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326 lines
13 KiB
C++
326 lines
13 KiB
C++
// Copyright 2016 Google Inc. All Rights Reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef CCTZ_CIVIL_TIME_H_
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#define CCTZ_CIVIL_TIME_H_
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#include "civil_time_detail.h"
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namespace cctz {
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// The term "civil time" refers to the legally recognized human-scale time
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// that is represented by the six fields YYYY-MM-DD hh:mm:ss. Modern-day civil
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// time follows the Gregorian Calendar and is a time-zone-independent concept.
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// A "date" is perhaps the most common example of a civil time (represented in
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// this library as cctz::civil_day). This library provides six classes and a
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// handful of functions that help with rounding, iterating, and arithmetic on
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// civil times while avoiding complications like daylight-saving time (DST).
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//
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// The following six classes form the core of this civil-time library:
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//
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// * civil_second
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// * civil_minute
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// * civil_hour
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// * civil_day
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// * civil_month
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// * civil_year
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//
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// Each class is a simple value type with the same interface for construction
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// and the same six accessors for each of the civil fields (year, month, day,
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// hour, minute, and second, aka YMDHMS). These classes differ only in their
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// alignment, which is indicated by the type name and specifies the field on
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// which arithmetic operates.
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//
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// Each class can be constructed by passing up to six optional integer
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// arguments representing the YMDHMS fields (in that order) to the
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// constructor. Omitted fields are assigned their minimum valid value. Hours,
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// minutes, and seconds will be set to 0, month and day will be set to 1, and
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// since there is no minimum valid year, it will be set to 1970. So, a
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// default-constructed civil-time object will have YMDHMS fields representing
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// "1970-01-01 00:00:00". Fields that are out-of-range are normalized (e.g.,
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// October 32 -> November 1) so that all civil-time objects represent valid
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// values.
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//
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// Each civil-time class is aligned to the civil-time field indicated in the
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// class's name after normalization. Alignment is performed by setting all the
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// inferior fields to their minimum valid value (as described above). The
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// following are examples of how each of the six types would align the fields
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// representing November 22, 2015 at 12:34:56 in the afternoon. (Note: the
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// string format used here is not important; it's just a shorthand way of
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// showing the six YMDHMS fields.)
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//
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// civil_second 2015-11-22 12:34:56
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// civil_minute 2015-11-22 12:34:00
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// civil_hour 2015-11-22 12:00:00
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// civil_day 2015-11-22 00:00:00
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// civil_month 2015-11-01 00:00:00
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// civil_year 2015-01-01 00:00:00
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//
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// Each civil-time type performs arithmetic on the field to which it is
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// aligned. This means that adding 1 to a civil_day increments the day field
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// (normalizing as necessary), and subtracting 7 from a civil_month operates
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// on the month field (normalizing as necessary). All arithmetic produces a
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// valid civil time. Difference requires two similarly aligned civil-time
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// objects and returns the scalar answer in units of the objects' alignment.
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// For example, the difference between two civil_hour objects will give an
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// answer in units of civil hours.
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//
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// In addition to the six civil-time types just described, there are
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// a handful of helper functions and algorithms for performing common
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// calculations. These are described below.
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//
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// Note: In C++14 and later, this library is usable in a constexpr context.
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//
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// CONSTRUCTION:
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//
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// Each of the civil-time types can be constructed in two ways: by directly
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// passing to the constructor up to six (optional) integers representing the
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// YMDHMS fields, or by copying the YMDHMS fields from a differently aligned
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// civil-time type.
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//
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// civil_day default_value; // 1970-01-01 00:00:00
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//
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// civil_day a(2015, 2, 3); // 2015-02-03 00:00:00
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// civil_day b(2015, 2, 3, 4, 5, 6); // 2015-02-03 00:00:00
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// civil_day c(2015); // 2015-01-01 00:00:00
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//
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// civil_second ss(2015, 2, 3, 4, 5, 6); // 2015-02-03 04:05:06
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// civil_minute mm(ss); // 2015-02-03 04:05:00
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// civil_hour hh(mm); // 2015-02-03 04:00:00
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// civil_day d(hh); // 2015-02-03 00:00:00
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// civil_month m(d); // 2015-02-01 00:00:00
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// civil_year y(m); // 2015-01-01 00:00:00
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//
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// m = civil_month(y); // 2015-01-01 00:00:00
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// d = civil_day(m); // 2015-01-01 00:00:00
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// hh = civil_hour(d); // 2015-01-01 00:00:00
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// mm = civil_minute(hh); // 2015-01-01 00:00:00
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// ss = civil_second(mm); // 2015-01-01 00:00:00
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//
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// ALIGNMENT CONVERSION:
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//
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// The alignment of a civil-time object cannot change, but the object may be
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// used to construct a new object with a different alignment. This is referred
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// to as "realigning". When realigning to a type with the same or more
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// precision (e.g., civil_day -> civil_second), the conversion may be
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// performed implicitly since no information is lost. However, if information
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// could be discarded (e.g., civil_second -> civil_day), the conversion must
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// be explicit at the call site.
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//
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// void fun(const civil_day& day);
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//
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// civil_second cs;
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// fun(cs); // Won't compile because data may be discarded
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// fun(civil_day(cs)); // OK: explicit conversion
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//
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// civil_day cd;
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// fun(cd); // OK: no conversion needed
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//
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// civil_month cm;
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// fun(cm); // OK: implicit conversion to civil_day
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//
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// NORMALIZATION:
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//
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// Integer arguments passed to the constructor may be out-of-range, in which
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// case they are normalized to produce a valid civil-time object. This enables
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// natural arithmetic on constructor arguments without worrying about the
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// field's range. Normalization guarantees that there are no invalid
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// civil-time objects.
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//
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// civil_day d(2016, 10, 32); // Out-of-range day; normalized to 2016-11-01
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//
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// Note: If normalization is undesired, you can signal an error by comparing
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// the constructor arguments to the normalized values returned by the YMDHMS
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// properties.
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//
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// PROPERTIES:
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//
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// All civil-time types have accessors for all six of the civil-time fields:
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// year, month, day, hour, minute, and second. Recall that fields inferior to
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// the type's aligment will be set to their minimum valid value.
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//
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// civil_day d(2015, 6, 28);
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// // d.year() == 2015
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// // d.month() == 6
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// // d.day() == 28
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// // d.hour() == 0
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// // d.minute() == 0
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// // d.second() == 0
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//
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// COMPARISON:
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//
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// Comparison always considers all six YMDHMS fields, regardless of the type's
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// alignment. Comparison between differently aligned civil-time types is
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// allowed.
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//
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// civil_day feb_3(2015, 2, 3); // 2015-02-03 00:00:00
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// civil_day mar_4(2015, 3, 4); // 2015-03-04 00:00:00
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// // feb_3 < mar_4
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// // civil_year(feb_3) == civil_year(mar_4)
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//
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// civil_second feb_3_noon(2015, 2, 3, 12, 0, 0); // 2015-02-03 12:00:00
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// // feb_3 < feb_3_noon
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// // feb_3 == civil_day(feb_3_noon)
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//
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// // Iterates all the days of February 2015.
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// for (civil_day d(2015, 2, 1); d < civil_month(2015, 3); ++d) {
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// // ...
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// }
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//
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// STREAMING:
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//
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// Each civil-time type may be sent to an output stream using operator<<().
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// The output format follows the pattern "YYYY-MM-DDThh:mm:ss" where fields
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// inferior to the type's alignment are omitted.
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//
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// civil_second cs(2015, 2, 3, 4, 5, 6);
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// std::cout << cs << "\n"; // Outputs: 2015-02-03T04:05:06
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//
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// civil_day cd(cs);
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// std::cout << cd << "\n"; // Outputs: 2015-02-03
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//
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// civil_year cy(cs);
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// std::cout << cy << "\n"; // Outputs: 2015
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//
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// ARITHMETIC:
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//
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// Civil-time types support natural arithmetic operators such as addition,
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// subtraction, and difference. Arithmetic operates on the civil-time field
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// indicated in the type's name. Difference requires arguments with the same
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// alignment and returns the answer in units of the alignment.
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//
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// civil_day a(2015, 2, 3);
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// ++a; // 2015-02-04 00:00:00
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// --a; // 2015-02-03 00:00:00
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// civil_day b = a + 1; // 2015-02-04 00:00:00
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// civil_day c = 1 + b; // 2015-02-05 00:00:00
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// int n = c - a; // n = 2 (civil days)
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// int m = c - civil_month(c); // Won't compile: different types.
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//
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// EXAMPLE: Adding a month to January 31.
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//
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// One of the classic questions that arises when considering a civil-time
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// library (or a date library or a date/time library) is this: "What happens
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// when you add a month to January 31?" This is an interesting question
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// because there could be a number of possible answers:
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//
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// 1. March 3 (or 2 if a leap year). This may make sense if the operation
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// wants the equivalent of February 31.
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// 2. February 28 (or 29 if a leap year). This may make sense if the operation
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// wants the last day of January to go to the last day of February.
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// 3. Error. The caller may get some error, an exception, an invalid date
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// object, or maybe false is returned. This may make sense because there is
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// no single unambiguously correct answer to the question.
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//
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// Practically speaking, any answer that is not what the programmer intended
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// is the wrong answer.
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//
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// This civil-time library avoids the problem by making it impossible to ask
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// ambiguous questions. All civil-time objects are aligned to a particular
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// civil-field boundary (such as aligned to a year, month, day, hour, minute,
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// or second), and arithmetic operates on the field to which the object is
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// aligned. This means that in order to "add a month" the object must first be
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// aligned to a month boundary, which is equivalent to the first day of that
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// month.
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//
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// Of course, there are ways to compute an answer the question at hand using
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// this civil-time library, but they require the programmer to be explicit
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// about the answer they expect. To illustrate, let's see how to compute all
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// three of the above possible answers to the question of "Jan 31 plus 1
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// month":
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//
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// const civil_day d(2015, 1, 31);
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//
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// // Answer 1:
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// // Add 1 to the month field in the constructor, and rely on normalization.
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// const auto ans_normalized = civil_day(d.year(), d.month() + 1, d.day());
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// // ans_normalized == 2015-03-03 (aka Feb 31)
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//
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// // Answer 2:
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// // Add 1 to month field, capping to the end of next month.
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// const auto next_month = civil_month(d) + 1;
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// const auto last_day_of_next_month = civil_day(next_month + 1) - 1;
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// const auto ans_capped = std::min(ans_normalized, last_day_of_next_month);
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// // ans_capped == 2015-02-28
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//
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// // Answer 3:
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// // Signal an error if the normalized answer is not in next month.
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// if (civil_month(ans_normalized) != next_month) {
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// // error, month overflow
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// }
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//
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using civil_year = detail::civil_year;
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using civil_month = detail::civil_month;
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using civil_day = detail::civil_day;
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using civil_hour = detail::civil_hour;
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using civil_minute = detail::civil_minute;
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using civil_second = detail::civil_second;
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// An enum class with members monday, tuesday, wednesday, thursday, friday,
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// saturday, and sunday. These enum values may be sent to an output stream
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// using operator<<(). The result is the full weekday name in English with a
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// leading capital letter.
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//
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// weekday wd = weekday::thursday;
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// std::cout << wd << "\n"; // Outputs: Thursday
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//
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using detail::weekday;
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// Returns the weekday for the given civil_day.
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//
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// civil_day a(2015, 8, 13);
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// weekday wd = get_weekday(a); // wd == weekday::thursday
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//
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using detail::get_weekday;
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// Returns the civil_day that strictly follows or precedes the given
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// civil_day, and that falls on the given weekday.
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//
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// For example, given:
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//
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// August 2015
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// Su Mo Tu We Th Fr Sa
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// 1
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// 2 3 4 5 6 7 8
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// 9 10 11 12 13 14 15
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// 16 17 18 19 20 21 22
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// 23 24 25 26 27 28 29
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// 30 31
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//
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// civil_day a(2015, 8, 13); // get_weekday(a) == weekday::thursday
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// civil_day b = next_weekday(a, weekday::thursday); // b = 2015-08-20
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// civil_day c = prev_weekday(a, weekday::thursday); // c = 2015-08-06
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//
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// civil_day d = ...
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// // Gets the following Thursday if d is not already Thursday
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// civil_day thurs1 = prev_weekday(d, weekday::thursday) + 7;
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// // Gets the previous Thursday if d is not already Thursday
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// civil_day thurs2 = next_weekday(d, weekday::thursday) - 7;
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//
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using detail::next_weekday;
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using detail::prev_weekday;
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// Returns the day-of-year for the given civil_day.
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//
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// civil_day a(2015, 1, 1);
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// int yd_jan_1 = get_yearday(a); // yd_jan_1 = 1
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// civil_day b(2015, 12, 31);
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// int yd_dec_31 = get_yearday(b); // yd_dec_31 = 365
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//
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using detail::get_yearday;
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} // namespace cctz
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#endif // CCTZ_CIVIL_TIME_H_
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