = i;
    std::cout << bits.to_string() << "\n";
}

Upvotes: 4

jkerian
jkerian

Reputation: 17036

There are doubtless other ways of doing it, but I would probably go with the simplest:

std::vector<unsigned char> integers; // Has your list of bytes
integers.push_back(0x02);
integers.push_back(0xFF);
integers.push_back(0x00);
integers.push_back(0x10);
integers.push_back(0x01);
std::string str;                     // Will have your resulting string
for(unsigned int i=0; i < integers.size(); i++)
    for(int j=0; j<8; j++)
        str += ((integers[i]<<j) & 0x80 ? "1" : "0");
std::cout << str << "\n";
size_t begin = str.find("1");
if(begin > 0) str.erase(0,begin);
std::cout << str << "\n";

I wrote this up before you mentioned that you were using long ints or whatnot, but that doesn't actually change very much of this. The mask needs to change, and the j loop variable, but otherwise the above should work.

Upvotes: 1

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