-- This template is a copy of the ISXN validation code from [[Module:Citation/CS1]] -- which allows for validating ISBN, ISMN, and ISSN without invoking a citation template  local p = {}  --[[--------------------------< IS _ V A L I D _ I S X N >-----------------------------------------------------  ISBN-10 and ISSN validator code calculates checksum across all isbn/issn digits including the check digit. ISBN-13 is checked in check_isbn(). If the number is valid the result will be 0. Before calling this function, issbn/issn must be checked for length and stripped of dashes, spaces and other non-isxn characters.  ]]  local function is_valid_isxn (isxn_str, len)  local temp = 0;  isxn_str = { isxn_str:byte(1, len) }; -- make a table of byte values '0' → 0x30 .. '9' → 0x39, 'X' → 0x58  len = len+1; -- adjust to be a loop counter  for i, v in ipairs( isxn_str ) do -- loop through all of the bytes and calculate the checksum  if v == string.byte( "X" ) then -- if checkdigit is X (compares the byte value of 'X' which is 0x58)  temp = temp + 10*( len - i ); -- it represents 10 decimal  else  temp = temp + tonumber( string.char(v) )*(len-i);  end  end  return temp % 11 == 0; -- returns true if calculation result is zero end   --[[--------------------------< IS _ V A L I D _ I S X N _ 1 3 >----------------------------------------------  ISBN-13 and ISMN validator code calculates checksum across all 13 isbn/ismn digits including the check digit. If the number is valid, the result will be 0. Before calling this function, isbn-13/ismn must be checked for length and stripped of dashes, spaces and other non-isxn-13 characters.  ]]  local function is_valid_isxn_13 (isxn_str)  local temp=0;   isxn_str = { isxn_str:byte(1, 13) }; -- make a table of byte values '0' → 0x30 .. '9' → 0x39  for i, v in ipairs( isxn_str ) do  temp = temp + (3 - 2*(i % 2)) * tonumber( string.char(v) ); -- multiply odd index digits by 1, even index digits by 3 and sum; includes check digit  end  return temp % 10 == 0; -- sum modulo 10 is zero when isbn-13/ismn is correct end  --[[--------------------------< C H E C K _ I S B N >------------------------------------------------------------  Determines whether an ISBN string is valid  ]]  local function check_isbn( isbn_str, error_string )  if nil ~= isbn_str:match("[^%s-0-9X]") then -- fail if isbn_str contains anything but digits, hyphens, or the uppercase X  return error_string;  end  isbn_str = isbn_str:gsub( "-", "" ):gsub( " ", "" ); -- remove hyphens and spaces  local len = isbn_str:len();   if len ~= 10 and len ~= 13 then  return error_string;  end   if len == 10 then  if isbn_str:match( "^%d*X?$" ) == nil then  return error_string;  end  return is_valid_isxn(isbn_str, 10) and '' or error_string;  else  local temp = 0;  if isbn_str:match( "^97[89]%d*$" ) == nil then -- isbn13 begins with 978 or 979; ismn begins with 979  return error_string;  end  return is_valid_isxn_13 (isbn_str) and '' or error_string;  end end  --[[--------------------------< C H E C K _ I S M N >------------------------------------------------------------  Determines whether an ISMN string is valid. Similar to isbn-13, ismn is 13 digits begining 979-0-... and uses the same check digit calculations. See http://www.ismn-international.org/download/Web_ISMN_Users_Manual_2008-6.pdf section 2, pages 9–12.  ]]  local function check_ismn (id, error_string)  local text;  local valid_ismn = true;   id=id:gsub( "[%s-–]", "" ); -- strip spaces, hyphens, and endashes from the ismn   if 13 ~= id:len() or id:match( "^9790%d*$" ) == nil then -- ismn must be 13 digits and begin 9790  valid_ismn = false;  else  valid_ismn=is_valid_isxn_13 (id); -- validate ismn  end   return valid_ismn and '' or error_string end  --[[--------------------------< I S S N >----------------------------------------------------------------------  Validate and format an issn. This code fixes the case where an editor has included an ISSN in the citation but has separated the two groups of four digits with a space. When that condition occurred, the resulting link looked like this:   |issn=0819 4327 gives: [http://www.worldcat.org/issn/0819 4327 0819 4327] -- can't have spaces in an external link   This code now prevents that by inserting a hyphen at the issn midpoint. It also validates the issn for length and makes sure that the checkdigit agrees with the calculated value. Incorrect length (8 digits), characters other than 0-9 and X, or checkdigit / calculated value mismatch will all cause a check issn error message.  ]]  local function check_issn(id, error_string)  local issn_copy = id; -- save a copy of unadulterated issn; use this version for display if issn does not validate  local text;  local valid_issn = true;   if not id:match ('^%d%d%d%d%-%d%d%d[%dX]$') then  return error_string;  end   id=id:gsub( "[%s-–]", "" ); -- strip spaces, hyphens, and endashes from the issn   if 8 ~= id:len() or nil == id:match( "^%d*X?$" ) then -- validate the issn: 8 digits long, containing only 0-9 or X in the last position  valid_issn=false; -- wrong length or improper character  else  valid_issn=is_valid_isxn(id, 8); -- validate issn  end   return valid_issn and '' or error_string end   ------------------------------< E N T R Y P O I N T S >--------------------------------------------------====  function p.check_isbn(frame)  return check_isbn(frame.args[1] or frame:getParent().args[1], frame.args['error'] or frame:getParent().args['error'] or 'error') end  function p.check_ismn(frame)  return check_ismn(frame.args[1] or frame:getParent().args[1], frame.args['error'] or frame:getParent().args['error'] or 'error') end  function p.check_issn(frame)  return check_issn(frame.args[1] or frame:getParent().args[1], frame.args['error'] or frame:getParent().args['error'] or 'error') end  return p