// 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 { 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); // std::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 } // namespace absl #endif // ABSL_TIME_CIVIL_TIME_H_