#include <iostream>
#include <openssl/md5.h> /* md5sum */
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <cstring>


/**
* Get the size of the file by its file descriptor
*
* @param file_descript
*/
unsigned long get_size_by_fd(int fd)
{
    struct stat statbuf;
    if (fstat(fd, &statbuf) < 0)
    {
      exit(EXIT_FAILURE);
    }
    return statbuf.st_size;
}

/**
* Print the MD5 sum as hex-digits.
*
* @param MD5 checksum
*/
void print_md5_sum(unsigned char* md5)
{
    for (unsigned int i = 0; i < MD5_DIGEST_LENGTH; i++)
    {
      printf("%02x", md5[i]);
    }
}

/**
* Converts MD5 checksum to hex.
*
* @param MD5 checksum
*/
std::string to_hex(unsigned char* md5)
{
    static const char digits[] = "0123456789abcdef";
    std::string result;

    for (unsigned int i = 0; i < MD5_DIGEST_LENGTH; i++)
    {
        result += digits[md5[i] / MD5_DIGEST_LENGTH];
        result += digits[md5[i] % MD5_DIGEST_LENGTH];
    }

    return result;
}

/**
* Computes a MD5 checksum and compares it to its corresponding calibrated checksum.
*
* @param File path to compute checksum, corresponding calibrated checksum
*/
void md5sum(std::string file_path, unsigned char md5[MD5_DIGEST_LENGTH])
{
  /*
  * Variable initialisation
  */
  int file_descript;
  unsigned long file_size;
  unsigned char* file_buffer;

  /* Checks if the file path can be opened. */
  file_descript = open(file_path.c_str(), O_RDONLY);
  if (file_descript < 0)
  {
    std::cerr << "Unable to open " << file_path << std::endl;
    exit(EXIT_FAILURE);
  }

  /* Grabs the size of file */
  file_size = get_size_by_fd(file_descript);

  /* Generates the buffer */
  file_buffer = (unsigned char*) mmap((caddr_t)0, file_size, PROT_READ, MAP_SHARED, file_descript, 0);

  /* Computes the MD5 checksum to result */
  MD5(file_buffer, file_size, md5);

  /* Removes fime_buffer and file_size */
  munmap(file_buffer, file_size);
}