//upc / gtin validation
function checkGTIN(curFld) {
	if (ChkFldNums(curFld)== false) {
		// UPC not all numbers, return false
		alert ('UPC/GTIN number contains one or more letters.');
		return false;
	}
	var fldval = curFld.value;
	
	// get the last digit for the check number, get the rest for the upc/gtin to verify
	var chkgtin = fldval.substr(0, fldval.length-1);		// the gtin number of fldval / upc passed - the upc minus the check number 
	var chkdigit = fldval.substr(fldval.length-1, fldval.length); 	// the upc/gtin to verify, all the chk digit of fldval
	// length must be 8, 12, 13, 14, or 18 including trailing check number
	if (fldval.length !=8 && fldval.length !=12 && fldval.length !=13 && fldval.length !=14 && fldval.length !=18) {
		// cannot check this upc, invalid format
		alert ('UPC/GTIN number is not the correct length. Also, verify you entered the UPC with the check digit.')
		return false;
	}
	// if 8 digits, check that first digit is 0
	if (fldval.length == 8) {
		if(fldval.substring(0,1) != '0') {
			// must start w/0, return false
			alert ('An 8-digit U.S. UPC-E number must start with a leading 0.')
			return false;
		} else {
			// gtin-8 starts w/0 - valid for US product
			// convert to upc-a
			chkgtin = '0' + UPCEtoA(chkgtin);
			// replace field value
			curFld.value = '0' + chkgtin + chkdigit;
			// done, conversion, check digit is for upc-a, validation will be done w/gtin validation below
		}
	}
	// take gtin code (w/o check digit and get check digit)
	cc = GETGTINCheckDigit(chkgtin)
	
	// compare the checksum number and the final gtin/upc digit
	if (cc == chkdigit) {
		// match
		return true;
	} else {
		// not matched, not correct upc/gtin
		alert ('UPC number and its check digit do not match. Please check the number and re-enter it.');
		return false;
	}
}

// get UPC check digit for UPC w/o digit
function GETGTINCheckDigit(ccode) {
	ccode = ('' + ccode).split('');		// convert number to string if needed
	for (var i=0, z=0, len=ccode.length; i<len; ++i) {
		z += (i%2)? + ccode[i] : ccode[i]*3;
	}
	z=z%10;
	return (z)? 10-z : z;
}

// function converts upc-e 8 digit to standard upc-a
function UPCEtoA(chkgtin) {
	// chkgtin does not include check digit but does include leading 0
	var upca = '';
	switch (chkgtin.substr(chkgtin.length-1, chkgtin.length)) { 
		// 0 - 1st 2 digits after 0 + last digit (not check digit) + 0000 + digits 3 through 5
		case '0' : 
			upca = chkgtin.substr(1,2) + chkgtin.substr(6,1) + '0000' + chkgtin.substr(3,3);
			break; 
		// 1- 1st 2 digits after 0 + last digit (not check digit) + 0000 + digits 3 through 5
		case '1' : 
			upca = chkgtin.substr(1,2) + chkgtin.substr(6,1) + '0000' + chkgtin.substr(3,3);
			break; 
		// 2- 1st 2 digits after 0 + last digit (not check digit) + 0000 + digits 3 through 5
		case '2' : 
			upca = chkgtin.substr(1,2) + chkgtin.substr(6,1) + '0000' + chkgtin.substr(3,3);
			break; 
		// 3 - 1st 3 digits after 0 + 00000 + digits 4 and 5
		case '3' : 
			upca = chkgtin.substr(1,3) + '0000' + chkgtin.substr(4,2);
			break; 
		// 4 - 1st 4 digits after 0 + 00000 + digit 5
		case '4' : 
			upca = chkgtin.substr(1,4) + '00000' + chkgtin.substr(5,1);
			break; 
		// 5,6,7,8,9 - 1st 5 digits after 0 + 0000 + digit 6
		default : 
     		upca = chkgtin.substr(1,5) + '0000' + chkgtin.substr(6,1); 
		} 
		return upca;
}