Add SubSecondRound extension trait with impl for Timelike

src/lib.rs

@@ -409,6 +409,7 @@ pub use date::{Date, MIN_DATE, MAX_DATE};
 pub use datetime::{DateTime, SecondsFormat};
 #[cfg(feature = "rustc-serialize")] pub use datetime::rustc_serialize::TsSeconds;
 pub use format::{ParseError, ParseResult};
+pub use round::SubSecondRound;
 
 /// A convenience module appropriate for glob imports (`use chrono::prelude::*;`).
 pub mod prelude {
@@ -418,6 +419,7 @@ pub mod prelude {
     #[doc(no_inline)] pub use {NaiveDate, NaiveTime, NaiveDateTime};
     #[doc(no_inline)] pub use Date;
     #[doc(no_inline)] pub use {DateTime, SecondsFormat};
+    #[doc(no_inline)] pub use {SubSecondRound};
 }
 
 // useful throughout the codebase
@@ -464,6 +466,7 @@ pub mod naive {
 mod date;
 mod datetime;
 pub mod format;
+mod round;
 
 /// Serialization/Deserialization in alternate formats
 ///

src/round.rs

@@ -0,0 +1,162 @@
+// This is a part of Chrono.
+// See README.md and LICENSE.txt for details.
+
+use Timelike;
+use std::ops::{Add, Sub};
+use oldtime::Duration;
+
+/// Extension trait for subsecond rounding or truncation to a maximum number
+/// of digits. Rounding can be used to decrease the error variance when
+/// serializing/persisting to lower precision. Truncation is the default
+/// behavior in Chrono display formatting.  Either can be used to guarantee
+/// equality (e.g. for testing) when round-tripping through a lower precision
+/// format.
+pub trait SubSecondRound {
+    /// Return a copy rounded to the specified number of subsecond digits. With
+    /// 9 or more digits, self is returned unmodified. Halfway values are
+    /// rounded up (away from zero).
+    fn round(self, digits: u16) -> Self;
+
+    /// Return a copy truncated to the specified number of subsecond
+    /// digits. With 9 or more digits, self is returned unmodified.
+    fn trunc(self, digits: u16) -> Self;
+}
+
+impl<T> SubSecondRound for T
+where T: Timelike + Add<Duration, Output=T> + Sub<Duration, Output=T>
+{
+    fn round(self, digits: u16) -> T {
+        let span = span_for_digits(digits);
+        let rem = self.nanosecond() % span;
+        if rem > 0 {
+            let rev = span - rem;
+            if rev <= rem {
+                self + Duration::nanoseconds(rev.into()) // up
+            } else {
+                self - Duration::nanoseconds(rem.into()) // down
+            }
+        } else {
+            self // unchanged
+        }
+    }
+
+    fn trunc(self, digits: u16) -> T {
+        let span = span_for_digits(digits);
+        let rem = self.nanosecond() % span;
+        if rem > 0 {
+            self - Duration::nanoseconds(rem.into()) // truncate
+        } else {
+            self // unchanged
+        }
+    }
+}
+
+// Return the maximum span in nanoseconds for the target number of digits.
+fn span_for_digits(digits: u16) -> u32 {
+    // fast lookup form of: 10^(9-min(9,digits))
+    match digits {
+        0 => 1_000_000_000,
+        1 =>   100_000_000,
+        2 =>    10_000_000,
+        3 =>     1_000_000,
+        4 =>       100_000,
+        5 =>        10_000,
+        6 =>         1_000,
+        7 =>           100,
+        8 =>            10,
+        _ =>             1
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use Timelike;
+    use offset::{FixedOffset, TimeZone, Utc};
+    use super::SubSecondRound;
+
+    #[test]
+    fn test_round() {
+        let pst = FixedOffset::east(8 * 60 * 60);
+        let dt = pst.ymd(2018, 1, 11).and_hms_nano(10, 5, 13, 084_660_684);
+
+        assert_eq!(dt.round(10), dt);
+        assert_eq!(dt.round(9), dt);
+        assert_eq!(dt.round(8).nanosecond(), 084_660_680);
+        assert_eq!(dt.round(7).nanosecond(), 084_660_700);
+        assert_eq!(dt.round(6).nanosecond(), 084_661_000);
+        assert_eq!(dt.round(5).nanosecond(), 084_660_000);
+        assert_eq!(dt.round(4).nanosecond(), 084_700_000);
+        assert_eq!(dt.round(3).nanosecond(), 085_000_000);
+        assert_eq!(dt.round(2).nanosecond(), 080_000_000);
+        assert_eq!(dt.round(1).nanosecond(), 100_000_000);
+
+        assert_eq!(dt.round(0).nanosecond(), 0);
+        assert_eq!(dt.round(0).second(), 13);
+
+        let dt = Utc.ymd(2018, 1, 11).and_hms_nano(10, 5, 27, 750_500_000);
+        assert_eq!(dt.round(9), dt);
+        assert_eq!(dt.round(4), dt);
+        assert_eq!(dt.round(3).nanosecond(), 751_000_000);
+        assert_eq!(dt.round(2).nanosecond(), 750_000_000);
+        assert_eq!(dt.round(1).nanosecond(), 800_000_000);
+
+        assert_eq!(dt.round(0).nanosecond(), 0);
+        assert_eq!(dt.round(0).second(), 28);
+    }
+
+    #[test]
+    fn test_round_leap_nanos() {
+        let dt = Utc.ymd(2016, 12, 31).and_hms_nano(23, 59, 59, 1_750_500_000);
+        assert_eq!(dt.round(9), dt);
+        assert_eq!(dt.round(4), dt);
+        assert_eq!(dt.round(2).nanosecond(), 1_750_000_000);
+        assert_eq!(dt.round(1).nanosecond(), 1_800_000_000);
+        assert_eq!(dt.round(1).second(), 59);
+
+        assert_eq!(dt.round(0).nanosecond(), 0);
+        assert_eq!(dt.round(0).second(), 0);
+    }
+
+    #[test]
+    fn test_trunc() {
+        let pst = FixedOffset::east(8 * 60 * 60);
+        let dt = pst.ymd(2018, 1, 11).and_hms_nano(10, 5, 13, 084_660_684);
+
+        assert_eq!(dt.trunc(10), dt);
+        assert_eq!(dt.trunc(9), dt);
+        assert_eq!(dt.trunc(8).nanosecond(), 084_660_680);
+        assert_eq!(dt.trunc(7).nanosecond(), 084_660_600);
+        assert_eq!(dt.trunc(6).nanosecond(), 084_660_000);
+        assert_eq!(dt.trunc(5).nanosecond(), 084_660_000);
+        assert_eq!(dt.trunc(4).nanosecond(), 084_600_000);
+        assert_eq!(dt.trunc(3).nanosecond(), 084_000_000);
+        assert_eq!(dt.trunc(2).nanosecond(), 080_000_000);
+        assert_eq!(dt.trunc(1).nanosecond(), 0);
+
+        assert_eq!(dt.trunc(0).nanosecond(), 0);
+        assert_eq!(dt.trunc(0).second(), 13);
+
+        let dt = pst.ymd(2018, 1, 11).and_hms_nano(10, 5, 27, 750_500_000);
+        assert_eq!(dt.trunc(9), dt);
+        assert_eq!(dt.trunc(4), dt);
+        assert_eq!(dt.trunc(3).nanosecond(), 750_000_000);
+        assert_eq!(dt.trunc(2).nanosecond(), 750_000_000);
+        assert_eq!(dt.trunc(1).nanosecond(), 700_000_000);
+
+        assert_eq!(dt.trunc(0).nanosecond(), 0);
+        assert_eq!(dt.trunc(0).second(), 27);
+    }
+
+    #[test]
+    fn test_trunc_leap_nanos() {
+        let dt = Utc.ymd(2016, 12, 31).and_hms_nano(23, 59, 59, 1_750_500_000);
+        assert_eq!(dt.trunc(9), dt);
+        assert_eq!(dt.trunc(4), dt);
+        assert_eq!(dt.trunc(2).nanosecond(), 1_750_000_000);
+        assert_eq!(dt.trunc(1).nanosecond(), 1_700_000_000);
+        assert_eq!(dt.trunc(1).second(), 59);
+
+        assert_eq!(dt.trunc(0).nanosecond(), 1_000_000_000);
+        assert_eq!(dt.trunc(0).second(), 59);
+    }
+}