1 // Copyright 2016 Google Inc. All Rights Reserved. 2 // 3 // Licensed under the Apache License, Version 2.0 (the "License"); 4 // you may not use this file except in compliance with the License. 5 // You may obtain a copy of the License at 6 // 7 // https://www.apache.org/licenses/LICENSE-2.0 8 // 9 // Unless required by applicable law or agreed to in writing, software 10 // distributed under the License is distributed on an "AS IS" BASIS, 11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 // See the License for the specific language governing permissions and 13 // limitations under the License. 14 15 #ifndef ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_ 16 #define ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_ 17 18 #include "absl/base/config.h" 19 #include "absl/time/internal/cctz/include/cctz/civil_time_detail.h" 20 21 namespace absl { 22 ABSL_NAMESPACE_BEGIN 23 namespace time_internal { 24 namespace cctz { 25 26 // The term "civil time" refers to the legally recognized human-scale time 27 // that is represented by the six fields YYYY-MM-DD hh:mm:ss. Modern-day civil 28 // time follows the Gregorian Calendar and is a time-zone-independent concept. 29 // A "date" is perhaps the most common example of a civil time (represented in 30 // this library as cctz::civil_day). This library provides six classes and a 31 // handful of functions that help with rounding, iterating, and arithmetic on 32 // civil times while avoiding complications like daylight-saving time (DST). 33 // 34 // The following six classes form the core of this civil-time library: 35 // 36 // * civil_second 37 // * civil_minute 38 // * civil_hour 39 // * civil_day 40 // * civil_month 41 // * civil_year 42 // 43 // Each class is a simple value type with the same interface for construction 44 // and the same six accessors for each of the civil fields (year, month, day, 45 // hour, minute, and second, aka YMDHMS). These classes differ only in their 46 // alignment, which is indicated by the type name and specifies the field on 47 // which arithmetic operates. 48 // 49 // Each class can be constructed by passing up to six optional integer 50 // arguments representing the YMDHMS fields (in that order) to the 51 // constructor. Omitted fields are assigned their minimum valid value. Hours, 52 // minutes, and seconds will be set to 0, month and day will be set to 1, and 53 // since there is no minimum valid year, it will be set to 1970. So, a 54 // default-constructed civil-time object will have YMDHMS fields representing 55 // "1970-01-01 00:00:00". Fields that are out-of-range are normalized (e.g., 56 // October 32 -> November 1) so that all civil-time objects represent valid 57 // values. 58 // 59 // Each civil-time class is aligned to the civil-time field indicated in the 60 // class's name after normalization. Alignment is performed by setting all the 61 // inferior fields to their minimum valid value (as described above). The 62 // following are examples of how each of the six types would align the fields 63 // representing November 22, 2015 at 12:34:56 in the afternoon. (Note: the 64 // string format used here is not important; it's just a shorthand way of 65 // showing the six YMDHMS fields.) 66 // 67 // civil_second 2015-11-22 12:34:56 68 // civil_minute 2015-11-22 12:34:00 69 // civil_hour 2015-11-22 12:00:00 70 // civil_day 2015-11-22 00:00:00 71 // civil_month 2015-11-01 00:00:00 72 // civil_year 2015-01-01 00:00:00 73 // 74 // Each civil-time type performs arithmetic on the field to which it is 75 // aligned. This means that adding 1 to a civil_day increments the day field 76 // (normalizing as necessary), and subtracting 7 from a civil_month operates 77 // on the month field (normalizing as necessary). All arithmetic produces a 78 // valid civil time. Difference requires two similarly aligned civil-time 79 // objects and returns the scalar answer in units of the objects' alignment. 80 // For example, the difference between two civil_hour objects will give an 81 // answer in units of civil hours. 82 // 83 // In addition to the six civil-time types just described, there are 84 // a handful of helper functions and algorithms for performing common 85 // calculations. These are described below. 86 // 87 // Note: In C++14 and later, this library is usable in a constexpr context. 88 // 89 // CONSTRUCTION: 90 // 91 // Each of the civil-time types can be constructed in two ways: by directly 92 // passing to the constructor up to six (optional) integers representing the 93 // YMDHMS fields, or by copying the YMDHMS fields from a differently aligned 94 // civil-time type. 95 // 96 // civil_day default_value; // 1970-01-01 00:00:00 97 // 98 // civil_day a(2015, 2, 3); // 2015-02-03 00:00:00 99 // civil_day b(2015, 2, 3, 4, 5, 6); // 2015-02-03 00:00:00 100 // civil_day c(2015); // 2015-01-01 00:00:00 101 // 102 // civil_second ss(2015, 2, 3, 4, 5, 6); // 2015-02-03 04:05:06 103 // civil_minute mm(ss); // 2015-02-03 04:05:00 104 // civil_hour hh(mm); // 2015-02-03 04:00:00 105 // civil_day d(hh); // 2015-02-03 00:00:00 106 // civil_month m(d); // 2015-02-01 00:00:00 107 // civil_year y(m); // 2015-01-01 00:00:00 108 // 109 // m = civil_month(y); // 2015-01-01 00:00:00 110 // d = civil_day(m); // 2015-01-01 00:00:00 111 // hh = civil_hour(d); // 2015-01-01 00:00:00 112 // mm = civil_minute(hh); // 2015-01-01 00:00:00 113 // ss = civil_second(mm); // 2015-01-01 00:00:00 114 // 115 // ALIGNMENT CONVERSION: 116 // 117 // The alignment of a civil-time object cannot change, but the object may be 118 // used to construct a new object with a different alignment. This is referred 119 // to as "realigning". When realigning to a type with the same or more 120 // precision (e.g., civil_day -> civil_second), the conversion may be 121 // performed implicitly since no information is lost. However, if information 122 // could be discarded (e.g., civil_second -> civil_day), the conversion must 123 // be explicit at the call site. 124 // 125 // void fun(const civil_day& day); 126 // 127 // civil_second cs; 128 // fun(cs); // Won't compile because data may be discarded 129 // fun(civil_day(cs)); // OK: explicit conversion 130 // 131 // civil_day cd; 132 // fun(cd); // OK: no conversion needed 133 // 134 // civil_month cm; 135 // fun(cm); // OK: implicit conversion to civil_day 136 // 137 // NORMALIZATION: 138 // 139 // Integer arguments passed to the constructor may be out-of-range, in which 140 // case they are normalized to produce a valid civil-time object. This enables 141 // natural arithmetic on constructor arguments without worrying about the 142 // field's range. Normalization guarantees that there are no invalid 143 // civil-time objects. 144 // 145 // civil_day d(2016, 10, 32); // Out-of-range day; normalized to 2016-11-01 146 // 147 // Note: If normalization is undesired, you can signal an error by comparing 148 // the constructor arguments to the normalized values returned by the YMDHMS 149 // properties. 150 // 151 // PROPERTIES: 152 // 153 // All civil-time types have accessors for all six of the civil-time fields: 154 // year, month, day, hour, minute, and second. Recall that fields inferior to 155 // the type's alignment will be set to their minimum valid value. 156 // 157 // civil_day d(2015, 6, 28); 158 // // d.year() == 2015 159 // // d.month() == 6 160 // // d.day() == 28 161 // // d.hour() == 0 162 // // d.minute() == 0 163 // // d.second() == 0 164 // 165 // COMPARISON: 166 // 167 // Comparison always considers all six YMDHMS fields, regardless of the type's 168 // alignment. Comparison between differently aligned civil-time types is 169 // allowed. 170 // 171 // civil_day feb_3(2015, 2, 3); // 2015-02-03 00:00:00 172 // civil_day mar_4(2015, 3, 4); // 2015-03-04 00:00:00 173 // // feb_3 < mar_4 174 // // civil_year(feb_3) == civil_year(mar_4) 175 // 176 // civil_second feb_3_noon(2015, 2, 3, 12, 0, 0); // 2015-02-03 12:00:00 177 // // feb_3 < feb_3_noon 178 // // feb_3 == civil_day(feb_3_noon) 179 // 180 // // Iterates all the days of February 2015. 181 // for (civil_day d(2015, 2, 1); d < civil_month(2015, 3); ++d) { 182 // // ... 183 // } 184 // 185 // STREAMING: 186 // 187 // Each civil-time type may be sent to an output stream using operator<<(). 188 // The output format follows the pattern "YYYY-MM-DDThh:mm:ss" where fields 189 // inferior to the type's alignment are omitted. 190 // 191 // civil_second cs(2015, 2, 3, 4, 5, 6); 192 // std::cout << cs << "\n"; // Outputs: 2015-02-03T04:05:06 193 // 194 // civil_day cd(cs); 195 // std::cout << cd << "\n"; // Outputs: 2015-02-03 196 // 197 // civil_year cy(cs); 198 // std::cout << cy << "\n"; // Outputs: 2015 199 // 200 // ARITHMETIC: 201 // 202 // Civil-time types support natural arithmetic operators such as addition, 203 // subtraction, and difference. Arithmetic operates on the civil-time field 204 // indicated in the type's name. Difference requires arguments with the same 205 // alignment and returns the answer in units of the alignment. 206 // 207 // civil_day a(2015, 2, 3); 208 // ++a; // 2015-02-04 00:00:00 209 // --a; // 2015-02-03 00:00:00 210 // civil_day b = a + 1; // 2015-02-04 00:00:00 211 // civil_day c = 1 + b; // 2015-02-05 00:00:00 212 // int n = c - a; // n = 2 (civil days) 213 // int m = c - civil_month(c); // Won't compile: different types. 214 // 215 // EXAMPLE: Adding a month to January 31. 216 // 217 // One of the classic questions that arises when considering a civil-time 218 // library (or a date library or a date/time library) is this: "What happens 219 // when you add a month to January 31?" This is an interesting question 220 // because there could be a number of possible answers: 221 // 222 // 1. March 3 (or 2 if a leap year). This may make sense if the operation 223 // wants the equivalent of February 31. 224 // 2. February 28 (or 29 if a leap year). This may make sense if the operation 225 // wants the last day of January to go to the last day of February. 226 // 3. Error. The caller may get some error, an exception, an invalid date 227 // object, or maybe false is returned. This may make sense because there is 228 // no single unambiguously correct answer to the question. 229 // 230 // Practically speaking, any answer that is not what the programmer intended 231 // is the wrong answer. 232 // 233 // This civil-time library avoids the problem by making it impossible to ask 234 // ambiguous questions. All civil-time objects are aligned to a particular 235 // civil-field boundary (such as aligned to a year, month, day, hour, minute, 236 // or second), and arithmetic operates on the field to which the object is 237 // aligned. This means that in order to "add a month" the object must first be 238 // aligned to a month boundary, which is equivalent to the first day of that 239 // month. 240 // 241 // Of course, there are ways to compute an answer the question at hand using 242 // this civil-time library, but they require the programmer to be explicit 243 // about the answer they expect. To illustrate, let's see how to compute all 244 // three of the above possible answers to the question of "Jan 31 plus 1 245 // month": 246 // 247 // const civil_day d(2015, 1, 31); 248 // 249 // // Answer 1: 250 // // Add 1 to the month field in the constructor, and rely on normalization. 251 // const auto ans_normalized = civil_day(d.year(), d.month() + 1, d.day()); 252 // // ans_normalized == 2015-03-03 (aka Feb 31) 253 // 254 // // Answer 2: 255 // // Add 1 to month field, capping to the end of next month. 256 // const auto next_month = civil_month(d) + 1; 257 // const auto last_day_of_next_month = civil_day(next_month + 1) - 1; 258 // const auto ans_capped = std::min(ans_normalized, last_day_of_next_month); 259 // // ans_capped == 2015-02-28 260 // 261 // // Answer 3: 262 // // Signal an error if the normalized answer is not in next month. 263 // if (civil_month(ans_normalized) != next_month) { 264 // // error, month overflow 265 // } 266 // 267 using civil_year = detail::civil_year; 268 using civil_month = detail::civil_month; 269 using civil_day = detail::civil_day; 270 using civil_hour = detail::civil_hour; 271 using civil_minute = detail::civil_minute; 272 using civil_second = detail::civil_second; 273 274 // An enum class with members monday, tuesday, wednesday, thursday, friday, 275 // saturday, and sunday. These enum values may be sent to an output stream 276 // using operator<<(). The result is the full weekday name in English with a 277 // leading capital letter. 278 // 279 // weekday wd = weekday::thursday; 280 // std::cout << wd << "\n"; // Outputs: Thursday 281 // 282 using detail::weekday; 283 284 // Returns the weekday for the given civil-time value. 285 // 286 // civil_day a(2015, 8, 13); 287 // weekday wd = get_weekday(a); // wd == weekday::thursday 288 // 289 using detail::get_weekday; 290 291 // Returns the civil_day that strictly follows or precedes the given 292 // civil_day, and that falls on the given weekday. 293 // 294 // For example, given: 295 // 296 // August 2015 297 // Su Mo Tu We Th Fr Sa 298 // 1 299 // 2 3 4 5 6 7 8 300 // 9 10 11 12 13 14 15 301 // 16 17 18 19 20 21 22 302 // 23 24 25 26 27 28 29 303 // 30 31 304 // 305 // civil_day a(2015, 8, 13); // get_weekday(a) == weekday::thursday 306 // civil_day b = next_weekday(a, weekday::thursday); // b = 2015-08-20 307 // civil_day c = prev_weekday(a, weekday::thursday); // c = 2015-08-06 308 // 309 // civil_day d = ... 310 // // Gets the following Thursday if d is not already Thursday 311 // civil_day thurs1 = next_weekday(d - 1, weekday::thursday); 312 // // Gets the previous Thursday if d is not already Thursday 313 // civil_day thurs2 = prev_weekday(d + 1, weekday::thursday); 314 // 315 using detail::next_weekday; 316 using detail::prev_weekday; 317 318 // Returns the day-of-year for the given civil-time value. 319 // 320 // civil_day a(2015, 1, 1); 321 // int yd_jan_1 = get_yearday(a); // yd_jan_1 = 1 322 // civil_day b(2015, 12, 31); 323 // int yd_dec_31 = get_yearday(b); // yd_dec_31 = 365 324 // 325 using detail::get_yearday; 326 327 } // namespace cctz 328 } // namespace time_internal 329 ABSL_NAMESPACE_END 330 } // namespace absl 331 332 #endif // ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_ 333