--[[ 

Calculates the Gregorian date of a recurring holiday that varies year-to-year, but follows the rule "Nth [day of week] of [month]"

  "month" = month number (1 to 12)
  "weeknumber" = number of week (1 to 4, or -1 to mean "last")
  "dayofweek" = number that represents the day of the week, where 1 = Sunday and 7 = Saturday
  "year" = Gregorian calendar year
               
 ]]

require('strict')

local p = {}

function p.calculate(frame)
	local args = frame.args

	local ONE_DAY = 86400 -- number of seconds in one day
	local year = tonumber(args.year)
	local month = tonumber(args.month)
	local weeknumber = tonumber(args.weeknumber)
	local dayofweek = tonumber(args.dayofweek)

	local date = os.time{year=year, month=month, day=1}
	local dateparts = os.date("*t", date)
	
	if weeknumber > 0 then
		-- find the first [dayofweek] of this month
		while dateparts["wday"] ~= dayofweek do
			date = date + ONE_DAY
			dateparts = os.date("*t", date)
		end

		-- add the correct number of weeks
		if weeknumber > 1 then
			date = date + ((weeknumber - 1) * (7 * ONE_DAY))
		end
	else
		-- find the first day of the next month
		while dateparts["month"] == month do
			date = date + ONE_DAY
			dateparts = os.date("*t", date)
		end

		-- go back one day to get the last day of the month we want
		date = date - ONE_DAY
		dateparts = os.date("*t", date)

		-- go backwards until we find the right day of week
		while dateparts["wday"] ~= dayofweek do
			date = date - ONE_DAY
			dateparts = os.date("*t", date)
		end
	end
	local result = os.date("%Y-%m-%d", date)
	return result

end

return p