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

Code before:
 * algorithm implementation
 */
namespace sha256 {
/**
 * @brief Returns the character at pos after the input is padded
 * @param input Input string
 * @param pos Position of character to be returned
 * @param padded_input_size Size of the padded input
 * @return char Character at the index pos in the padded string
 */
char get_char(const std::string &input, std::size_t pos,
              size_t padded_input_size) {
    size_t input_size = input.length();
    char ch = 0;
    if (pos < input_size) {
        ch = input[pos];
    } else if (pos == input_size) {
        ch = '\x80';
    } else if (pos < padded_input_size - 8) {
        ch = '\x00';
    } else {
        ch =
            static_cast<char>((input_size * 8 >> (56 - (pos % 56) * 8)) & 0xFF);
    }
    return ch;
}

/**
 * @brief Rotates the bits of a 32-bit unsigned integer
 * @param n Integer to rotate
 * @param rotate Number of bits to rotate
 * @return uint32_t The rotated integer
 */
uint32_t right_rotate(uint32_t n, size_t rotate) {
    return (n >> rotate) | (n << (32 - rotate));
}

/**
 * @brief Computes the final hash array
 * @param padded_input Padded input string
 * @return std::array<uint32_t, 8> The final hash array
 */
std::array<uint32_t, 8> compute_hash(const std::string &input) {
    // Initialize array of hash values with first 32 bits of the fractional
    // parts of the square roots of the first 8 primes 2..19
    std::array<uint32_t, 8> hash = {0x6A09E667, 0xBB67AE85, 0x3C6EF372,
                                    0xA54FF53A, 0x510E527F, 0x9B05688C,
                                    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

Review comment (refactor):
```suggestion
char byte_as_char(const std::size_t in_value, const std::size_t byte_num)
{
    assert(byte_num < 8);
    return static_cast<char>((in_value * 8 >> (byte_num * 8)) & 0xFF);
}

/**
 * @brief Returns the character at pos after the input is padded
 * @param input Input string
 * @param pos Position of character to be returned
 * @param padded_input_size Size of the padded input
 * @return char Character at the index pos in the padded string
 */
char get_char(const std::string &input, std::size_t pos,
              size_t padded_input_size) {
    size_t input_size = input.length();
    if (pos < input_size) {
        return input[pos];
    }
    if (pos == input_size) {
        return '\x80';
    }
    if (pos < padded_input_size - 8) {
        return '\x00';
    }
    assert(pos < padded_input_size);
    return byte_as_char(input_size, padded_input_size-pos-1);
}
```
Others things to consider:
* replace `assert`s with throwing a proper exception (eg. [`std::out_of_range`](https://en.cppreference.com/w/cpp/error/out_of_range) or [`std::invalid_argument`](https://en.cppreference.com/w/cpp/error/invalid_argument)),
* instead of having something like `(in_value * 8 >> (byte_num * 8)) & 0xFF` what do you think about having a separate function like `extract_byte`. I see several advantages:
  - It might be used also in the function `hash_to_string`,
  - one can read _easily_ what happens (the name suggests it),
  -  it makes that code _testable_.

Code after:
 * @brief Functions for the [SHA-256](https://en.wikipedia.org/wiki/SHA-2)
 * algorithm implementation
 */
namespace sha256 {
/**
 * @class Hash
 * @brief Contains hash array and functions to update it and convert it to a
 * hexadecimal string
 */
class Hash {
    // Initialize array of hash values with first 32 bits of the fractional
    // parts of the square roots of the first 8 primes 2..19
    std::array<uint32_t, 8> hash = {0x6A09E667, 0xBB67AE85, 0x3C6EF372,
                                    0xA54FF53A, 0x510E527F, 0x9B05688C,
                                    0x1F83D9AB, 0x5BE0CD19};

 public:
    void update(const std::array<uint32_t, 64> &blocks);
    std::string to_string() const;
};

/**
 * @brief Rotates the bits of a 32-bit unsigned integer
 * @param n Integer to rotate
 * @param rotate Number of bits to rotate
 * @return uint32_t The rotated integer
 */
uint32_t right_rotate(uint32_t n, size_t rotate) {
    return (n >> rotate) | (n << (32 - rotate));
}

/**
 * @brief Updates the hash array
 * @param blocks Message schedule array
 * @return void
 */
void Hash::update(const std::array<uint32_t, 64> &blocks) {
    // 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> round_constants = {
        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};