Module:Citation/CS1/Date validation

    From Nonbinary Wiki
    < Module:Citation/CS1
    Revision as of 16:13, 26 September 2015 by m>Trappist the monk (Synch from sandbox;)

    Documentation for this module may be created at Module:Citation/CS1/Date validation/doc

    
    local p = {}
    
    --[[--------------------------< I S _ V A L I D _ A C C E S S D A T E >----------------------------------------
    
    returns true if:
    	Wikipedia start date <= accessdate < today + 2 days
    
    Wikipedia start date is 2001-01-15T00:00:00 UTC which is 979516800 seconds after 1970-01-01T00:00:00 UTC (the start of Unix time)
    accessdate is the date provided in |accessdate= at time 00:00:00 UTC
    today is the current date at time 00:00:00 UTC plus 48 hours
    	if today is 2015-01-01T00:00:00 then
    		adding 24 hours gives 2015-01-02T00:00:00 – one second more than today
    		adding 24 hours gives 2015-01-03T00:00:00 – one second more than tomorrow
    
    ]]
    
    local function is_valid_accessdate (accessdate)
    	local lang = mw.getContentLanguage();
    	local good1, good2;
    	local access_ts, tomorrow_ts;												-- to hold unix time stamps representing the dates
    		
    	good1, access_ts = pcall( lang.formatDate, lang, 'U', accessdate );			-- convert accessdate value to unix timesatmp 
    	good2, tomorrow_ts = pcall( lang.formatDate, lang, 'U', 'today + 2 days' );	-- today midnight + 2 days is one second more than all day tomorrow
    	
    	if good1 and good2 then
    		access_ts = tonumber (access_ts);										-- convert to numbers for the comparison
    		tomorrow_ts = tonumber (tomorrow_ts);
    	else
    		return false;															-- one or both failed to convert to unix time stamp
    	end
    	
    	if 979516800 <= access_ts and access_ts < tomorrow_ts then					-- Wikipedia start date <= accessdate < tomorrow's date
    		return true;
    	else
    		return false;															-- accessdate out of range
    	end
    end
    
    --[[--------------------------< G E T _ M O N T H _ N U M B E R >----------------------------------------------
    
    returns a number according to the month in a date: 1 for January, etc.  Capitalization and spelling must be correct. If not a valid month, returns 0
    
    ]]
    
    local function get_month_number (month)
    local long_months = {['January']=1, ['February']=2, ['March']=3, ['April']=4, ['May']=5, ['June']=6, ['July']=7, ['August']=8, ['September']=9, ['October']=10, ['November']=11, ['December']=12};
    local short_months = {['Jan']=1, ['Feb']=2, ['Mar']=3, ['Apr']=4, ['May']=5, ['Jun']=6, ['Jul']=7, ['Aug']=8, ['Sep']=9, ['Oct']=10, ['Nov']=11, ['Dec']=12};
    local temp;
    	temp=long_months[month];
    	if temp then return temp; end				-- if month is the long-form name
    	temp=short_months[month];
    	if temp then return temp; end				-- if month is the short-form name
    	return 0;									-- misspelled, improper case, or not a month name
    end
    
    --[[--------------------------< G E T _ S E A S O N _ N U M B E R >--------------------------------------------
    
    returns a number according to the sequence of seasons in a year: 1 for Winter, etc.  Capitalization and spelling must be correct. If not a valid season, returns 0
    
    ]]
    
    local function get_season_number (season)
    local season_list = {['Winter']=1, ['Spring']=2, ['Summer']=3, ['Fall']=4, ['Autumn']=4}
    local temp;
    	temp=season_list[season];
    	if temp then return temp; end				-- if season is a valid name return its number
    	return 0;									-- misspelled, improper case, or not a season name
    end
    
    --[[--------------------------< I S _ P R O P E R _ N A M E >--------------------------------------------------
    
    returns a non-zero number if date contains a recognized proper name.  Capitalization and spelling must be correct.
    
    ]]
    
    local function is_proper_name (name)
    local name_list = {['Christmas']=1}
    local temp;
    	temp=name_list[name];
    	if temp then return temp; end				-- if name is a valid name return its number
    	return 0;									-- misspelled, improper case, or not a proper name
    end
    
    --[[--------------------------< I S _ V A L I D _ M O N T H _ O R _ S E A S O N >------------------------------
    
    --returns true if month or season is valid (properly spelled, capitalized, abbreviated)
    
    ]]
    
    local function is_valid_month_or_season (month_season)
    	if 0 == get_month_number (month_season) then		-- if month text isn't one of the twelve months, might be a season
    		if 0 == get_season_number (month_season) then	-- not a month, is it a season?
    			return false;								-- return false not a month or one of the five seasons
    		end
    	end
    	return true;
    end
    
    
    --[[--------------------------< I S _ V A L I D _ Y E A R >----------------------------------------------------
    
    Function gets current year from the server and compares it to year from a citation parameter.  Years more than one year in the future are not acceptable.
    
    ]]
    
    local function is_valid_year(year)
    	if not is_set(year_limit) then
    		year_limit = tonumber(os.date("%Y"))+1;			-- global variable so we only have to fetch it once
    	end
    	return tonumber(year) <= year_limit;				-- false if year is in the future more than one year
    end
    
    --[[
    Returns true if day is less than or equal to the number of days in month and year is no farther into the future than next year; else returns false.
    
    Assumes Julian calendar prior to year 1582 and Gregorian calendar thereafter. Accounts for Julian calendar leap years before 1582 and Gregorian leap years after 1582.
    Where the two calendars overlap (1582 to approximately 1923) dates are assumed to be Gregorian.
    ]]
    local function is_valid_date (year, month, day)
    local days_in_month = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    local month_length;
    	if not is_valid_year(year) then	-- no farther into the future than next year
    		return false;
    	end
    	
    	if (2==month) then							-- if February
    		month_length = 28;						-- then 28 days unless
    		if 1582 > tonumber(year) then			-- Julian calendar
    			if 0==(year%4) then
    				month_length = 29;
    			end
    		else									-- Gregorian calendar
    			if (0==(year%4) and (0~=(year%100) or 0==(year%400))) then	-- is a leap year?
    				month_length = 29;				-- if leap year then 29 days in February
    			end
    		end
    	else
    		month_length=days_in_month[month];
    	end
    
    	if tonumber (day) > month_length then
    		return false;
    	end
    	return true;
    end
    
    --[[--------------------------< I S _ V A L I D _ M O N T H _ R A N G E _ S T Y L E >--------------------------
    
    Months in a range are expected to have the same style: Jan–Mar or October–December but not February–Mar or Jul–August. 
    There is a special test for May because it can be either short or long form.
    
    Returns true when style for both months is the same
    
    ]]
    
    local function is_valid_month_range_style (month1, month2)
    local len1 = month1:len();
    local len2 = month2:len();
    	if len1 == len2 then
    		return true;															-- both months are short form so return true
    	elseif 'May' == month1 or 'May'== month2 then
    		return true;															-- both months are long form so return true
    	elseif 3 == len1 or 3 == len2 then
    		return false;															-- months are mixed form so return false
    	else
    		return true;															-- both months are long form so return true
    	end
    end
    
    
    --[[--------------------------< I S _ V A L I D _ M O N T H _ S E A S O N _ R A N G E >------------------------
    
    Check a pair of months or seasons to see if both are valid members of a month or season pair.
    
    Month pairs are expected to be left to right, earliest to latest in time.
    
    Similarly, seasons are also left to right, earliest to latest in time.  There is an oddity with seasons: winter is assigned a value of 1, spring 2, ...,
    fall and autumn 4.  Because winter can follow fall/autumn at the end of a calender year, a special test is made to see if |date=Fall-Winter yyyy (4-1) is the date.
    
    ]]
    
    local function is_valid_month_season_range(range_start, range_end)
    	local range_start_number = get_month_number (range_start);
    	
    	if 0 == range_start_number then								-- is this a month range?
    		local range_start_number = get_season_number (range_start);		-- not a month; is it a season? get start season number
    		local range_end_number = get_season_number (range_end);			-- get end season number
    
    		if 0 ~= range_start_number then							-- is start of range a season?
    			if range_start_number < range_end_number then		-- range_start is a season
    				return true;									-- return true when range_end is also a season and follows start season; else false
    			end
    			if 4 == range_start_number and 1 == range_end_number then	-- special case when range is Fall-Winter or Autumn-Winter
    				return true;
    			end
    		end
    		return false;		-- range_start is not a month or a season; or range_start is a season and range_end is not; or improper season sequence
    	end
    
    	local range_end_number = get_month_number (range_end);		-- get end month number
    	if range_start_number < range_end_number then				-- range_start is a month; does range_start precede range_end?
    		if is_valid_month_range_style (range_start, range_end) then				-- do months have the same style?
    			return true;														-- proper order and same style
    		end
    	end
    	return false;												-- range_start month number is greater than or equal to range end number; or range end isn't a month
    end
    
    --[[--------------------------< C H E C K _ D A T E >----------------------------------------------------------
    
    Check date format to see that it is one of the formats approved by WP:DATESNO or WP:DATERANGE. Exception: only allowed range separator is endash.
    Additionally, check the date to see that it is a real date: no 31 in 30-day months; no 29 February when not a leap year.  Months, both long-form and three
    character abbreviations, and seasons must be spelled correctly. Future years beyond next year are not allowed.
    
    If the date fails the format tests, this function returns false and does not return values for anchor_year and COinS_date.  When this happens, the date parameter is
    used in the COinS metadata and the CITEREF identifier gets its year from the year parameter if present otherwise CITEREF does not get a date value.
    
    Inputs:
    	date_string - date string from date-holding parameters (date, year, accessdate, embargo, archivedate, etc.)
    
    Returns:
    	false if date string is not a real date; else
    	true, anchor_year, COinS_date
    		anchor_year can be used in CITEREF anchors
    		COinS_date is date_string without anchor_year disambiguator if any
    ]]
    local function check_date (date_string)
    	local year;			-- assume that year2, months, and days are not used;
    	local year2=0;		-- second year in a year range
    	local month=0;
    	local month2=0;		-- second month in a month range
    	local day=0;
    	local day2=0;		-- second day in a day range
    	local anchor_year;
    	local coins_date;
    
    	if date_string:match("^%d%d%d%d%-%d%d%-%d%d$") then										-- year-initial numerical year month day format
    		year, month, day=string.match(date_string, "(%d%d%d%d)%-(%d%d)%-(%d%d)");
    		month=tonumber(month);
    		if 12 < month or 1 > month or 1583 > tonumber(year) then return false; end			-- month number not valid or not Gregorian calendar
    		anchor_year = year;
    
    	elseif date_string:match("^%a+ +[1-9]%d?, +[1-9]%d%d%d%a?$") then						-- month-initial: month day, year
    		month, day, anchor_year, year=string.match(date_string, "(%a+)%s*(%d%d?),%s*((%d%d%d%d)%a?)");
    		month = get_month_number (month);
    		if 0 == month then return false; end												-- return false if month text isn't one of the twelve months
    				
    	elseif date_string:match("^%a+ +[1-9]%d?–[1-9]%d?, +[1-9]%d%d%d%a?$") then				-- month-initial day range: month day–day, year; days are separated by endash
    		month, day, day2, anchor_year, year=string.match(date_string, "(%a+) +(%d%d?)–(%d%d?), +((%d%d%d%d)%a?)");
    		if tonumber(day) >= tonumber(day2) then return false; end							-- date range order is left to right: earlier to later; dates may not be the same;
    		month = get_month_number (month);
    		if 0 == month then return false; end												-- return false if month text isn't one of the twelve months
    
    	elseif date_string:match("^[1-9]%d? +%a+ +[1-9]%d%d%d%a?$") then						-- day-initial: day month year
    		day, month, anchor_year, year=string.match(date_string, "(%d%d*)%s*(%a+)%s*((%d%d%d%d)%a?)");
    		month = get_month_number (month);
    		if 0 == month then return false; end												-- return false if month text isn't one of the twelve months
    
    	elseif date_string:match("^[1-9]%d?–[1-9]%d? +%a+ +[1-9]%d%d%d%a?$") then				-- day-range-initial: day–day month year; days are separated by endash
    		day, day2, month, anchor_year, year=string.match(date_string, "(%d%d?)–(%d%d?) +(%a+) +((%d%d%d%d)%a?)");
    		if tonumber(day) >= tonumber(day2) then return false; end							-- date range order is left to right: earlier to later; dates may not be the same;
    		month = get_month_number (month);
    		if 0 == month then return false; end												-- return false if month text isn't one of the twelve months
    
    	elseif date_string:match("^[1-9]%d? +%a+ – [1-9]%d? +%a+ +[1-9]%d%d%d%a?$") then		-- day initial month-day-range: day month - day month year; uses spaced endash
    		day, month, day2, month2, anchor_year, year=date_string:match("(%d%d?) +(%a+) – (%d%d?) +(%a+) +((%d%d%d%d)%a?)");
    		if (not is_valid_month_season_range(month, month2)) or not is_valid_year(year) then return false; end	-- date range order is left to right: earlier to later;
    		month = get_month_number (month);
    		month2 = get_month_number (month2);
    
    	elseif date_string:match("^%a+ +[1-9]%d? – %a+ +[1-9]%d?, +[1-9]%d%d%d?%a?$") then		-- month initial month-day-range: month day – month day, year;  uses spaced endash
    		month, day, month2, day2, anchor_year, year=date_string:match("(%a+) +(%d%d?) – (%a+) +(%d%d?), +((%d%d%d%d)%a?)");
    		if (not is_valid_month_season_range(month, month2)) or not is_valid_year(year) then return false; end
    		month = get_month_number (month);
    		month2 = get_month_number (month2);
    
    	elseif date_string:match("^[1-9]%d? +%a+ +[1-9]%d%d%d – [1-9]%d? +%a+ +[1-9]%d%d%d%a?$") then		-- day initial month-day-year-range: day month year - day month year; uses spaced endash
    		day, month, year, day2, month2, anchor_year, year2=date_string:match("(%d%d?) +(%a+) +(%d%d%d%d?) – (%d%d?) +(%a+) +((%d%d%d%d?)%a?)");
    		if tonumber(year2) <= tonumber(year) then return false; end							-- must be sequential years, left to right, earlier to later
    		if not is_valid_year(year2) or not is_valid_month_range_style(month, month2) then return false; end		-- year2 no more than one year in the future; months same style
    		month = get_month_number (month);
    		month2 = get_month_number (month2);
    
    	elseif date_string:match("^%a+ +[1-9]%d?, +[1-9]%d%d%d – %a+ +[1-9]%d?, +[1-9]%d%d%d%a?$") then		-- month initial month-day-year-range: month day, year – month day, year;  uses spaced endash
    		month, day, year, month2, day2, anchor_year, year2=date_string:match("(%a+) +(%d%d?), +(%d%d%d%d) – (%a+) +(%d%d?), +((%d%d%d%d)%a?)");
    		if tonumber(year2) <= tonumber(year) then return false; end							-- must be sequential years, left to right, earlier to later
    		if not is_valid_year(year2) or not is_valid_month_range_style(month, month2) then return false; end		-- year2 no more than one year in the future; months same style
    		month = get_month_number (month);
    		month2 = get_month_number (month2);
    
    	elseif date_string:match("^%a+ +[1-9]%d%d%d–%d%d%a?$") then								-- special case Winter/Summer year-year (YYYY-YY); year separated with unspaced endash
    		if nil == date_string:match("^Winter") and nil == date_string:match("^Summer") then return false end;	-- 'month' can only be Winter or Summer
    		local century;
    		year, century, anchor_year, year2=date_string:match("%a+ +((%d%d)%d%d)–((%d%d)%a?)");
    		anchor_year=year..'–'..anchor_year;													-- assemble anchor_year from both years
    		year2 = century..year2;																-- add the century to year2 for comparisons
    		if 1 ~= tonumber(year2) - tonumber(year) then return false; end						-- must be sequential years, left to right, earlier to later
    		if not is_valid_year(year2) then return false; end									-- no year farther in the future than next year
    
    	elseif date_string:match("^%a+ +[1-9]%d%d%d–[1-9]%d%d%d%a?$") then						-- special case Winter/Summer year-year; year separated with unspaced endash
    		if nil == date_string:match("^Winter") and nil == date_string:match("^Summer") then return false end;	-- 'month' can only be Winter or Summer
    		year, anchor_year, year2=date_string:match("%a+ +(%d%d%d%d)–((%d%d%d%d)%a?)");
    		anchor_year=year..'–'..anchor_year;													-- assemble anchor_year from both years
    		if 1 ~= tonumber(year2) - tonumber(year) then return false; end						-- must be sequential years, left to right, earlier to later
    		if not is_valid_year(year2) then return false; end									-- no year farther in the future than next year
    
    	elseif date_string:match("^%a+ +[1-9]%d%d%d% – %a+ +[1-9]%d%d%d%a?$") then				-- month/season year - month/season year; separated by spaced endash
    		month, year, month2, anchor_year, year2=date_string:match("(%a+) +(%d%d%d%d) – (%a+) +((%d%d%d%d)%a?)");
    		anchor_year=year..'–'..anchor_year;													-- assemble anchor_year from both years
    		if tonumber(year) >= tonumber(year2) then return false; end							-- left to right, earlier to later, not the same
    		if not is_valid_year(year2) then return false; end									-- no year farther in the future than next year
    		if not((0 ~= get_month_number(month) and 0 ~= get_month_number(month2) and is_valid_month_range_style(month, month2)) or 	-- both must be month year, same month style
    			(0 ~= get_season_number(month) and 0 ~= get_season_number(month2))) then return false; end								-- or season year, not mixed
    
    	elseif date_string:match ("^%a+–%a+ +[1-9]%d%d%d%a?$") then								-- month/season range year; months separated by endash 
    		month, month2, anchor_year, year=date_string:match ("(%a+)–(%a+)%s*((%d%d%d%d)%a?)");
    		if (not is_valid_month_season_range(month, month2)) or (not is_valid_year(year)) then
    			return false;
    		end
    		
    	elseif date_string:match("^%a+ +%d%d%d%d%a?$") then							-- month/season year or proper-name year
    		month, anchor_year, year=date_string:match("(%a+)%s*((%d%d%d%d)%a?)");
    		if not is_valid_year(year) then return false; end
    		if not is_valid_month_or_season (month) and 0 == is_proper_name (month) then return false; end
    
    	elseif date_string:match("^[1-9]%d%d%d?–[1-9]%d%d%d?%a?$") then				-- Year range: YYY-YYY or YYY-YYYY or YYYY–YYYY; separated by unspaced endash; 100-9999
    		year, anchor_year, year2=date_string:match("(%d%d%d%d?)–((%d%d%d%d?)%a?)");
    		anchor_year=year..'–'..anchor_year;										-- assemble anchor year from both years
    		if tonumber(year) >= tonumber(year2) then return false; end				-- left to right, earlier to later, not the same
    		if not is_valid_year(year2) then return false; end						-- no year farther in the future than next year
    
    	elseif date_string:match("^[1-9]%d%d%d–%d%d%a?$") then						-- Year range: YYYY–YY; separated by unspaced endash
    		local century;
    		year, century, anchor_year, year2=date_string:match("((%d%d)%d%d)–((%d%d)%a?)");
    		anchor_year=year..'–'..anchor_year;										-- assemble anchor year from both years
    		if 13 > tonumber(year2) then return false; end							-- don't allow 2003-05 which might be May 2003
    		year2 = century..year2;													-- add the century to year2 for comparisons
    		if tonumber(year) >= tonumber(year2) then return false; end				-- left to right, earlier to later, not the same
    		if not is_valid_year(year2) then return false; end						-- no year farther in the future than next year
    
    	elseif date_string:match("^[1-9]%d%d%d?%a?$") then							-- year; here accept either YYY or YYYY
    		anchor_year, year=date_string:match("((%d%d%d%d?)%a?)");
    		if false == is_valid_year(year) then
    			return false;
    		end
    
    	else
    		return false;											-- date format not one of the MOS:DATE approved formats
    	end
    
    	local result=true;											-- check whole dates for validity; assume true because not all dates will go through this test
    	if 0 ~= year and 0 ~= month and 0 ~= day and 0 == year2 and 0 == month2 and 0 == day2 then		-- YMD (simple whole date)
    		result=is_valid_date(year,month,day);
    
    	elseif 0 ~= year and 0 ~= month and 0 ~= day and 0 == year2 and 0 == month2 and 0 ~= day2 then	-- YMD-d (day range)
    		result=is_valid_date(year,month,day);
    		result=result and is_valid_date(year,month,day2);
    
    	elseif 0 ~= year and 0 ~= month and 0 ~= day and 0 == year2 and 0 ~= month2 and 0 ~= day2 then	-- YMD-md (day month range)
    		result=is_valid_date(year,month,day);
    		result=result and is_valid_date(year,month2,day2);
    
    	elseif 0 ~= year and 0 ~= month and 0 ~= day and 0 ~= year2 and 0 ~= month2 and 0 ~= day2 then	-- YMD-ymd (day month year range)
    		result=is_valid_date(year,month,day);
    		result=result and is_valid_date(year2,month2,day2);
    	end
    	
    	if false == result then return false; end
    																-- if here, then date_string is valid; get coins_date from date_string (leave CITEREF disambiguator) ...
    	coins_date=date_string:match("^(.+%d)%a?$");				-- last character of valid disambiguatable date is always a digit
    	coins_date= mw.ustring.gsub(coins_date, "–", "-" );			-- ... and replace any ndash with a hyphen
    	
    	return true, anchor_year, coins_date;						-- format is good and date string represents a real date
    end	
    
    --[[--------------------------< D A T E S >--------------------------------------------------------------------
    
    Cycle the date-holding parameters in passed table date_parameters_list through check_date() to check compliance with MOS:DATE. For all valid dates, check_date() returns
    true. The |date= parameter test is unique, it is the only date holding parameter from which values for anchor_year (used in CITEREF identifiers) and COinS_date (used in
    the COinS metadata) are derived.  The |date= parameter is the only date-holding parameter that is allowed to contain the no-date keywords "n.d." or "nd" (without quotes).
    
    Unlike most error messages created in this module, only one error message is created by this function. Because all of the date holding parameters are processed serially,
    a single error message is created as the dates are tested.
    
    ]]
    
    --function p.dates(date_parameters_list)
    local function dates(date_parameters_list)
    	local anchor_year;		-- will return as nil if the date being tested is not |date=
    	local COinS_date;		-- will return as nil if the date being tested is not |date=
    	local error_message = "";
    	local mismatch = 0;
    	local good_date = false;
    	
    	for k, v in pairs(date_parameters_list) do										-- for each date-holding parameter in the list
    		if is_set(v) then															-- if the parameter has a value
    			if v:match("^c%. [1-9]%d%d%d?%a?$") then								-- special case for c. year or with or without CITEREF disambiguator - only |date= and |year=
    				local year = v:match("c%. ([1-9]%d%d%d?)%a?");						-- get the year portion so it can be tested
    				if 'date'==k then
    					anchor_year, COinS_date = v:match("((c%. [1-9]%d%d%d?)%a?)");	-- anchor year and COinS_date only from |date= parameter
    					good_date = is_valid_year(year);
    				elseif 'year'==k then
    					good_date = is_valid_year(year);
    				end
    			elseif 'date'==k then													-- if the parameter is |date=
    				if v:match("^n%.d%.%a?") then										-- if |date=n.d. with or without a CITEREF disambiguator
    					good_date, anchor_year, COinS_date = true, v:match("((n%.d%.)%a?)");	--"n.d."; no error when date parameter is set to no date
    				elseif v:match("^nd%a?$") then										-- if |date=nd with or without a CITEREF disambiguator
    					good_date, anchor_year, COinS_date = true, v:match("((nd)%a?)");	--"nd";	no error when date parameter is set to no date
    				else
    					good_date, anchor_year, COinS_date = check_date (v);			-- go test the date
    				end
    			elseif 'access-date'==k then												-- if the parameter is |date=
    				good_date = check_date (v);											-- go test the date
    				if true == good_date then											-- if the date is a valid date
    					good_date = is_valid_accessdate (v);							-- is Wikipedia start date < accessdate < tomorrow's date?
    				end
    			else																	-- any other date-holding parameter
    				good_date = check_date (v);											-- go test the date
    			end
    			if false==good_date then												-- assemble one error message so we don't add the tracking category multiple times
    				if is_set(error_message) then										-- once we've added the first portion of the error message ...
    					error_message=error_message .. ", ";							-- ... add a comma space separator
    				end
    				error_message=error_message .. "&#124;" .. k .. "=";				-- add the failed parameter
    			end
    		end
    	end
    	return anchor_year, COinS_date, error_message, mismatch;					-- and done
    end
    
    --[[--------------------------< Y E A R _ D A T E _ C H E C K >------------------------------------------------
    
    Compare the value provided in |year= with the year value(s) provided in |date=.  This function returns a numeric value:
    	0 - year value does not match the year value in date
    	1 - (default) year value matches the year value in date or one of the year values when date contains two years
    	2 - year value matches the year value in date when date is in the form YYYY-MM-DD and year is disambiguated (|year=YYYYx)
    
    ]]
    
    local function year_date_check (year_string, date_string)
    	local year;
    	local date1;
    	local date2;
    	local result = 1;															-- result of the test; assume that the test passes
    	
    	year = year_string:match ('(%d%d%d%d?)');
    
    	if date_string:match ('%d%d%d%d%-%d%d%-%d%d') and year_string:match ('%d%d%d%d%a') then	--special case where date and year required YYYY-MM-DD and YYYYx
    		date1 = date_string:match ('(%d%d%d%d)');
    		year = year_string:match ('(%d%d%d%d)');
    		if year ~= date1 then
    			result = 0;															-- years don't match
    		else
    			result = 2;															-- years match; but because disambiguated, don't add to maint cat
    		end
    		
    	elseif date_string:match ("%d%d%d%d?.-%d%d%d%d?") then						-- any of the standard formats of date with two three- or four-digit years
    		date1, date2 = date_string:match ("(%d%d%d%d?).-(%d%d%d%d?)");
    		if year ~= date1 and year ~= date2 then
    			result = 0;
    		end
    
    	elseif date_string:match ("%d%d%d%d[%s%-–]+%d%d") then						-- YYYY-YY date ranges
    		local century;
    		date1, century, date2 = date_string:match ("((%d%d)%d%d)[%s%-–]+(%d%d)");
    		date2 = century..date2;													-- convert YY to YYYY
    		if year ~= date1 and year ~= date2 then
    			result = 0;
    		end
    
    	elseif date_string:match ("%d%d%d%d?") then									-- any of the standard formats of date with one year
    		date1 = date_string:match ("(%d%d%d%d?)");
    		if year ~= date1 then
    			result = 0;
    		end
    	end
    	return result;
    end
    
    return {dates = dates, year_date_check = year_date_check}						-- return exported functions