C++ array of 192 bitsets to 4 24char array

Question

I want to convert between the following to formats:

#include <bitset>

std::bitset<4> input[192]; // (4 * 192 bits = 768 bits)

char output[4][24];        // (4 * 24 bytes = 768 bits)

Specifically, I would like to convert each of the four bits, in the bitsets, as separated char[24] arrays.

How would do that the easiest and the fastest way?

Answer 1

I would simply use a nested for loop:

constexpr int kSize1 = 4;
constexpr int kSize2 = 24;
constexpr int kBitsPerChar = 8;

void convert(std::array<std::bitset<kSize1>, kSize2 *  kBitsPerChar> &input,
             std::array<std::array<char, kSize2>, kSize1> &output)
{
    for (int i = 0; i < kSize1; ++i)
    {
        for (int j = 0; j < kSize2; ++j)
        {
            char &c = output[i][j];
            c = 0;
            for (int bit = 0; bit < kBitsPerChar; ++bit)
            {
                c |= input[j * kBitsPerChar + bit][i] << bit;
            }
        }
    }
}

What I did to make it reasonably safe is defining constants for all dimensions, which I use in both the array declarations and the for loop. You should choose more meaningful names than kSize1, depending on your context.

The line that actually "does the work" is quite straight forward: I fetch one bit from the source array, shift it to the correct position within the current char and update the destination array.