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