Convert array of bits to an array of bytes

Question

I want to convert an array of bits (bool* bitArray) where the values are 1s and 0s into an array of bytes (unsigned char* byteArray) where the values at each index would be one byte. For ex, index 0~7 in bitArray would go into byteArray[1].

How would I go about doing this? Assuming that I already have an array of bits (but the amount would be subject to change based on the incoming data). I am not worried about having it divisible by 8 because I will just add padding at the end of the bitArray to make it divisible by 8.

Answer 1

Just just use bit shifts or a lookup array and and combine numbers with 1 bit set each with bitwise or for 8 bits at a time:

int main() {
    bool input[] = {
        false, false, false, true, true, true, false, false, false,
        false, false, false, true, true, true, false, false, false,
        false, false, false, true, true, true, false, false, false,
        false, false, false, true, true, true, false, false, false,
    };

    constexpr auto len = sizeof(input) / sizeof(*input);
    constexpr size_t outLen = ((len % 8 == 0) ? 0 : 1) + len / 8;

    uint8_t out[outLen];

    bool* inPos = input;
    uint8_t* outPos = out;

    size_t remaining = len;

    // output bytes where there are all 8 bits available
    for (; remaining >= 8; remaining -= 8, ++outPos)
    {
        uint8_t value = 0;
        for (size_t i = 0; i != 8; ++i, ++inPos)
        {
            if (*inPos)
            {
                value |= (1 << (7 - i));
            }
        }
        *outPos = value;
    }

    if (remaining != 0)
    {
        // output byte that requires padding
        uint8_t value = 0;
        for (size_t i = 0; i != remaining; ++i, ++inPos)
        {
            if (*inPos)
            {
                value |= (1 << (7 - i));
            }
        }
        *outPos = value;
    }

    for (auto v : out)
    {
        std::cout << static_cast<int>(v) << '\n';
    }

    return 0;
}

The rhs of the |= operator could also be replaced with a lookup in the following array, if you consider this simpler to understand:

constexpr uint8_t Bits[8]
{
    0b1000'0000,
    0b0100'0000,
    0b0010'0000,
    0b0001'0000,
    0b0000'1000,
    0b0000'0100,
    0b0000'0010,
    0b0000'0001,
};

...
value |= Bits[i];
...