indexing
	description: "Durations of date"
	status: "See notice at end of class"
	date: "$Date$"
	revision: "$Revision$"
	access: date

class interface
	DATE_DURATION

create 

	make (y, m, d: INTEGER)
			-- Set year, month and day to y, m and d respectively.
		ensure
			year_set: year = y
			month_set: month = m
			day_set: day = d

	make_by_days (d: INTEGER)
			-- Set day to d.
			-- The duration is definite
		ensure
			day_set: day = d
			definite_duration: definite

feature -- Initialization

	make (y, m, d: INTEGER)
			-- Set year, month and day to y, m and d respectively.
		ensure
			year_set: year = y
			month_set: month = m
			day_set: day = d

	make_by_days (d: INTEGER)
			-- Set day to d.
			-- The duration is definite
		ensure
			day_set: day = d
			definite_duration: definite
	
feature -- Access

	date_default_format_string: STRING
			-- Default output format for dates
			-- (from DATE_CONSTANTS)

	days_in_i_th_month (i, y: INTEGER): INTEGER
			-- Number of days in the i th month at year y
			-- (from DATE_CONSTANTS)
		require -- from DATE_CONSTANTS
			i_large_enough: i >= 1
			i_small_enough: i <= months_in_year

	Days_in_leap_year: INTEGER is 366
			-- Number of days in a leap year
			-- (from DATE_CONSTANTS)

	Days_in_non_leap_year: INTEGER is 365
			-- Number of days in a non-leap year
			-- (from DATE_CONSTANTS)

	Days_in_week: INTEGER is 7
			-- Number of days in a week
			-- (from DATE_CONSTANTS)

	days_text: ARRAY [STRING]
			-- Short text representation of days
			-- (from DATE_CONSTANTS)

	long_days_text: ARRAY [STRING]
			-- Long text representation of days
			-- (from DATE_CONSTANTS)

	long_months_text: ARRAY [STRING]
			-- Long text representation of months
			-- (from DATE_CONSTANTS)

	Max_weeks_in_year: INTEGER is 53
			-- Maximun number of weeks in a year
			-- (from DATE_CONSTANTS)

	Months_in_year: INTEGER is 12
			-- Number of months in year
			-- (from DATE_CONSTANTS)

	months_text: ARRAY [STRING]
			-- Short text representation of months
			-- (from DATE_CONSTANTS)

	zero: DATE_DURATION
			-- Neutral element for "+" and "-"
		ensure -- from GROUP_ELEMENT
			result_exists: Result /= void
		ensure then
			definite: Result.definite
	
feature -- Comparison

	is_equal (other: like Current): BOOLEAN
			-- Are the current object and other equal?
		require -- from ANY
			other_not_void: other /= void
		ensure -- from ANY
			symmetric: Result implies other.is_equal (Current)
			consistent: standard_is_equal (other) implies Result
		ensure then
			result_definition: Result = (year = other.year and then month = other.month and then day = other.day)

	infix "<" (other: like Current): BOOLEAN
			-- Is the current object smaller than other?
			-- It is impossible to compare not definite duration.
		require -- from PART_COMPARABLE
			other_exists: other /= void
		ensure then
			definite_duration: (definite and then other.definite) implies Result = (day < other.day)
			non_definite_duration: (not definite or else not other.definite) implies Result = False

	infix "<=" (other: like Current): BOOLEAN
			-- Is current object less than or equal to other?
			-- (from PART_COMPARABLE)
		require -- from PART_COMPARABLE
			other_exists: other /= void

	infix ">" (other: like Current): BOOLEAN
			-- Is current object greater than other?
			-- (from PART_COMPARABLE)
		require -- from PART_COMPARABLE
			other_exists: other /= void

	infix ">=" (other: like Current): BOOLEAN
			-- Is current object greater than or equal to other?
			-- (from PART_COMPARABLE)
		require -- from PART_COMPARABLE
			other_exists: other /= void
	
feature -- Status report

	canonical (date: DATE): BOOLEAN
			-- Is duration expressed minimally for adding to date, i.e.
			-- 	If addition will yield a date after date, then:
			--		year positive,
			--		month between 0 and `Months_in_year - 1',
			--		day between 0 and (number of days of the month before
			--		the yielded) - 1?
			-- 	If addition will yield a date before date, then:
			--		year negative,
			--		month between `1 - Months_in_year' and 0,
			--		day between (number of days of the month before the
			--		yielded) and 0?
		require
			date_exist: date /= void

	definite: BOOLEAN
			-- Is the duration is independant of the date
			-- on which it applies? (use of day only)?
			-- or not (use of year, month and day)?
		ensure
			result_definition: Result = ((year = 0) and then (month = 0))

	has_origin_date: BOOLEAN
			-- Has `origin date' been set?

	is_leap_year (y: INTEGER): BOOLEAN
			-- Is year y a leap year?
			-- (from DATE_CONSTANTS)

	is_negative: BOOLEAN
			-- Is duration negative?
			-- (from DURATION)

	is_positive: BOOLEAN
			-- Is duration positive?

	is_zero: BOOLEAN
			-- Is duration zero?
			-- (from DURATION)
	
feature -- Status setting

	set_origin_date (d: DATE)
			-- Set origin_date to d.
		ensure
			origin_date_set: origin_date = d
	
feature -- Element change

	prefix "+": like Current
			-- Unary plus
			-- (from DURATION)
		ensure -- from GROUP_ELEMENT
			result_exists: Result /= void
			result_definition: Result.is_equal (Current)

	infix "-" (other: like Current): like Current
			-- Difference with other
			-- (from DURATION)
		require -- from GROUP_ELEMENT
			other_exists: other /= void
		ensure -- from GROUP_ELEMENT
			result_exists: Result /= void
	
feature -- Conversion

	to_canonical (start_date: DATE): like Current
			-- A new duration, equivalent to current one
			-- and canonical for date
		ensure
			canonical_result: Result.canonical (start_date)
			duration_not_changed: equal (start_date + Current, start_date + Result)

	to_date_time: DATE_TIME_DURATION
			-- Date-time version, with a zero time component
		ensure
			result_exists: Result /= void
			year_set: Result.year = year
			month_set: Result.month = month
			day_set: Result.day = day

	to_definite (date: DATE)
			-- Make current duration definite.
		require
			date_exists: date /= void
		ensure
			definite_result: definite
	
feature -- Basic operations

	infix "+" (other: like Current): like Current
			-- Sum of current object with other
		require -- from GROUP_ELEMENT
			other_exists: other /= void
		ensure -- from GROUP_ELEMENT
			result_exists: Result /= void
			commutative: Result.is_equal (other + Current)
		ensure then
			origin_equal: equal (origin_date, Result.origin_date)

	prefix "-": like Current
			-- Unary minus
		ensure -- from GROUP_ELEMENT
			result_exists: Result /= void
			result_definition: (Result + Current).is_equal (zero)
		ensure then
			origin_equal: equal (origin_date, Result.origin_date)
	
feature -- Attribute

	day: INTEGER

	days_count: INTEGER
			-- Number of days in duration
		require
			origin_date_set: has_origin_date

	month: INTEGER

	origin_date: DATE
			-- Origin date of duration

	year: INTEGER
	
feature -- Element Change

	day_add (d: INTEGER)
			-- Add d days to Current.
		ensure
			day_set: day = old day + d

	month_add (m: INTEGER)
			-- Add m months to Current.
		ensure
			month_set: month = old month + m

	set_day (d: INTEGER)
			-- Set day to d.
		require -- from DATE_MEASUREMENT
			d_large_enough: d >= 1
			d_small_enough: d <= days_in_month
		ensure -- from DATE_MEASUREMENT
			day_set: day = d

	set_month (m: INTEGER)
			-- Set month to m.
		require -- from DATE_MEASUREMENT
			m_large_enough: m >= 1
			m_small_enough: m <= months_in_year
			d_small_enough: day <= days_in_i_th_month (m, year)
		ensure -- from DATE_MEASUREMENT
			month_set: month = m

	set_year (y: INTEGER)
			-- Set year to y.
		require -- from DATE_MEASUREMENT
			can_not_cut_29th_feb: day <= days_in_i_th_month (month, y)
		ensure -- from DATE_MEASUREMENT
			year_set: year = y

	year_add (y: INTEGER)
			-- Add y years to Current.
		ensure
			year_set: year = old year + y
	
feature -- Status Report

	days_in_month: INTEGER
			-- Number of days in month 'month'.
			-- (from DATE_MEASUREMENT)
		ensure -- from DATE_MEASUREMENT
			positive_result: Result > 0
	
invariant

	equal_signs: (has_origin_date and then canonical (origin_date)) implies (day >= 0 and month >= 0 and year >= 0) or (day <= 0 and month <= 0 and year <= 0)
		-- from ANY
	reflexive_equality: standard_is_equal (Current)
	reflexive_conformance: conforms_to (Current)
		-- from DURATION
	sign_correctness: is_positive xor is_negative xor is_zero
		-- from GROUP_ELEMENT
	neutral_addition: Current.is_equal (Current + zero)
	self_subtraction: zero.is_equal (Current - Current)

end -- class DATE_DURATION