about summary refs log blame commit diff
path: root/absl/time/civil_time.h
blob: f6f76cd65cda3fea62da51c0d4baae6c49d8eed2 (plain) (tree)
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486










































































                                                                                


























































































































































































































































































































































































































                                                                                
// 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);
//   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_