C++11 : OpenSSL : SHA256

/*
g++ test.cxx -o test -std=c++11 -lcrypto
*/

#include <cstring>
#include <ctime>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <string>

#include "openssl/sha.h"

std::string sha256_generic(const std::function<void(const void ** data, size_t * length)> & callback) {
    unsigned char hash[SHA256_DIGEST_LENGTH];
    SHA256_CTX ctx;
    SHA256_Init(&ctx);

    const void * data;
    size_t length;
    while (true) {
        callback(&data, &length);
        if (length == 0 || data == nullptr)
            break;

        SHA256_Update(&ctx, data, length);
    }

    SHA256_Final(hash, &ctx);

    std::ostringstream ostr;
    ostr << std::hex << std::setfill('0') << std::right;
    for (int i = 0; i < SHA256_DIGEST_LENGTH; ++i) {
        ostr << std::setw(2) << static_cast<int>(hash[i]);
    }

    return ostr.str();
}

std::string sha256_string(const std::string & str) {
    bool done = false;

    return sha256_generic(
    [&](const void ** data, size_t * length) {
        if (done) {
            *data = nullptr;
            *length = 0;
        } else {
            *data = static_cast<const void*>(str.c_str());
            *length = str.length();
            done = true;
        }
    } );
}

std::string sha256_file(const std::string & filename, size_t buffer_size = 65536) {
    std::ifstream ifile;
    ifile.open(filename, std::ios::in | std::ios::binary);
    if (! ifile.is_open()) {
        return "";
    }

    char * buffer = new char[buffer_size];

    return sha256_generic(
    [&](const void ** data, size_t * length) {
        std::streamsize read = ifile.readsome(buffer, static_cast<std::streamsize>(buffer_size));
        if (read > 0) {
            *data = buffer;
            *length = static_cast<size_t>(read);
        } else {
            *data = nullptr;
            *length = 0;

            delete [] buffer;

            ifile.close();
        }
    } );
}

int main(int argc, char* argv[]) {
    std::cout << sha256_string("") << std::endl;
    std::cout << sha256_string("Hello, World!") << std::endl;

    std::cout << sha256_file("test.cxx") << std::endl;
    std::cout << sha256_file("test") << std::endl;
    return 0;
}