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diff --git a/absl/time/civil_time.h b/absl/time/civil_time.h new file mode 100644 index 00000000..f6cc3ff8 --- /dev/null +++ b/absl/time/civil_time.h @@ -0,0 +1,488 @@ +// Copyright 2018 The Abseil Authors. +// +// 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. +// +// ----------------------------------------------------------------------------- +// File: civil_time.h +// ----------------------------------------------------------------------------- +// +// This header file defines abstractions for computing with "civil time". +// 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`. A "date" +// is perhaps the most common example of a civil time (represented here as +// an `absl::CivilDay`). +// +// Modern-day civil time follows the Gregorian Calendar and is a +// time-zone-independent concept: a civil time of "2015-06-01 12:00:00", for +// example, is not tied to a time zone. Put another way, a civil time does not +// map to a unique point in time; a civil time must be mapped to an absolute +// time *through* a time zone. +// +// Because a civil time is what most people think of as "time," it is common to +// map absolute times to civil times to present to users. +// +// Time zones define the relationship between absolute and civil times. Given an +// absolute or civil time and a time zone, you can compute the other time: +// +// Civil Time = F(Absolute Time, Time Zone) +// Absolute Time = G(Civil Time, Time Zone) +// +// The Abseil time library allows you to construct such civil times from +// absolute times; consult time.h for such functionality. +// +// This library provides six classes for constructing civil-time objects, and +// provides several helper functions for rounding, iterating, and performing +// arithmetic on civil-time objects, while avoiding complications like +// daylight-saving time (DST): +// +// * `absl::CivilSecond` +// * `absl::CivilMinute` +// * `absl::CivilHour` +// * `absl::CivilDay` +// * `absl::CivilMonth` +// * `absl::CivilYear` +// +// Example: +// +// // Construct a civil-time object for a specific day +// const absl::CivilDay cd(1969, 07, 20); +// +// // Construct a civil-time object for a specific second +// const absl::CivilSecond cd(2018, 8, 1, 12, 0, 1); +// +// Note: In C++14 and later, this library is usable in a constexpr context. +// +// Example: +// +// // Valid in C++14 +// constexpr absl::CivilDay cd(1969, 07, 20); +// + +#ifndef ABSL_TIME_CIVIL_TIME_H_ +#define ABSL_TIME_CIVIL_TIME_H_ + +#include <string> + +#include "absl/strings/string_view.h" +#include "absl/time/internal/cctz/include/cctz/civil_time.h" + +namespace absl { +inline namespace lts_2018_12_18 { + +namespace time_internal { +struct second_tag : cctz::detail::second_tag {}; +struct minute_tag : second_tag, cctz::detail::minute_tag {}; +struct hour_tag : minute_tag, cctz::detail::hour_tag {}; +struct day_tag : hour_tag, cctz::detail::day_tag {}; +struct month_tag : day_tag, cctz::detail::month_tag {}; +struct year_tag : month_tag, cctz::detail::year_tag {}; +} // namespace time_internal + +// ----------------------------------------------------------------------------- +// CivilSecond, CivilMinute, CivilHour, CivilDay, CivilMonth, CivilYear +// ----------------------------------------------------------------------------- +// +// Each of these civil-time types is a simple value type with the same +// interface for construction and the same six accessors for each of the civil +// time 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. +// +// CONSTRUCTION +// +// Each of the civil-time types can be constructed in two ways: by directly +// passing to the constructor up to six integers representing the YMDHMS fields, +// or by copying the YMDHMS fields from a differently aligned civil-time type. +// 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. Since there is no +// minimum year, the default is 1970. +// +// Examples: +// +// absl::CivilDay default_value; // 1970-01-01 00:00:00 +// +// absl::CivilDay a(2015, 2, 3); // 2015-02-03 00:00:00 +// absl::CivilDay b(2015, 2, 3, 4, 5, 6); // 2015-02-03 00:00:00 +// absl::CivilDay c(2015); // 2015-01-01 00:00:00 +// +// absl::CivilSecond ss(2015, 2, 3, 4, 5, 6); // 2015-02-03 04:05:06 +// absl::CivilMinute mm(ss); // 2015-02-03 04:05:00 +// absl::CivilHour hh(mm); // 2015-02-03 04:00:00 +// absl::CivilDay d(hh); // 2015-02-03 00:00:00 +// absl::CivilMonth m(d); // 2015-02-01 00:00:00 +// absl::CivilYear y(m); // 2015-01-01 00:00:00 +// +// m = absl::CivilMonth(y); // 2015-01-01 00:00:00 +// d = absl::CivilDay(m); // 2015-01-01 00:00:00 +// hh = absl::CivilHour(d); // 2015-01-01 00:00:00 +// mm = absl::CivilMinute(hh); // 2015-01-01 00:00:00 +// ss = absl::CivilSecond(mm); // 2015-01-01 00:00:00 +// +// 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.) +// +// absl::CivilSecond : 2015-11-22 12:34:56 +// absl::CivilMinute : 2015-11-22 12:34:00 +// absl::CivilHour : 2015-11-22 12:00:00 +// absl::CivilDay : 2015-11-22 00:00:00 +// absl::CivilMonth : 2015-11-01 00:00:00 +// absl::CivilYear : 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 an absl::CivilDay increments the day +// field (normalizing as necessary), and subtracting 7 from an absl::CivilMonth +// 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 absl::CivilHour objects +// will give an answer in units of civil hours. +// +// 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., absl::CivilDay -> absl::CivilSecond), the conversion may be +// performed implicitly since no information is lost. However, if information +// could be discarded (e.g., CivilSecond -> CivilDay), the conversion must +// be explicit at the call site. +// +// Examples: +// +// void UseDay(absl::CivilDay day); +// +// absl::CivilSecond cs; +// UseDay(cs); // Won't compile because data may be discarded +// UseDay(absl::CivilDay(cs)); // OK: explicit conversion +// +// absl::CivilDay cd; +// UseDay(cd); // OK: no conversion needed +// +// absl::CivilMonth cm; +// UseDay(cm); // OK: implicit conversion to absl::CivilDay +// +// NORMALIZATION +// +// Normalization takes invalid values and adjusts them to produce valid values. +// Within the civil-time library, integer arguments passed to the Civil* +// constructors may be out-of-range, in which case they are normalized by +// carrying overflow into a field of courser granularity to produce valid +// civil-time objects. This normalization enables natural arithmetic on +// constructor arguments without worrying about the field's range. +// +// Examples: +// +// // Out-of-range; normalized to 2016-11-01 +// absl::CivilDay d(2016, 10, 32); +// // Out-of-range, negative: normalized to 2016-10-30T23 +// absl::CivilHour h1(2016, 10, 31, -1); +// // Normalization is cumulative: normalized to 2016-10-30T23 +// absl::CivilHour h2(2016, 10, 32, -25); +// +// Note: If normalization is undesired, you can signal an error by comparing +// the constructor arguments to the normalized values returned by the YMDHMS +// properties. +// +// COMPARISON +// +// Comparison between civil-time objects considers all six YMDHMS fields, +// regardless of the type's alignment. Comparison between differently aligned +// civil-time types is allowed. +// +// Examples: +// +// absl::CivilDay feb_3(2015, 2, 3); // 2015-02-03 00:00:00 +// absl::CivilDay mar_4(2015, 3, 4); // 2015-03-04 00:00:00 +// // feb_3 < mar_4 +// // absl::CivilYear(feb_3) == absl::CivilYear(mar_4) +// +// absl::CivilSecond feb_3_noon(2015, 2, 3, 12, 0, 0); // 2015-02-03 12:00:00 +// // feb_3 < feb_3_noon +// // feb_3 == absl::CivilDay(feb_3_noon) +// +// // Iterates all the days of February 2015. +// for (absl::CivilDay d(2015, 2, 1); d < absl::CivilMonth(2015, 3); ++d) { +// // ... +// } +// +// 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 operators require arguments with +// the same alignment and return the answer in units of the alignment. +// +// Example: +// +// absl::CivilDay a(2015, 2, 3); +// ++a; // 2015-02-04 00:00:00 +// --a; // 2015-02-03 00:00:00 +// absl::CivilDay b = a + 1; // 2015-02-04 00:00:00 +// absl::CivilDay c = 1 + b; // 2015-02-05 00:00:00 +// int n = c - a; // n = 2 (civil days) +// int m = c - absl::CivilMonth(c); // Won't compile: different types. +// +// ACCESSORS +// +// Each civil-time type has accessors for all six of the civil-time fields: +// year, month, day, hour, minute, and second. +// +// civil_year_t year() +// int month() +// int day() +// int hour() +// int minute() +// int second() +// +// Recall that fields inferior to the type's aligment will be set to their +// minimum valid value. +// +// Example: +// +// absl::CivilDay d(2015, 6, 28); +// // d.year() == 2015 +// // d.month() == 6 +// // d.day() == 28 +// // d.hour() == 0 +// // d.minute() == 0 +// // d.second() == 0 +// +// CASE STUDY: 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 is the result of adding a month to January 31?" +// This is an interesting question because it is unclear what is meant by a +// "month", and several different answers are possible, depending on context: +// +// 1. March 3 (or 2 if a leap year), if "add a month" means to add a month to +// the current month, and adjust the date to overflow the extra days into +// March. In this case the result of "February 31" would be normalized as +// within the civil-time library. +// 2. February 28 (or 29 if a leap year), if "add a month" means to add a +// month, and adjust the date while holding the resulting month constant. +// In this case, the result of "February 31" would be truncated to the last +// day in February. +// 3. An error. The caller may get some error, an exception, an invalid date +// object, or perhaps return `false`. 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. +// +// The Abseil time library avoids this 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 Abseil 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": +// +// Example: +// +// const absl::CivilDay d(2015, 1, 31); +// +// // Answer 1: +// // Add 1 to the month field in the constructor, and rely on normalization. +// const auto normalized = absl::CivilDay(d.year(), d.month() + 1, d.day()); +// // 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 = absl::CivilMonth(d) + 1; +// const auto last_day_of_next_month = absl::CivilDay(next_month + 1) - 1; +// const auto capped = std::min(normalized, last_day_of_next_month); +// // capped == 2015-02-28 +// +// // Answer 3: +// // Signal an error if the normalized answer is not in next month. +// if (absl::CivilMonth(normalized) != next_month) { +// // error, month overflow +// } +// +using CivilSecond = + time_internal::cctz::detail::civil_time<time_internal::second_tag>; +using CivilMinute = + time_internal::cctz::detail::civil_time<time_internal::minute_tag>; +using CivilHour = + time_internal::cctz::detail::civil_time<time_internal::hour_tag>; +using CivilDay = + time_internal::cctz::detail::civil_time<time_internal::day_tag>; +using CivilMonth = + time_internal::cctz::detail::civil_time<time_internal::month_tag>; +using CivilYear = + time_internal::cctz::detail::civil_time<time_internal::year_tag>; + +// civil_year_t +// +// Type alias of a civil-time year value. This type is guaranteed to (at least) +// support any year value supported by `time_t`. +// +// Example: +// +// absl::CivilSecond cs = ...; +// absl::civil_year_t y = cs.year(); +// cs = absl::CivilSecond(y, 1, 1, 0, 0, 0); // CivilSecond(CivilYear(cs)) +// +using civil_year_t = time_internal::cctz::year_t; + +// civil_diff_t +// +// Type alias of the difference between two civil-time values. +// This type is used to indicate arguments that are not +// normalized (such as parameters to the civil-time constructors), the results +// of civil-time subtraction, or the operand to civil-time addition. +// +// Example: +// +// absl::civil_diff_t n_sec = cs1 - cs2; // cs1 == cs2 + n_sec; +// +using civil_diff_t = time_internal::cctz::diff_t; + +// Weekday::monday, Weekday::tuesday, Weekday::wednesday, Weekday::thursday, +// Weekday::friday, Weekday::saturday, Weekday::sunday +// +// The Weekday enum class represents the civil-time concept of a "weekday" with +// members for all days of the week. +// +// absl::Weekday wd = absl::Weekday::thursday; +// +using Weekday = time_internal::cctz::weekday; + +// GetWeekday() +// +// Returns the absl::Weekday for the given absl::CivilDay. +// +// Example: +// +// absl::CivilDay a(2015, 8, 13); +// absl::Weekday wd = absl::GetWeekday(a); // wd == absl::Weekday::thursday +// +inline Weekday GetWeekday(CivilDay cd) { + return time_internal::cctz::get_weekday(cd); +} + +// NextWeekday() +// PrevWeekday() +// +// Returns the absl::CivilDay that strictly follows or precedes a given +// absl::CivilDay, and that falls on the given absl::Weekday. +// +// Example, given the following month: +// +// 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 +// +// absl::CivilDay a(2015, 8, 13); +// // absl::GetWeekday(a) == absl::Weekday::thursday +// absl::CivilDay b = absl::NextWeekday(a, absl::Weekday::thursday); +// // b = 2015-08-20 +// absl::CivilDay c = absl::PrevWeekday(a, absl::Weekday::thursday); +// // c = 2015-08-06 +// +// absl::CivilDay d = ... +// // Gets the following Thursday if d is not already Thursday +// absl::CivilDay thurs1 = absl::PrevWeekday(d, absl::Weekday::thursday) + 7; +// // Gets the previous Thursday if d is not already Thursday +// absl::CivilDay thurs2 = absl::NextWeekday(d, absl::Weekday::thursday) - 7; +// +inline CivilDay NextWeekday(CivilDay cd, Weekday wd) { + return CivilDay(time_internal::cctz::next_weekday(cd, wd)); +} +inline CivilDay PrevWeekday(CivilDay cd, Weekday wd) { + return CivilDay(time_internal::cctz::prev_weekday(cd, wd)); +} + +// GetYearDay() +// +// Returns the day-of-year for the given absl::CivilDay. +// +// Example: +// +// absl::CivilDay a(2015, 1, 1); +// int yd_jan_1 = absl::GetYearDay(a); // yd_jan_1 = 1 +// absl::CivilDay b(2015, 12, 31); +// int yd_dec_31 = absl::GetYearDay(b); // yd_dec_31 = 365 +// +inline int GetYearDay(CivilDay cd) { + return time_internal::cctz::get_yearday(cd); +} + +// FormatCivilTime() +// +// Formats the given civil-time value into a string value of the following +// format: +// +// Type | Format +// --------------------------------- +// CivilSecond | YYYY-MM-DDTHH:MM:SS +// CivilMinute | YYYY-MM-DDTHH:MM +// CivilHour | YYYY-MM-DDTHH +// CivilDay | YYYY-MM-DD +// CivilMonth | YYYY-MM +// CivilYear | YYYY +// +// Example: +// +// absl::CivilDay d = absl::CivilDay(1969, 7, 20); +// string day_string = absl::FormatCivilTime(d); // "1969-07-20" +// +std::string FormatCivilTime(CivilSecond c); +std::string FormatCivilTime(CivilMinute c); +std::string FormatCivilTime(CivilHour c); +std::string FormatCivilTime(CivilDay c); +std::string FormatCivilTime(CivilMonth c); +std::string FormatCivilTime(CivilYear c); + +namespace time_internal { // For functions found via ADL on civil-time tags. + +// Streaming Operators +// +// Each civil-time type may be sent to an output stream using operator<<(). +// The result matches the string produced by `FormatCivilTime()`. +// +// Example: +// +// absl::CivilDay d = absl::CivilDay("1969-07-20"); +// std::cout << "Date is: " << d << "\n"; +// +std::ostream& operator<<(std::ostream& os, CivilYear y); +std::ostream& operator<<(std::ostream& os, CivilMonth m); +std::ostream& operator<<(std::ostream& os, CivilDay d); +std::ostream& operator<<(std::ostream& os, CivilHour h); +std::ostream& operator<<(std::ostream& os, CivilMinute m); +std::ostream& operator<<(std::ostream& os, CivilSecond s); + +} // namespace time_internal + +} // inline namespace lts_2018_12_18 +} // namespace absl + +#endif // ABSL_TIME_CIVIL_TIME_H_ |