/** funkce pro prevod ISBN, EAN, ... na 13-ti mistny format  **/  


function check_isbn($value)
{

 	/* Remove spaces, dots, and hyphens from the ISBN */
	$isbn = str_replace(array(' ', '-', '.'), '', $value);
	/* Figure out how long the remaining string is */
	$length = strlen($isbn);
 	/* Set the checkdigit to the last character in the string */
	$checkdigit = substr($isbn, -1);

	/* Take action based on the length of the string */
	switch ($length) {

      	case "10":  // If the isbn is 10 digits, do the following

			/* Check to see if the first 9 digits are numeric */
			if (is_numeric(substr($isbn, -10, 9))) {

			/* Check to see if the checkdigit is a number -- if it is,
				leave it alone.  If it is not, convert it to uppercase. */
			$checkdigit = (!is_numeric($checkdigit)) ? $checkdigit : strtoupper($checkdigit);

			/* Check to see if the checkdigit is X -- if it is, set it to
				10, if it isn't, leave it alone. */
			$checkdigit = ($checkdigit == "X") ? "10" : $checkdigit;

			/* Initialize $sum to 0 */
			$sum = 0;

			/* Cycle through, adding up the numbers */
			for ($i = 0; $i < 9; $i++) {
				$sum = $sum + ($isbn[$i] * (10 - $i));
			}

			/* Add the checkdigit at the end */
			$sum = $sum + $checkdigit;

			/* Does it divide like it is supposed to? */
			$mod = $sum % 11;

			if ($mod == 0) {	// If so, display a success message
				//print "<p>" . $isbn . " is a good ISBN.<br />\n";

        /* Now figure out the 13 digit equivalent */
        $newisbn = substr($isbn, -10, 9);	// drop the checkdigit
				$newisbn = "978" . $newisbn; // Add 978 prefix

				/* Run through the new first 12 digits, weighting them alternately by 1 and 3, sum them up */
            $sum = $newisbn[0] + ($newisbn[1] * 3) + $newisbn[2] + ($newisbn[3] * 3) +
						$newisbn[4] + ($newisbn[5] * 3) + $newisbn[6] + ($newisbn[7] * 3) +
						$newisbn[8] + ($newisbn[9] * 3) + $newisbn[10] + ($newisbn[11] * 3);

				/* Get the remainder when the sum is divided by 10 */
                $mod = $sum % 10;

				/* Calculate the new checkdigit */
                $checkdigit = 10 - $mod;

				/* If $checkdigit is 10, change it to 0. Otherwise leave as-is. */
				//$checkdigit = ($checkdigit == "10") ? "0" : $correct_checkdigit;
        if  ($checkdigit == "10") $checkdigit = 0; 

				/* Append it to the first 12 digits */
				$newisbn = $newisbn . $checkdigit;

				/* Print it out */
				//print "When converted to ISBN-13, " . $isbn . " becomes "  . $newisbn . ".</p>\n";
        $vystup = $newisbn;
        
			} // end if checking to see if the 10 digit isbn was good
			} // end if is_numeric(substr($isbn, -10, 9)

			else {
				//display_error("bad checkdigit");   // If not, display an error
			   $vystup = "error";
      }

		  	break;
   
    
      	case "12":  // If the isbn is 10 digits, do the following

             $chech_num = 10-(($isbn[0]*1+$isbn[1]*3+$isbn[2]*1+$isbn[3]*3+$isbn[4]*1+$isbn[5]*3+$isbn[6]*1+$isbn[7]*3+$isbn[8]*1+$isbn[9]*3+$isbn[10]*1+$isbn[11]*3) % 10);
             $vystup = $isbn.$chech_num; 
             //echo "doplnit na 13 znaku";
             //https://en.wikipedia.org/wiki/International_Standard_Book_Number
		  	break;        
        

		case "13":  // If the isbn is 13 digits, do the following

			/* Run through the new first 12 digits, weighting them  alternately by 1 and 3, sum them up */
			$sum = $isbn[0] + ($isbn[1] * 3) + $isbn[2] + ($isbn[3] * 3) +
						$isbn[4] + ($isbn[5] * 3) + $isbn[6] + ($isbn[7] * 3) +
						$isbn[8] + ($isbn[9] * 3) + $isbn[10] + ($isbn[11] * 3);

			/* Get the remainder when the sum is divided by 10 */
			$mod = $sum % 10;

			/* Calculate the new checkdigit */
			$correct_checkdigit = 10 - $mod;

			/* If $checkdigit is 10, change it to 0. Otherwise leave as-is. */
			$correct_checkdigit = ($correct_checkdigit == "10") ? "0" : $correct_checkdigit;

			/* Compare the two checkdigits */
			if ($checkdigit == $correct_checkdigit) {
            	$vystup = $isbn;
              //print "<p>\n" . $isbn . " is a valid ISBN.</p>\n\n";  // Success
			}

			else {
				$vystup = "error";
        //display_error("bad checkdigit");
			}
			break;
		default:	// If the isbn is neither 10 nor 13 digits, display an error
			$vystup = "error";
      //display_error("invalid length");
			break;
	} // end switch
  
  return $vystup;

}






/** funkce na prevod ISSN na 13-ti mistny format   **/

function issn_convert($value)
{
  //https://cs.wikipedia.org/wiki/International_Standard_Serial_Number       //info
  //https://www.barcodefaq.com/1d/issn/    // postup
  $vystup = "977".$value[0].$value[1].$value[2].$value[3].$value[5].$value[6].$value[7]."00";
  $check = (((($vystup[1]+$vystup[3]+$vystup[5]+$vystup[7]+$vystup[9]+$vystup[11]) * 3) + ($vystup[0]+$vystup[2]+$vystup[4]+$vystup[6]+$vystup[8]+$vystup[10])));
  
  $round = round($check, -1, PHP_ROUND_HALF_UP);
  if ($round <= $check) $round=$round+10;
  $value = $round - $check;
  if ($value == '10') $value = 1; 
  $value = $vystup.$value;
  
  return $value;    
}