# Module:Check isxn

-- 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*$" ) == nil then -- ismn must be 13 digits and begin 9790
valid_ismn = false;
else
valid_ismn=is_valid_isxn_13 (id); -- validate ismn
end  
return valid_ismn and '' or error_string
end  
--issn=0819 4327 gives: 4327 0819 4327 -- can't have spaces in an external link  
This code now prevents that by inserting a hyphen at the issn midpoint. It also validates the issn for length and makes sure that the checkdigit agrees
with the calculated value. Incorrect length (8 digits), characters other than 0-9 and X, or checkdigit / calculated value mismatch will all cause a check issn
error message.  
local function check_issn(id, error_string)
local issn_copy = id; -- save a copy of unadulterated issn; use this version for display if issn does not validate
local text;
local valid_issn = true;  
if not id:match ('^%d%d%d%d%-%d%d%d[%dX]$') 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
return check_isbn(.args[1] or :getParent.args[1], .args['error'] or :getParent.args['error'] or 'error')
end  
function p.check_ismn
return check_ismn(.args[1] or :getParent.args[1], .args['error'] or :getParent.args['error'] or 'error')
end  
function p.check_issn
return check_issn(.args[1] or :getParent.args[1], .args['error'] or :getParent.args['error'] or 'error')
end  
return p