local strformat = string.format
local floor = math.floor
local function idiv(n, d)
	return floor(n / d)
end

local c_locale = {
	abday = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
	day = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
	abmon = {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
	mon = {"January", "February", "March", "April", "May", "June",
		"July", "August", "September", "October", "November", "December"};
	am_pm = {"AM", "PM"};
}

--- ISO-8601 week logic
-- ISO 8601 weekday as number with Monday as 1 (1-7)
local function iso_8601_weekday(wday)
	if wday == 1 then
		return 7
	else
		return wday - 1
	end
end
local iso_8601_week do
	-- Years that have 53 weeks according to ISO-8601
	local long_years = {}
	for _, v in ipairs {
		  4,   9,  15,  20,  26,  32,  37,  43,  48,  54,  60,  65,  71,  76,  82,
		 88,  93,  99, 105, 111, 116, 122, 128, 133, 139, 144, 150, 156, 161, 167,
		172, 178, 184, 189, 195, 201, 207, 212, 218, 224, 229, 235, 240, 246, 252,
		257, 263, 268, 274, 280, 285, 291, 296, 303, 308, 314, 320, 325, 331, 336,
		342, 348, 353, 359, 364, 370, 376, 381, 387, 392, 398
	} do
		long_years[v] = true
	end
	local function is_long_year(year)
		return long_years[year % 400]
	end
	function iso_8601_week(self)
		local wday = iso_8601_weekday(self.wday)
		local n = self.yday - wday
		local year = self.year
		if n < -3 then
			year = year - 1
			if is_long_year(year) then
				return year, 53, wday
			else
				return year, 52, wday
			end
		elseif n >= 361 and not is_long_year(year) then
			return year + 1, 1, wday
		else
			return year, idiv(n + 10, 7), wday
		end
	end
end

--- Specifiers
local t = {}
function t:a(locale)
	return "%s", locale.abday[self.wday]
end
function t:A(locale)
	return "%s", locale.day[self.wday]
end
function t:b(locale)
	return "%s", locale.abmon[self.month]
end
function t:B(locale)
	return "%s", locale.mon[self.month]
end
function t:c(locale)
	return "%.3s %.3s%3d %.2d:%.2d:%.2d %d",
		locale.abday[self.wday], locale.abmon[self.month],
		self.day, self.hour, self.min, self.sec, self.year
end
-- Century
function t:C()
	return "%02d", idiv(self.year, 100)
end
function t:d()
	return "%02d", self.day
end
-- Short MM/DD/YY date, equivalent to %m/%d/%y
function t:D()
	return "%02d/%02d/%02d", self.month, self.day, self.year % 100
end
function t:e()
	return "%2d", self.day
end
-- Short YYYY-MM-DD date, equivalent to %Y-%m-%d
function t:F()
	return "%d-%02d-%02d", self.year, self.month, self.day
end
-- Week-based year, last two digits (00-99)
function t:g()
	return "%02d", iso_8601_week(self) % 100
end
-- Week-based year
function t:G()
	return "%d", iso_8601_week(self)
end
t.h = t.b
function t:H()
	return "%02d", self.hour
end
function t:I()
	return "%02d", (self.hour-1) % 12 + 1
end
function t:j()
	return "%03d", self.yday
end
function t:m()
	return "%02d", self.month
end
function t:M()
	return "%02d", self.min
end
-- New-line character ('\n')
function t:n() -- luacheck: ignore 212
	return "\n"
end
function t:p(locale)
	return self.hour < 12 and locale.am_pm[1] or locale.am_pm[2]
end
-- TODO: should respect locale
function t:r(locale)
	return "%02d:%02d:%02d %s",
		(self.hour-1) % 12 + 1, self.min, self.sec,
		self.hour < 12 and locale.am_pm[1] or locale.am_pm[2]
end
-- 24-hour HH:MM time, equivalent to %H:%M
function t:R()
	return "%02d:%02d", self.hour, self.min
end
function t:s()
	return "%d", self:timestamp()
end
function t:S()
	return "%02d", self.sec
end
-- Horizontal-tab character ('\t')
function t:t() -- luacheck: ignore 212
	return "\t"
end
-- ISO 8601 time format (HH:MM:SS), equivalent to %H:%M:%S
function t:T()
	return "%02d:%02d:%02d", self.hour, self.min, self.sec
end
function t:u()
	return "%d", iso_8601_weekday(self.wday)
end
-- Week number with the first Sunday as the first day of week one (00-53)
function t:U()
	return "%02d", idiv(self.yday - self.wday + 7, 7)
end
-- ISO 8601 week number (00-53)
function t:V()
	return "%02d", select(2, iso_8601_week(self))
end
-- Weekday as a decimal number with Sunday as 0 (0-6)
function t:w()
	return "%d", self.wday - 1
end
-- Week number with the first Monday as the first day of week one (00-53)
function t:W()
	return "%02d", idiv(self.yday - iso_8601_weekday(self.wday) + 7, 7)
end
-- TODO make t.x and t.X respect locale
t.x = t.D
t.X = t.T
function t:y()
	return "%02d", self.year % 100
end
function t:Y()
	return "%d", self.year
end
-- TODO timezones
function t:z() -- luacheck: ignore 212
	return "+0000"
end
function t:Z() -- luacheck: ignore 212
	return "GMT"
end
-- A literal '%' character.
t["%"] = function(self) -- luacheck: ignore 212
	return "%%"
end

local function strftime(format_string, timetable)
	return (string.gsub(format_string, "%%([EO]?)(.)", function(locale_modifier, specifier)
		local func = t[specifier]
		if func then
			return strformat(func(timetable, c_locale))
		else
			error("invalid conversation specifier '%"..locale_modifier..specifier.."'", 3)
		end
	end))
end

local function asctime(timetable)
	-- Equivalent to the format string "%c\n"
	return strformat(t.c(timetable, c_locale)) .. "\n"
end

return {
	strftime = strftime;
	asctime = asctime;
}