civil: civil time types

A minimal implementation of the types Date, Time and DateTime, which represent
times without a corresponding location.

The main motivation for these types to to represent values in
database-like storage systems, like BigQuery. All three types support
conversion to and from RFC3339 string formats, modified only to deal
with nanoseconds.

All three types can be constructed from a time.Time.

All three types support equality via the "==" operator.

The Date and DateTime types can also be converted to a time.Time with
a location, and can be compared with the Before method.

The Date type also supports some day-based arithmetic.

We deliberately avoid introducing additional operations, no matter how
tempting. We can always add them later if there is demand. Also, users
can always convert to time.Time by providing a location, and then can
avail themselves of the richer operations that type provides.

Change-Id: I20ad82c1f266928be5508299a5df1e62203c0b79
Reviewed-on: https://code-review.googlesource.com/9566
Reviewed-by: Damien Neil <dneil@google.com>

civil/civil.go

@@ -0,0 +1,235 @@
+// Copyright 2016 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package civil implements types for civil time, a time-zone-independent
+// representation of time that follows the rules of the proleptic
+// Gregorian calendar with exactly 24-hour days, 60-minute hours, and 60-second
+// minutes.
+//
+// Because they lack location information, these types do not represent unique
+// moments or intervals of time. Use time.Time for that purpose.
+package civil
+
+import (
+	"fmt"
+	"time"
+)
+
+// A Date represents a date (year, month, day).
+//
+// This type does not include location information, and therefore does not
+// describe a unique 24-hour timespan.
+type Date struct {
+	Year  int        // Year (e.g., 2014).
+	Month time.Month // Month of the year (January = 1, ...).
+	Day   int        // Day of the month, starting at 1.
+}
+
+// DateOf returns the Date in which a time occurs in that time's location.
+func DateOf(t time.Time) Date {
+	var d Date
+	d.Year, d.Month, d.Day = t.Date()
+	return d
+}
+
+// ParseDate parses a string in RFC3339 full-date format and returns the date value it represents.
+func ParseDate(s string) (Date, error) {
+	t, err := time.Parse("2006-01-02", s)
+	if err != nil {
+		return Date{}, err
+	}
+	return DateOf(t), nil
+}
+
+// String returns the date in RFC3339 full-date format.
+func (d Date) String() string {
+	return fmt.Sprintf("%04d-%02d-%02d", d.Year, d.Month, d.Day)
+}
+
+// IsValid reports whether the date is valid.
+func (d Date) IsValid() bool {
+	return DateOf(d.In(time.UTC)) == d
+}
+
+// In returns the time corresponding to time 00:00:00 of the date in the location.
+//
+// In is always consistent with time.Date, even when time.Date returns a time
+// on a different day. For example, if loc is America/Indiana/Vincennes, then both
+//     time.Date(1955, time.May, 1, 0, 0, 0, 0, loc)
+// and
+//     civil.Date{Year: 1955, Month: time.May, Day: 1}.In(loc)
+// return 23:00:00 on April 30, 1955.
+//
+// In panics if loc is nil.
+func (d Date) In(loc *time.Location) time.Time {
+	return time.Date(d.Year, d.Month, d.Day, 0, 0, 0, 0, loc)
+}
+
+// AddDays returns the date that is n days in the future.
+// n can also be negative to go into the past.
+func (d Date) AddDays(n int) Date {
+	return DateOf(d.In(time.UTC).AddDate(0, 0, n))
+}
+
+// DaysSince returns the signed number of days between the date and s, not including the end day.
+// This is the inverse operation to AddDays.
+func (d Date) DaysSince(s Date) (days int) {
+	// We convert to Unix time so we do not have to worry about leap seconds:
+	// Unix time increases by exactly 86400 seconds per day.
+	deltaUnix := d.In(time.UTC).Unix() - s.In(time.UTC).Unix()
+	return int(deltaUnix / 86400)
+}
+
+// Before reports whether d1 occurs before d2.
+func (d1 Date) Before(d2 Date) bool {
+	if d1.Year != d2.Year {
+		return d1.Year < d2.Year
+	}
+	if d1.Month != d2.Month {
+		return d1.Month < d2.Month
+	}
+	return d1.Day < d2.Day
+}
+
+// After reports whether d1 occurs after d2.
+func (d1 Date) After(d2 Date) bool {
+	return d2.Before(d1)
+}
+
+// A Time represents a time with nanosecond precision.
+//
+// This type does not include location information, and therefore does not
+// describe a unique moment in time.
+//
+// This type exists to represent the TIME type in storage-based APIs like BigQuery.
+// Most operations on Times are unlikely to be meaningful. Prefer the DateTime type.
+type Time struct {
+	Hour       int // The hour of the day in 24-hour format; range [0-23]
+	Minute     int // The minute of the hour; range [0-59]
+	Second     int // The second of the minute; range [0-59]
+	Nanosecond int // The nanosecond of the second; range [0-999999999]
+}
+
+// TimeOf returns the Time representing the time of day in which a time occurs
+// in that time's location. It ignores the date.
+func TimeOf(t time.Time) Time {
+	var tm Time
+	tm.Hour, tm.Minute, tm.Second = t.Clock()
+	tm.Nanosecond = t.Nanosecond()
+	return tm
+}
+
+// ParseTime parses a string and returns the time value it represents.
+// ParseTime accepts an extended form of the RFC3339 partial-time format. After
+// the HH:MM:SS part of the string, an optional fractional part may appear,
+// consisting of a decimal point followed by one to nine decimal digits.
+// (RFC3339 admits only one digit after the decimal point).
+func ParseTime(s string) (Time, error) {
+	t, err := time.Parse("15:04:05.999999999", s)
+	if err != nil {
+		return Time{}, err
+	}
+	return TimeOf(t), nil
+}
+
+// String returns the date in the format described in ParseTime. If Nanoseconds
+// is zero, no fractional part will be generated. Otherwise, the result will
+// end with a fractional part consisting of a decimal point and nine digits.
+func (t Time) String() string {
+	s := fmt.Sprintf("%02d:%02d:%02d", t.Hour, t.Minute, t.Second)
+	if t.Nanosecond == 0 {
+		return s
+	}
+	return s + fmt.Sprintf(".%09d", t.Nanosecond)
+}
+
+// IsValid reports whether the time is valid.
+func (t Time) IsValid() bool {
+	// Construct a non-zero time.
+	tm := time.Date(2, 2, 2, t.Hour, t.Minute, t.Second, t.Nanosecond, time.UTC)
+	return TimeOf(tm) == t
+}
+
+// A DateTime represents a date and time.
+//
+// This type does not include location information, and therefore does not
+// describe a unique moment in time.
+type DateTime struct {
+	Date Date
+	Time Time
+}
+
+// Note: We deliberately do not embed Date into DateTime, to avoid promoting AddDays and Sub.
+
+// DateTimeOf returns the DateTime in which a time occurs in that time's location.
+func DateTimeOf(t time.Time) DateTime {
+	return DateTime{
+		Date: DateOf(t),
+		Time: TimeOf(t),
+	}
+}
+
+// ParseDateTime parses a string and returns the DateTime it represents.
+// ParseDateTime accepts a variant of the RFC3339 date-time format that omits
+// the time offset but includes an optional fractional time, as described in
+// ParseTime. Informally, the accepted format is
+//     YYYY-MM-DDTHH:MM:SS[.FFFFFFFFF]
+// where the 'T' may be a lower-case 't'.
+func ParseDateTime(s string) (DateTime, error) {
+	t, err := time.Parse("2006-01-02T15:04:05.999999999", s)
+	if err != nil {
+		t, err = time.Parse("2006-01-02t15:04:05.999999999", s)
+		if err != nil {
+			return DateTime{}, err
+		}
+	}
+	return DateTimeOf(t), nil
+}
+
+// String returns the date in the format described in ParseDate.
+func (dt DateTime) String() string {
+	return dt.Date.String() + "T" + dt.Time.String()
+}
+
+// IsValid reports whether the datetime is valid.
+func (dt DateTime) IsValid() bool {
+	return dt.Date.IsValid() && dt.Time.IsValid()
+}
+
+// In returns the time corresponding to the DateTime in the given location.
+//
+// If the time is missing or ambigous at the location, In returns the same
+// result as time.Date. For example, if loc is America/Indiana/Vincennes, then
+// both
+//     time.Date(1955, time.May, 1, 0, 30, 0, 0, loc)
+// and
+//     civil.DateTime{
+//         civil.Date{Year: 1955, Month: time.May, Day: 1}},
+//         civil.Time{Minute: 30}}.In(loc)
+// return 23:30:00 on April 30, 1955.
+//
+// In panics if loc is nil.
+func (dt DateTime) In(loc *time.Location) time.Time {
+	return time.Date(dt.Date.Year, dt.Date.Month, dt.Date.Day, dt.Time.Hour, dt.Time.Minute, dt.Time.Second, dt.Time.Nanosecond, loc)
+}
+
+// Before reports whether dt1 occurs before dt2.
+func (dt1 DateTime) Before(dt2 DateTime) bool {
+	return dt1.In(time.UTC).Before(dt2.In(time.UTC))
+}
+
+// After reports whether dt1 occurs after dt2.
+func (dt1 DateTime) After(dt2 DateTime) bool {
+	return dt2.Before(dt1)
+}