Repository: TheAlgorithms/C-Plus-Plus
File: hashing/sha256.cpp (C++)

Code before:
                                    0x1F83D9AB, 0x5BE0CD19};

    // Initialize array of round constants with first 32 bits of the fractional
    // parts of the cube roots of the first 64 primes 2..311
    const std::array<uint32_t, 64> k = {
        0x428A2F98, 0x71374491, 0xB5C0FBCF, 0xE9B5DBA5, 0x3956C25B, 0x59F111F1,
        0x923F82A4, 0xAB1C5ED5, 0xD807AA98, 0x12835B01, 0x243185BE, 0x550C7DC3,
        0x72BE5D74, 0x80DEB1FE, 0x9BDC06A7, 0xC19BF174, 0xE49B69C1, 0xEFBE4786,
        0x0FC19DC6, 0x240CA1CC, 0x2DE92C6F, 0x4A7484AA, 0x5CB0A9DC, 0x76F988DA,
        0x983E5152, 0xA831C66D, 0xB00327C8, 0xBF597FC7, 0xC6E00BF3, 0xD5A79147,
        0x06CA6351, 0x14292967, 0x27B70A85, 0x2E1B2138, 0x4D2C6DFC, 0x53380D13,
        0x650A7354, 0x766A0ABB, 0x81C2C92E, 0x92722C85, 0xA2BFE8A1, 0xA81A664B,
        0xC24B8B70, 0xC76C51A3, 0xD192E819, 0xD6990624, 0xF40E3585, 0x106AA070,
        0x19A4C116, 0x1E376C08, 0x2748774C, 0x34B0BCB5, 0x391C0CB3, 0x4ED8AA4A,
        0x5B9CCA4F, 0x682E6FF3, 0x748F82EE, 0x78A5636F, 0x84C87814, 0x8CC70208,
        0x90BEFFFA, 0xA4506CEB, 0xBEF9A3F7, 0xC67178F2};

    size_t input_size = input.length();  // Input size in bytes

    // Calculate size of the padded input in bytes
    size_t padded_input_size = 0;
    if (input_size % 64 < 56) {
        padded_input_size = input_size + 64 - (input_size % 64);
    } else {
        padded_input_size = input_size + 128 - (input_size % 64);
    }

    // Process message in successive 512-bit (64-byte) chunks
    for (size_t i = 0; i < padded_input_size; i += 64) {
        std::array<uint32_t, 64> blocks{};

        // Copy chunk into first 16 words of the message schedule array
        for (size_t j = 0; j < 16; ++j) {
            blocks[j] = (static_cast<uint8_t>(
                             get_char(input, i + j * 4, padded_input_size))
                         << 24) |
                        (static_cast<uint8_t>(
                             get_char(input, i + j * 4 + 1, padded_input_size))
                         << 16) |
                        (static_cast<uint8_t>(
                             get_char(input, i + j * 4 + 2, padded_input_size))
                         << 8) |
                        static_cast<uint8_t>(
                            get_char(input, i + j * 4 + 3, padded_input_size));
        }

        for (size_t j = 16; j < 64; ++j) {
            uint32_t s0 = right_rotate(blocks[j - 15], 7) ^
                          right_rotate(blocks[j - 15], 18) ^
                          (blocks[j - 15] >> 3);
            uint32_t s1 = right_rotate(blocks[j - 2], 17) ^

Review comment (refactor):
This could be abstracted into a separate function of the type
```cpp
std::size_t compute_padded_size(const std::size_t input_size);
```
What do you think about the computational cost of calling it? Maybe one could simplify the signature of the function `get_char` by removing the argument `padded_input_size` and calling `compute_padded_size` inside it?

When having both of these, one could get rid of the variable `padded_input_size` completely.

Code after:
        0xC24B8B70, 0xC76C51A3, 0xD192E819, 0xD6990624, 0xF40E3585, 0x106AA070,
        0x19A4C116, 0x1E376C08, 0x2748774C, 0x34B0BCB5, 0x391C0CB3, 0x4ED8AA4A,
        0x5B9CCA4F, 0x682E6FF3, 0x748F82EE, 0x78A5636F, 0x84C87814, 0x8CC70208,
        0x90BEFFFA, 0xA4506CEB, 0xBEF9A3F7, 0xC67178F2};

    // Initialize working variables
    auto a = hash[0];
    auto b = hash[1];
    auto c = hash[2];
    auto d = hash[3];
    auto e = hash[4];
    auto f = hash[5];
    auto g = hash[6];
    auto h = hash[7];

    // Compression function main loop
    for (size_t block_num = 0; block_num < 64; ++block_num) {
        const auto s1 =
            right_rotate(e, 6) ^ right_rotate(e, 11) ^ right_rotate(e, 25);
        const auto ch = (e & f) ^ (~e & g);
        const auto temp1 =
            h + s1 + ch + round_constants[block_num] + blocks[block_num];
        const auto s0 =
            right_rotate(a, 2) ^ right_rotate(a, 13) ^ right_rotate(a, 22);
        const auto maj = (a & b) ^ (a & c) ^ (b & c);
        const auto temp2 = s0 + maj;

        h = g;
        g = f;
        f = e;
        e = d + temp1;
        d = c;
        c = b;
        b = a;
        a = temp1 + temp2;
    }

    // Update hash values
    hash[0] += a;
    hash[1] += b;
    hash[2] += c;
    hash[3] += d;
    hash[4] += e;
    hash[5] += f;
    hash[6] += g;
    hash[7] += h;
}

/**
 * @brief Convert the hash to a hexadecimal string
 * @return std::string Final hash value