added `chrono::format` module and `format` methods.

this is a saner replacement for `time::strftime`; it does not allocate
the additional memory.

src/date.rs

@@ -15,6 +15,7 @@ use naive;
 use naive::date::NaiveDate;
 use naive::time::NaiveTime;
 use datetime::DateTime;
+use format::DelayedFormat;
 
 /// ISO 8601 calendar date with timezone.
 #[deriving(Clone)]
@@ -158,6 +159,13 @@ impl<Off:Offset> Date<Off> {
         self.date.pred_opt().map(|date| Date::from_utc(date, self.offset.clone()))
     }
 
+    /// Formats the date in the specified format string.
+    /// See the `format` module on the supported escape sequences.
+    #[inline]
+    pub fn format<'a>(&'a self, fmt: &'a str) -> DelayedFormat<'a> {
+        DelayedFormat::new_with_offset(Some(self.local()), None, &self.offset, fmt)
+    }
+
     /// Returns a view to the local date.
     fn local(&self) -> NaiveDate {
         self.offset.to_local_date(&self.date)

src/datetime.rs

@@ -14,6 +14,7 @@ use duration::Duration;
 use naive::datetime::NaiveDateTime;
 use time::Time;
 use date::Date;
+use format::DelayedFormat;
 
 /// ISO 8601 combined date and time with timezone.
 #[deriving(Clone)]
@@ -48,6 +49,14 @@ impl<Off:Offset> DateTime<Off> {
         self.datetime.num_seconds_from_unix_epoch()
     }
 
+    /// Formats the combined date and time in the specified format string.
+    /// See the `format` module on the supported escape sequences.
+    #[inline]
+    pub fn format<'a>(&'a self, fmt: &'a str) -> DelayedFormat<'a> {
+        let local = self.local();
+        DelayedFormat::new_with_offset(Some(local.date()), Some(local.time()), &self.offset, fmt)
+    }
+
     /// Returns a view to the local datetime.
     fn local(&self) -> NaiveDateTime {
         self.offset.to_local_datetime(&self.datetime)

src/format.rs

@@ -0,0 +1,215 @@
+// This is a part of rust-chrono.
+// Copyright (c) 2014, Kang Seonghoon.
+// See README.md and LICENSE.txt for details.
+
+/*!
+ * Formatting utilities for date and time.
+ */
+
+use std::fmt;
+use std::str::MaybeOwned;
+use std::io::{IoResult, IoError, InvalidInput};
+
+use {Datelike, Timelike};
+use duration::Duration;
+use offset::Offset;
+use naive::date::NaiveDate;
+use naive::time::NaiveTime;
+
+/// The internal workhouse for `DelayedFormat`.
+fn format(w: &mut Writer, date: Option<&NaiveDate>, time: Option<&NaiveTime>,
+          off: Option<&(MaybeOwned<'static>, Duration)>, fmt: &str) -> IoResult<()> {
+    static SHORT_MONTHS: [&'static str, ..12] =
+        ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
+    static LONG_MONTHS: [&'static str, ..12] =
+        ["January", "February", "March", "April", "May", "June",
+         "July", "August", "September", "October", "November", "December"];
+    static SHORT_WEEKDAYS: [&'static str, ..7] =
+        ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
+    static LONG_WEEKDAYS: [&'static str, ..7] =
+        ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"];
+
+    let mut parts = fmt.split('%');
+    match parts.next() {
+        Some(first) => try!(write!(w, "{}", first)),
+        None => return Ok(()),
+    }
+
+    let mut last_was_percent = false;
+    for part in parts {
+        if last_was_percent { // `%%<part>`
+            last_was_percent = false;
+            try!(write!(w, "%{}", part));
+            continue;
+        }
+
+        let (head, tail) = part.slice_shift_char();
+        match (head, date, time, off) {
+            // year
+            (Some('Y'), Some(d), _, _) => try!(write!(w, "{}", d.year())),
+            (Some('C'), Some(d), _, _) => try!(write!(w, "{:02}", d.year() / 100)),
+            (Some('y'), Some(d), _, _) => try!(write!(w, "{:02}", d.year() % 100)),
+            (Some('G'), Some(d), _, _) => try!(write!(w, "{:04}", d.isoweekdate().val0())),
+            (Some('g'), Some(d), _, _) => try!(write!(w, "{:02}", d.isoweekdate().val0() % 100)),
+
+            // month
+            (Some('m'), Some(d), _, _) => try!(write!(w, "{:02}", d.month())),
+            (Some('b'), Some(d), _, _) | (Some('h'), Some(d), _, _) =>
+                try!(write!(w, "{}", SHORT_MONTHS[d.month0() as uint])),
+            (Some('B'), Some(d), _, _) =>
+                try!(write!(w, "{}", LONG_MONTHS[d.month0() as uint])),
+
+            // day of month
+            (Some('d'), Some(d), _, _) => try!(write!(w, "{:02}", d.day())),
+            (Some('e'), Some(d), _, _) => try!(write!(w, "{:2}", d.day())),
+
+            // week
+            (Some('U'), Some(d), _, _) =>
+                try!(write!(w, "{:02}", (d.ordinal() - d.weekday().num_days_from_sunday()
+                                                     + 7) / 7)),
+            (Some('W'), Some(d), _, _) =>
+                try!(write!(w, "{:02}", (d.ordinal() - d.weekday().num_days_from_monday()
+                                                     + 7) / 7)),
+            (Some('V'), Some(d), _, _) => try!(write!(w, "{:02}", d.isoweekdate().val1())),
+
+            // day of week
+            (Some('a'), Some(d), _, _) =>
+                try!(write!(w, "{}", SHORT_WEEKDAYS[d.weekday().num_days_from_monday() as uint])),
+            (Some('A'), Some(d), _, _) =>
+                try!(write!(w, "{}", LONG_WEEKDAYS[d.weekday().num_days_from_monday() as uint])),
+            (Some('w'), Some(d), _, _) => try!(write!(w, "{}", d.weekday().num_days_from_sunday())),
+            (Some('u'), Some(d), _, _) => try!(write!(w, "{}", d.weekday().number_from_monday())),
+
+            // day of year
+            (Some('j'), Some(d), _, _) => try!(write!(w, "{:03}", d.ordinal())),
+
+            // combined date
+            (Some('D'), Some(d), _, _) | (Some('x'), Some(d), _, _) => // `%m/%d/%y`
+                try!(write!(w, "{:02}/{:02}/{:02}", d.month(), d.day(), d.year() % 100)),
+            (Some('F'), Some(d), _, _) => // `%Y-%m-%d'
+                try!(write!(w, "{:04}-{:02}-{:02}", d.year(), d.month(), d.day())),
+            (Some('v'), Some(d), _, _) => // `%e-%b-%Y'
+                try!(write!(w, "{:2}-{}-{:04}", d.day(), SHORT_MONTHS[d.month0() as uint],
+                                                d.year())),
+
+            // hour
+            (Some('H'), _, Some(t), _) => try!(write!(w, "{:02}", t.hour())),
+            (Some('k'), _, Some(t), _) => try!(write!(w, "{:2}", t.hour())),
+            (Some('I'), _, Some(t), _) => try!(write!(w, "{:02}", t.hour12().val1())),
+            (Some('l'), _, Some(t), _) => try!(write!(w, "{:2}", t.hour12().val1())),
+            (Some('P'), _, Some(t), _) =>
+                try!(write!(w, "{}", if t.hour12().val0() {"pm"} else {"am"})),
+            (Some('p'), _, Some(t), _) =>
+                try!(write!(w, "{}", if t.hour12().val0() {"PM"} else {"AM"})),
+
+            // minute
+            (Some('M'), _, Some(t), _) => try!(write!(w, "{:02}", t.minute())),
+
+            // second and below
+            (Some('S'), _, Some(t), _) => try!(write!(w, "{:02}", t.second())),
+            (Some('f'), _, Some(t), _) => try!(write!(w, "{:09}", t.nanosecond())),
+
+            // combined time
+            (Some('R'), _, Some(t), _) => // `%H:%M`
+                try!(write!(w, "{:02}:{:02}", t.hour(), t.minute())),
+            (Some('T'), _, Some(t), _) | (Some('X'), _, Some(t), _) => // `%H:%M:%S`
+                try!(write!(w, "{:02}:{:02}:{:02}", t.hour(), t.minute(), t.second())),
+            (Some('r'), _, Some(t), _) => { // `%I:%M:%S %p`
+                let (is_pm, hour12) = t.hour12();
+                try!(write!(w, "{:02}:{:02}:{:02} {}", hour12, t.minute(), t.second(),
+                               if is_pm {"PM"} else {"AM"}))
+            },
+
+            // timezone
+            (Some('Z'), _, _, Some(&(ref name, _))) => try!(write!(w, "{}", *name)),
+            (Some('z'), _, _, Some(&(_, ref local_minus_utc))) => {
+                let off = local_minus_utc.num_minutes();
+                let (sign, off) = if off < 0 {('-', -off)} else {('+', off)};
+                try!(write!(w, "{}{:02}{:02}", sign, off / 60, off % 60))
+            },
+
+            /*
+            // timestamp
+            (Some('s'), Some(d), Some(t), Some(o)) => { // XXX
+                let datetime = o.from_local_datetime(&d.and_time(t.clone())).unwrap();
+                try!(write!(w, "{}", datetime.num_seconds_from_unix_epoch()))
+            },
+            */
+
+            // combined date and time
+            (Some('c'), Some(d), Some(t), _) => // `%a %b %e %T %Y`
+                try!(write!(w, "{} {} {:2} {:02}:{:02}:{:02} {:04}",
+                               SHORT_WEEKDAYS[d.weekday().num_days_from_monday() as uint],
+                               SHORT_MONTHS[d.month0() as uint], d.day(),
+                               t.hour(), t.minute(), t.second(), d.year())),
+            (Some('+'), Some(d), Some(t),
+                        Some(&(_, ref local_minus_utc))) => { // `%Y-%m-%dT%H:%M:%S` plus tz
+                let off = local_minus_utc.num_minutes();
+                let (sign, off) = if off < 0 {('-', -off)} else {('+', off)};
+                try!(write!(w, "{}-{:02}-{:02}T{:02}:{:02}:{:02}{}{:02}:{:02}",
+                               d.year(), d.month(), d.day(), t.hour(), t.minute(), t.second(),
+                               sign, off / 60, off % 60))
+            },
+
+            // special characters
+            (Some('t'), _, _, _) => try!(write!(w, "\t")),
+            (Some('n'), _, _, _) => try!(write!(w, "\n")),
+
+            (Some(c), _, _, _) => {
+                return Err(IoError { kind: InvalidInput, desc: "invalid date/time format",
+                                     detail: Some(format!("unsupported escape sequence %{}", c)) });
+            }
+
+            (None, _, _, _) => {
+                // if there is the next part, a single `%` and that part should be printed
+                // in verbatim. otherwise it is an error (the stray `%`).
+                last_was_percent = true;
+            }
+        }
+
+        try!(write!(w, "{}", tail));
+    }
+
+    if last_was_percent { // a stray `%`
+        Err(IoError { kind: InvalidInput,
+                      desc: "invalid date/time format: stray `%`", detail: None })
+    } else {
+        Ok(())
+    }
+}
+
+/// A *temporary* object which can be used as an argument to `format!` or others.
+/// This is normally constructed via `format` methods of each date and time type.
+pub struct DelayedFormat<'a> {
+    /// The date view, if any.
+    date: Option<NaiveDate>,
+    /// The time view, if any.
+    time: Option<NaiveTime>,
+    /// The name and local-to-UTC difference for the offset (timezone), if any.
+    off: Option<(MaybeOwned<'static>, Duration)>,
+    /// The format string.
+    fmt: &'a str,
+}
+
+impl<'a> DelayedFormat<'a> {
+    /// Makes a new `DelayedFormat` value out of local date and time.
+    pub fn new(date: Option<NaiveDate>, time: Option<NaiveTime>,
+               fmt: &'a str) -> DelayedFormat<'a> {
+        DelayedFormat { date: date, time: time, off: None, fmt: fmt }
+    }
+
+    /// Makes a new `DelayedFormat` value out of local date and time and UTC offset.
+    pub fn new_with_offset<Off:Offset>(date: Option<NaiveDate>, time: Option<NaiveTime>,
+                                       offset: &Off, fmt: &'a str) -> DelayedFormat<'a> {
+        let name_and_diff = (offset.name(), offset.local_minus_utc());
+        DelayedFormat { date: date, time: time, off: Some(name_and_diff), fmt: fmt }
+    }
+}
+
+impl<'a> fmt::Show for DelayedFormat<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let ret = format(f, self.date.as_ref(), self.time.as_ref(), self.off.as_ref(), self.fmt);
+        ret.map_err(|_| fmt::WriteError) // XXX
+    }
+}
+

src/lib.rs

@@ -39,6 +39,7 @@ pub mod naive {
 pub mod date;
 pub mod time;
 pub mod datetime;
+pub mod format;
 
 /// The day of week (DOW).
 ///

src/naive/date.rs

@@ -13,6 +13,7 @@ use {Weekday, Datelike};
 use duration::Duration;
 use naive::time::NaiveTime;
 use naive::datetime::NaiveDateTime;
+use format::DelayedFormat;
 
 use self::internals::{DateImpl, Of, Mdf, YearFlags};
 
@@ -296,6 +297,13 @@ impl NaiveDate {
     pub fn pred_opt(&self) -> Option<NaiveDate> {
         self.with_of(self.of().pred()).or_else(|| NaiveDate::from_ymd_opt(self.year() - 1, 12, 31))
     }
+
+    /// Formats the date in the specified format string.
+    /// See the `format` module on the supported escape sequences.
+    #[inline]
+    pub fn format<'a>(&'a self, fmt: &'a str) -> DelayedFormat<'a> {
+        DelayedFormat::new(Some(self.clone()), None, fmt)
+    }
 }
 
 impl Datelike for NaiveDate {
@@ -736,6 +744,27 @@ mod tests {
         assert_eq!(format!("{:+30}", NaiveDate::from_ymd(1234, 5, 6)), "1234-05-06".to_string());
         assert_eq!(format!("{:30}", NaiveDate::from_ymd(12345, 6, 7)), "+12345-06-07".to_string());
     }
+
+    #[test]
+    fn test_date_format() {
+        let d = NaiveDate::from_ymd(2012, 3, 4);
+        assert_eq!(d.format("%Y,%C,%y,%G,%g").to_string(), "2012,20,12,2012,12".to_string());
+        assert_eq!(d.format("%m,%b,%h,%B").to_string(), "03,Mar,Mar,March".to_string());
+        assert_eq!(d.format("%d,%e").to_string(), "04, 4".to_string());
+        assert_eq!(d.format("%U,%W,%V").to_string(), "10,09,09".to_string());
+        assert_eq!(d.format("%a,%A,%w,%u").to_string(), "Sun,Sunday,0,7".to_string());
+        assert_eq!(d.format("%j").to_string(), "064".to_string()); // since 2012 is a leap year
+        assert_eq!(d.format("%D,%x").to_string(), "03/04/12,03/04/12".to_string());
+        assert_eq!(d.format("%F").to_string(), "2012-03-04".to_string());
+        assert_eq!(d.format("%v").to_string(), " 4-Mar-2012".to_string());
+        assert_eq!(d.format("%t%n%%%n%t").to_string(), "\t\n%\n\t".to_string());
+
+        // corner cases
+        assert_eq!(NaiveDate::from_ymd(2007, 12, 31).format("%G,%g,%U,%W,%V").to_string(),
+                   "2008,08,53,53,01".to_string());
+        assert_eq!(NaiveDate::from_ymd(2010, 1, 3).format("%G,%g,%U,%W,%V").to_string(),
+                   "2009,09,01,00,53".to_string());
+    }
 }
 
 /**

src/naive/datetime.rs

@@ -13,6 +13,7 @@ use {Weekday, Timelike, Datelike};
 use duration::Duration;
 use naive::time::NaiveTime;
 use naive::date::NaiveDate;
+use format::DelayedFormat;
 
 /// ISO 8601 combined date and time without timezone.
 #[deriving(PartialEq, Eq, PartialOrd, Ord, Clone, Hash)]
@@ -77,6 +78,13 @@ impl NaiveDateTime {
         let nseconds = self.time.num_seconds_from_midnight() as i64;
         (ndays - 719163) * 86400 + nseconds
     }
+
+    /// Formats the combined date and time in the specified format string.
+    /// See the `format` module on the supported escape sequences.
+    #[inline]
+    pub fn format<'a>(&'a self, fmt: &'a str) -> DelayedFormat<'a> {
+        DelayedFormat::new(Some(self.date.clone()), Some(self.time.clone()), fmt)
+    }
 }
 
 impl Datelike for NaiveDateTime {
@@ -247,5 +255,12 @@ mod tests {
         assert_eq!(to_timestamp(2001, 9, 9, 1, 46, 40), 1_000_000_000);
         assert_eq!(to_timestamp(2038, 1, 19, 3, 14, 7), 0x7fffffff);
     }
+
+    #[test]
+    fn test_datetime_format() {
+        let dt = NaiveDate::from_ymd(2010, 9, 8).and_hms_milli(7, 6, 54, 321);
+        assert_eq!(dt.format("%c").to_string(), "Wed Sep  8 07:06:54 2010".to_string());
+        assert_eq!(dt.format("%t%n%%%n%t").to_string(), "\t\n%\n\t".to_string());
+    }
 }
 

src/naive/time.rs

@@ -12,6 +12,7 @@ use num::Integer;
 use Timelike;
 use offset::Offset;
 use duration::Duration;
+use format::DelayedFormat;
 
 /// ISO 8601 time without timezone.
 /// Allows for the nanosecond precision and optional leap second representation.
@@ -115,6 +116,13 @@ impl NaiveTime {
         Some(NaiveTime { secs: secs, frac: nano })
     }
 
+    /// Formats the time in the specified format string.
+    /// See the `format` module on the supported escape sequences.
+    #[inline]
+    pub fn format<'a>(&'a self, fmt: &'a str) -> DelayedFormat<'a> {
+        DelayedFormat::new(None, Some(self.clone()), fmt)
+    }
+
     /// Returns a triple of the hour, minute and second numbers.
     fn hms(&self) -> (u32, u32, u32) {
         let (mins, sec) = self.secs.div_mod_floor(&60);
@@ -350,5 +358,21 @@ mod tests {
         assert_eq!(format!("{:30}", NaiveTime::from_hms_milli(3, 5, 7, 9)),
                    "03:05:07.009".to_string());
     }
+
+    #[test]
+    fn test_time_format() {
+        let t = NaiveTime::from_hms_nano(3, 5, 7, 98765432);
+        assert_eq!(t.format("%H,%k,%I,%l,%P,%p").to_string(), "03, 3,03, 3,am,AM".to_string());
+        assert_eq!(t.format("%M").to_string(), "05".to_string());
+        assert_eq!(t.format("%S,%f").to_string(), "07,098765432".to_string());
+        assert_eq!(t.format("%R").to_string(), "03:05".to_string());
+        assert_eq!(t.format("%T,%X").to_string(), "03:05:07,03:05:07".to_string());
+        assert_eq!(t.format("%r").to_string(), "03:05:07 AM".to_string());
+        assert_eq!(t.format("%t%n%%%n%t").to_string(), "\t\n%\n\t".to_string());
+
+        // corner cases
+        assert_eq!(NaiveTime::from_hms(13, 57, 9).format("%r").to_string(),
+                   "01:57:09 PM".to_string());
+    }
 }
 

src/offset.rs

@@ -7,6 +7,7 @@
  */
 
 use std::fmt;
+use std::str::MaybeOwned;
 use stdtime;
 use num::Integer;
 
@@ -214,6 +215,12 @@ pub trait Offset: Clone + fmt::Show {
         }
     }
 
+    /// Returns a name or abbreviation of this offset.
+    fn name(&self) -> MaybeOwned<'static>;
+
+    /// Returns the *current* offset from UTC to the local time.
+    fn local_minus_utc(&self) -> Duration;
+
     /// Converts the local `NaiveDate` to the timezone-aware `Date` if possible.
     fn from_local_date(&self, local: &NaiveDate) -> LocalResult<Date<Self>>;
 
@@ -253,6 +260,9 @@ impl UTC {
 }
 
 impl Offset for UTC {
+    fn name(&self) -> MaybeOwned<'static> { "UTC".into_maybe_owned() }
+    fn local_minus_utc(&self) -> Duration { Duration::zero() }
+
     fn from_local_date(&self, local: &NaiveDate) -> LocalResult<Date<UTC>> {
         Single(Date::from_utc(local.clone(), UTC))
     }
@@ -321,6 +331,9 @@ impl FixedOffset {
 }
 
 impl Offset for FixedOffset {
+    fn name(&self) -> MaybeOwned<'static> { "UTC".into_maybe_owned() } // XXX
+    fn local_minus_utc(&self) -> Duration { Duration::seconds(self.local_minus_utc) }
+
     fn from_local_date(&self, local: &NaiveDate) -> LocalResult<Date<FixedOffset>> {
         Single(Date::from_utc(local.clone(), self.clone()))
     }
@@ -410,6 +423,9 @@ impl Local {
 }
 
 impl Offset for Local {
+    fn name(&self) -> MaybeOwned<'static> { "LMT".into_maybe_owned() } // XXX XXX
+    fn local_minus_utc(&self) -> Duration { self.cached.local_minus_utc() }
+
     fn from_local_date(&self, local: &NaiveDate) -> LocalResult<Date<Local>> {
         match self.from_local_datetime(&local.and_hms(0, 0, 0)) {
             NoResult => NoResult,

src/time.rs

@@ -12,6 +12,7 @@ use Timelike;
 use offset::Offset;
 use duration::Duration;
 use naive::time::NaiveTime;
+use format::DelayedFormat;
 
 /// ISO 8601 time with timezone.
 #[deriving(Clone)]
@@ -28,6 +29,13 @@ impl<Off:Offset> Time<Off> {
         Time { time: time, offset: offset }
     }
 
+    /// Formats the time in the specified format string.
+    /// See the `format` module on the supported escape sequences.
+    #[inline]
+    pub fn format<'a>(&'a self, fmt: &'a str) -> DelayedFormat<'a> {
+        DelayedFormat::new_with_offset(None, Some(self.local()), &self.offset, fmt)
+    }
+
     /// Returns a view to the local time.
     fn local(&self) -> NaiveTime {
         self.offset.to_local_time(&self.time)