Module:Sandbox/trappist the monk/check digit

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 = { 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

local function check_digit_calc (id_str) local temp=0; id_str = { id_str:byte(1, 12) };											-- 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 else end

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 end end

return {main = main}