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    // This is a part of Chrono.
    // See README.md and LICENSE.txt for details.

    //! ISO 8601 week.

    use core::fmt;

    use super::internals::{DateImpl, Of, YearFlags};

    /// ISO 8601 week.
    ///
    /// This type, combined with [`Weekday`](../enum.Weekday.html),
    /// constitues the ISO 8601 [week date](./struct.NaiveDate.html#week-date).
    /// One can retrieve this type from the existing [`Datelike`](../trait.Datelike.html) types
    /// via the [`Datelike::iso_week`](../trait.Datelike.html#tymethod.iso_week) method.
    #[derive(PartialEq, Eq, PartialOrd, Ord, Copy, Clone)]
    pub struct IsoWeek {
        // note that this allows for larger year range than `NaiveDate`.
        // this is crucial because we have an edge case for the first and last week supported,
        // which year number might not match the calendar year number.
        ywf: DateImpl, // (year << 10) | (week << 4) | flag
    }

    /// Returns the corresponding `IsoWeek` from the year and the `Of` internal value.
    //
    // internal use only. we don't expose the public constructor for `IsoWeek` for now,
    // because the year range for the week date and the calendar date do not match and
    // it is confusing to have a date that is out of range in one and not in another.
    // currently we sidestep this issue by making `IsoWeek` fully dependent of `Datelike`.
    pub fn iso_week_from_yof(year: i32, of: Of) -> IsoWeek {
        let (rawweek, _) = of.isoweekdate_raw();
        let (year, week) = if rawweek < 1 {
            // previous year
            let prevlastweek = YearFlags::from_year(year - 1).nisoweeks();
            (year - 1, prevlastweek)
        } else {
            let lastweek = of.flags().nisoweeks();
            if rawweek > lastweek {
                // next year
                (year + 1, 1)
            } else {
                (year, rawweek)
            }
        };
        IsoWeek { ywf: (year << 10) | (week << 4) as DateImpl | DateImpl::from(of.flags().0) }
    }

    impl IsoWeek {
        /// Returns the year number for this ISO week.
        ///
        /// # Example
        ///
        /// ~~~~
        /// use chrono::{NaiveDate, Datelike, Weekday};
        ///
        /// let d = NaiveDate::from_isoywd(2015, 1, Weekday::Mon);
        /// assert_eq!(d.iso_week().year(), 2015);
        /// ~~~~
        ///
        /// This year number might not match the calendar year number.
        /// Continuing the example...
        ///
        /// ~~~~
        /// # use chrono::{NaiveDate, Datelike, Weekday};
        /// # let d = NaiveDate::from_isoywd(2015, 1, Weekday::Mon);
        /// assert_eq!(d.year(), 2014);
        /// assert_eq!(d, NaiveDate::from_ymd(2014, 12, 29));
        /// ~~~~
        #[inline]
        pub fn year(&self) -> i32 {
            self.ywf >> 10
        }

        /// Returns the ISO week number starting from 1.
        ///
        /// The return value ranges from 1 to 53. (The last week of year differs by years.)
        ///
        /// # Example
        ///
        /// ~~~~
        /// use chrono::{NaiveDate, Datelike, Weekday};
        ///
        /// let d = NaiveDate::from_isoywd(2015, 15, Weekday::Mon);
        /// assert_eq!(d.iso_week().week(), 15);
        /// ~~~~
        #[inline]
        pub fn week(&self) -> u32 {
            ((self.ywf >> 4) & 0x3f) as u32
        }

        /// Returns the ISO week number starting from 0.
        ///
        /// The return value ranges from 0 to 52. (The last week of year differs by years.)
        ///
        /// # Example
        ///
        /// ~~~~
        /// use chrono::{NaiveDate, Datelike, Weekday};
        ///
        /// let d = NaiveDate::from_isoywd(2015, 15, Weekday::Mon);
        /// assert_eq!(d.iso_week().week0(), 14);
        /// ~~~~
        #[inline]
        pub fn week0(&self) -> u32 {
            ((self.ywf >> 4) & 0x3f) as u32 - 1
        }
    }

    /// The `Debug` output of the ISO week `w` is the same as
    /// [`d.format("%G-W%V")`](../format/strftime/index.html)
    /// where `d` is any `NaiveDate` value in that week.
    ///
    /// # Example
    ///
    /// ~~~~
    /// use chrono::{NaiveDate, Datelike};
    ///
    /// assert_eq!(format!("{:?}", NaiveDate::from_ymd(2015,  9,  5).iso_week()), "2015-W36");
    /// assert_eq!(format!("{:?}", NaiveDate::from_ymd(   0,  1,  3).iso_week()), "0000-W01");
    /// assert_eq!(format!("{:?}", NaiveDate::from_ymd(9999, 12, 31).iso_week()), "9999-W52");
    /// ~~~~
    ///
    /// ISO 8601 requires an explicit sign for years before 1 BCE or after 9999 CE.
    ///
    /// ~~~~
    /// # use chrono::{NaiveDate, Datelike};
    /// assert_eq!(format!("{:?}", NaiveDate::from_ymd(    0,  1,  2).iso_week()),  "-0001-W52");
    /// assert_eq!(format!("{:?}", NaiveDate::from_ymd(10000, 12, 31).iso_week()), "+10000-W52");
    /// ~~~~
    impl fmt::Debug for IsoWeek {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let year = self.year();
            let week = self.week();
            if 0 <= year && year <= 9999 {
                write!(f, "{:04}-W{:02}", year, week)
            } else {
                // ISO 8601 requires the explicit sign for out-of-range years
                write!(f, "{:+05}-W{:02}", year, week)
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use naive::{internals, MAX_DATE, MIN_DATE};
        use Datelike;

        #[test]
        fn test_iso_week_extremes() {
            let minweek = MIN_DATE.iso_week();
            let maxweek = MAX_DATE.iso_week();

            assert_eq!(minweek.year(), internals::MIN_YEAR);
            assert_eq!(minweek.week(), 1);
            assert_eq!(minweek.week0(), 0);
            assert_eq!(format!("{:?}", minweek), MIN_DATE.format("%G-W%V").to_string());

            assert_eq!(maxweek.year(), internals::MAX_YEAR + 1);
            assert_eq!(maxweek.week(), 1);
            assert_eq!(maxweek.week0(), 0);
            assert_eq!(format!("{:?}", maxweek), MAX_DATE.format("%G-W%V").to_string());
        }
    }