Module:Sandbox/trappist the monk/check digit explained

require('strict');

local function err_msg_make (msg) return '

error: ' .. msg .. ''end

local function is_valid_isxn_13 (isxn_str) local temp=0; isxn_str = ; -- 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 correctend

local function check_digit_calc (id_str) local temp=0; id_str = ; -- make a table of byte values '0' → 0x30 .. '9' → 0x39 for i, v in ipairs(id_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 temp = 10 - (temp % 10); -- subtract sum modulo 10 from 10 to get the check digit return (10

temp) and 0 or temp; -- if the result is 10, return 0; return result elseend

local function main (frame) local id = frame.args[1]; -- get the id string local id_str = id; -- copy that we will modify to do check digit calculations local check_digit; if id_str:match ('%D') then -- look for characters that are not digits return err_msg_make ('id has non-digit characters: ' .. id_str); end if

id_str then -- make sure id is not the empty string return err_msg_make ('id is empty'); end if 12 < id_str:len then -- make sure that the id is no longer than 12 digits return err_msg_make ('id has too many characters (' .. id_str:len .. '): ' .. id_str); end

if 0

tonumber(id_str) then -- make sure that the id is not 0 return err_msg_make ('id may not be zero'); end

id_str = string.rep ('0', 12-id:len) .. id; -- left fill with 0s to a string length of 12 check_digit = check_digit_calc (id_str); -- calculate the check digit if is_valid_isxn_13 (id_str .. check_digit) then -- append check digit to 12-digit id and validate return id .. '.' .. check_digit; -- return original id with dot-separated check digit else return err_msg_make ('calculation failure: ' .. check_digit) -- did not validate endend

return