-- Examples showing how to use [[Module:Date]]. See the talk page for output.
local Date = require('Module:Date')._Date

local show  -- function defined below to display results

-- A date can be constructed using various input formats.
local function make_a_date()
	show('Make a date')
	local same_dates = {
		Date(2016, 3, 9),
		Date('2016-03-09'),
		Date('2016-3-9'),
		Date('9 March 2016'),
		Date('09 mar 2016'),
		Date('MAR 09 2016'),
		Date('March 9, 2016'),
		Date('March 9, 2016 CE'),
		Date('March 9, 2016 A.D.'),
		Date('3:45 pm 9 March 2016'),
		Date('3:45 p.m. 9 March 2016'),
		Date(2016, 3, 9, 15, 45),
		Date('9 March 2016 15:45'),
	}
	for _, date in ipairs(same_dates) do
		show(nil, date:text() .. ' or ' .. date:text('mdy'))
	end
	local more_dates = {
		Date('4 October 1582', 'Julian'),
		Date('juliandate', 2299160),
		Date('15 October 1582'),
	}
	for _, date in ipairs(more_dates) do
		show(nil, date.dayname .. ' ' .. date:text() .. ' in the ' ..
			date.calendar .. ' calendar is Julian day ' .. date.jd)
	end
end

-- A date can be displayed using various output formats.
local function show_a_date()
	show('Show a date')
	local dates = {
		Date(2016, 3, 9),
		Date('9 March 2016 BC'),
	}
	local format_option = {
		{ 'ymd' },
		{ 'mdy' },
		{ 'dmy' },
		{ 'dmy', 'era=B.C.E.' },
		{ '%A %B %-d, %Y %{era}' },
		{ '%A %B %-d, %Y %{era}', 'era=A.D.' },
		{ 'a %{dayname} in %{monthname} %Y %{era}' },
	}
	for _, t in ipairs(format_option) do
		local format = t[1]
		local option = t[2]
		for _, date in ipairs(dates) do
			show(nil, date:text(format, option))
		end
	end
end

-- When an input date is parsed, its format is stored.
local function keep_format()
	show('Keep format of input date')
	local somedate = Date('March 9, 2016')
	local dates = {
		Date(2016, 3, 9),
		Date('2016-3-9'),
		Date('9 Mar 2016'),
		Date('March 9, 2016'),
		Date(somedate, {day = 1}),  -- somedate with day changed
		somedate + 23,              -- 23 days after somedate
		somedate - '3 months',      -- 3 months before somedate
		Date('3:45 p.m. March 9, 2016'),
	}
	show(nil, 'Format of somedate was ' .. somedate.format)
	for _, date in ipairs(dates) do
		show(nil, date:text() .. ' or ' .. date:text(date.format))
	end
end

-- Using Date to get the current date, or current date and time.
local function current_date()
	local now_date = Date('currentdate')
	local now_datetime = Date('currentdatetime')
	show('Current date showing when this page was last purged',
		now_date:text(),                -- 7 March 2016 (for example)
		now_date:text('mdy'),           -- March 7, 2016
		now_date:text('ymd'),           -- 2016-03-07
		now_date:text('%A %-d %B %-Y'), -- Monday 7 March 2016
		now_datetime:text(),            -- 21:32:45 7 March 2016
		now_datetime:text('hms'),       -- 21:32:45
		now_datetime:text('%c')         -- 9:32 pm 7 March 2016
	)
end

-- Using current to provide default values.
local function current_as_default(year, month, day, hour, minute, second)
	local current = require('Module:Date')._current
	year = year or current.year
	month = month or current.month
	day = day or current.day
	hour = hour or current.hour
	minute = minute or current.minute
	second = second or current.second
	show('Using the current date as a default',
		year, month, day, hour, minute, second)
	-- Alternatively, a date can be constructed with specified items overridden.
	show(nil, Date('currentdatetime', {
		year = year,
		month = month,
		day = day,
		hour = hour,
		minute = minute,
		second = second }):text('%c'))
end

-- Make a date from the day number in a year.
local function date_from_day_of_year()
	-- Example: day 123 in 2015 and in 2016.
	show('Make a date from the day number in a year')
	local offset = 123 - 1  -- 1 January has day-of-year = 1
	for _, year in ipairs({ 2015, 2016 }) do
		local date = Date(year, 1, 1) + offset
		show(nil, 'Day ' .. date.dayofyear .. ' in ' .. year .. ' is ' .. date:text())
	end
end

-- Number of days in a month for Gregorian (default) and Julian calendars.
local function days_in_month(year, month, calendar_name)
	local title = 'Days in month'
	if calendar_name then
		title = title .. ' (' .. calendar_name .. ' calendar)'
	end
	local monthdays = require('Module:Date')._days_in_month
	show(title, monthdays(year, month, calendar_name))
	-- Alternative method, using Date.
	local date = Date(year, month, 1, calendar_name)
	show(nil, date.monthname .. ' ' .. date.year .. ' had ' .. date.monthdays .. ' days')
	show(nil, date:text('%{monthname} %{year} had %{monthdays} days'))  -- same
end

-- Julian day number and date arithmetic.
local function julian_date(year, month, count, calendar_name)
	if calendar_name then
		show('Julian date (' .. calendar_name .. ')')
	else
		show('Julian date (Gregorian)')  -- Gregorian calendar by default
		local date = Date('24 November 4714 BCE')
		show(nil, 'Julian day number was ' .. date.jd .. ' on ' .. date:text('mdy'))
	end
	local first_of_month = Date(year, month, 1, calendar_name)
	for _ = 1, count do
		first_of_month = first_of_month + '1m'  -- next month
		local date = first_of_month - '1d'  -- last day of month
		show(nil,
			'Last day in month (' .. date:text() .. ' ' .. date.calendar ..
			' calendar) had Julian day number ' .. date.jd
		)
	end
end

-- Number of days a date was in the past, or will be in the future.
local function how_long(date_text)
	local now_date = Date('currentdate')
	local then_date = Date(date_text)
	if not then_date then
		show('How long', 'Invalid date: ' .. (date_text or ''))
		return
	end
	local fmt = '%A %B %-d, %-Y %{era} (day of year = %{dayofyear}, serial day = %{gsd}) '
	local info = then_date:text(fmt)
	if then_date == now_date then
		fmt = 'is now (%d day%s)'
	elseif then_date > now_date then
		fmt = 'will be in %d day%s'
	else
		fmt = 'was %d day%s ago'
	end
	local diff = then_date - now_date
	local days = diff.age_days
	local s = days == 1 and '' or 's'
	show('How long', info .. string.format(fmt, days, s))
	local y, m, d = diff:age('ymd')  -- age in years, months, days
	show(nil, string.format('(%d years + %d months + %d days)', y, m, d))
end

-- First and third Fridays in each month of the given year.
local function fridays(year)
	show('First and third Fridays in each month of ' .. year)
	for month = 1, 12 do
		local dates = Date(year, month, 1):list('Friday >=')
		show(nil, dates[1] .. ', ' .. dates[3])
	end
end

-- Next Friday after or before a particular date.
local function next_friday()
	show('Next Friday and last Friday for certain dates')
	local dates = {
		Date('1 Jan 2016'),
		Date('2 Jan 2016'),
		Date('3 Mar 1980'),
	}
	for _, date in ipairs(dates) do
		-- 1 = number of Fridays that are wanted in the list
		local after = date:list('1 Friday')[1]
		local before = date:list('1 Friday <')[1]
		local format = '%A %-d %B %-Y'
		show(nil,
			'For ' .. date:text(format) ..
			', the next is ' .. after:text(format) ..
			', and the last is ' .. before:text(format)
		)
	end
end

-- Results are held in the lines table.
local lines
function show(title, ...)  -- for forward declaration above
	if title then
		if lines[1] then
			table.insert(lines, '')
		end
		table.insert(lines, "'''" .. title .. "'''")
	end
	for _, text in ipairs({...}) do
		table.insert(lines, ':' .. tostring(text))
	end
end

local function main()
	lines = {}
	make_a_date()
	show_a_date()
	keep_format()
	current_date()
	current_as_default(nil, nil, nil, 14, 30, 0)  -- 2:30 pm today
	date_from_day_of_year()
	days_in_month(1900, 2)
	days_in_month(1900, 2, 'Julian')
	julian_date(1899, 11, 4)
	julian_date(-120, 11, 4, 'Julian')
	how_long('29 Feb 2100')  -- an invalid date
	how_long('currentdate')
	how_long('29 Feb 2400')
	how_long('29 Feb 2401 BCE')
	fridays(2016)
	next_friday()
	return table.concat(lines, '\n')
end

return { main = main }