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diff --git a/src/naive/isoweek.rs b/src/naive/isoweek.rs
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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());
+    }
+}