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| -- Date functions for use by other modules.
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| -- I18N and time zones are not supported.
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| local MINUS = '−' -- Unicode U+2212 MINUS SIGN
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| local floor = math.floor
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|
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| local Date, DateDiff, diffmt -- forward declarations
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| local uniq = { 'unique identifier' }
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|
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| local function is_date(t)
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| -- The system used to make a date read-only means there is no unique
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| -- metatable that is conveniently accessible to check.
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| return type(t) == 'table' and t._id == uniq
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| end
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|
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| local function is_diff(t)
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| return type(t) == 'table' and getmetatable(t) == diffmt
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| end
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|
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| local function _list_join(list, sep)
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| return table.concat(list, sep)
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| end
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|
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| local function collection()
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| -- Return a table to hold items.
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| return {
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| n = 0,
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| add = function (self, item)
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| self.n = self.n + 1
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| self[self.n] = item
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| end,
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| join = _list_join,
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| }
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| end
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|
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| local function strip_to_nil(text)
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| -- If text is a string, return its trimmed content, or nil if empty.
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| -- Otherwise return text (convenient when Date fields are provided from
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| -- another module which may pass a string, a number, or another type).
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| if type(text) == 'string' then
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| text = text:match('(%S.-)%s*$')
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| end
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| return text
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| end
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|
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| local function is_leap_year(year, calname)
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| -- Return true if year is a leap year.
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| if calname == 'Julian' then
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| return year % 4 == 0
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| end
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| return (year % 4 == 0 and year % 100 ~= 0) or year % 400 == 0
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| end
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|
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| local function days_in_month(year, month, calname)
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| -- Return number of days (1..31) in given month (1..12).
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| if month == 2 and is_leap_year(year, calname) then
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| return 29
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| end
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| return ({ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 })[month]
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| end
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|
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| local function h_m_s(time)
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| -- Return hour, minute, second extracted from fraction of a day.
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| time = floor(time * 24 * 3600 + 0.5) -- number of seconds
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| local second = time % 60
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| time = floor(time / 60)
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| return floor(time / 60), time % 60, second
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| end
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|
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| local function hms(date)
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| -- Return fraction of a day from date's time, where (0 <= fraction < 1)
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| -- if the values are valid, but could be anything if outside range.
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| return (date.hour + (date.minute + date.second / 60) / 60) / 24
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| end
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|
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| local function julian_date(date)
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| -- Return jd, jdz from a Julian or Gregorian calendar date where
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| -- jd = Julian date and its fractional part is zero at noon
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| -- jdz = same, but assume time is 00:00:00 if no time given
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| -- http://www.tondering.dk/claus/cal/julperiod.php#formula
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| -- Testing shows this works for all dates from year -9999 to 9999!
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| -- JDN 0 is the 24-hour period starting at noon UTC on Monday
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| -- 1 January 4713 BC = (-4712, 1, 1) Julian calendar
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| -- 24 November 4714 BC = (-4713, 11, 24) Gregorian calendar
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| local offset
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| local a = floor((14 - date.month)/12)
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| local y = date.year + 4800 - a
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| if date.calendar == 'Julian' then
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| offset = floor(y/4) - 32083
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| else
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| offset = floor(y/4) - floor(y/100) + floor(y/400) - 32045
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| end
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| local m = date.month + 12*a - 3
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| local jd = date
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