CRC calculation - bit by bit - C++

Question

I need to create a program, that calculates CRC from file. It needs to be done bit by bit.

The way I would like to read a file:

unsigned char byte;
ifstream file;
bool result;
int number;

file.open("test.txt", ios::binary);

while(true)
{
    byte = file.get();
    number = (int)byte;

    result = file.good();
    if(!result) 
    break;  
}

However, I don't know how to read it bit by bit.

My CRC's divisor (called a "polynomial") is 0x04C11DB7 and I need to import 1 new bit from file each time I calculate my buffer.

My idea is to add first 4 bytes to variable (for let's say "1234" it would be 0x31323334), then remove last bit (by moving the number 1 bit to the left), but I don't know how to add a new bit from the next char.

Read a byte and then process every bit in a byte one by one. — Slava
Given your edit, I think that you are asking how to move a 4 byte "window" through a datastream, one bit at a time. Is that your question? — Drew Dormann
You can't read a bit at a time. Read a byte at a time (or more), then process those bytes bit by bit. — Jesper Juhl
That linked example is sort of brutal. Most CRCs are done at the byte-level or above. To get what you want you will have to do some bit masking on bytes. eg: val &0x80 and then val &0x40 and then val &0x20, etc... walking the you are inspecting along one bit at a time. — user4581301
use boost seems to be an universal answer for everything these days... — Timo

Victor Havin Victor Havin · Accepted Answer · 2018-06-07T20:44:42

Do you mean something along these lines? The CRC calculation may vary, but the focus here is on getting the file content "bit by bit".

#include <iostream>
#include <fstream>

int main(int argc, char* argv[])
{
    unsigned char next;
    unsigned long crc = 0;
    if (argc < 2)
        return -1;
    std::fstream fs(argv[1], std::fstream::in);
    while (!fs.bad() && !fs.eof())
    {
        fs >> next;
        for (int i = 0; i < 8; i++)
        {
            crc += next & 1;
            next >>= 1;
        }
    }
    std::cout << "CRC " << crc << std::endl;
    return 0;
}

CRC calculation - bit by bit - C++

2 Answers