[libc] create TimeReader to look at a struct tm (#126138)

michaelrj-google · Icohedron · commit e7846641f27d · 2025-02-11T23:36:39.000Z
In the process of adding strftime (#122556) I wrote this utility class to simplify reading from a struct tm. It provides helper functions that return basically everything needed by strftime. It's not tested directly, but it is thoroughly exercised by the strftime tests.
diff --git a/libc/include/llvm-libc-types/struct_tm.h b/libc/include/llvm-libc-types/struct_tm.h
@@ -19,6 +19,7 @@ struct tm {
   int tm_wday;  // days since Sunday
   int tm_yday;  // days since January
   int tm_isdst; // Daylight Saving Time flag
+  // TODO: add tm_gmtoff and tm_zone? (posix extensions)
 };
 
 #endif // LLVM_LIBC_TYPES_STRUCT_TM_H
diff --git a/libc/src/time/CMakeLists.txt b/libc/src/time/CMakeLists.txt
@@ -22,6 +22,8 @@ add_object_library(
   DEPENDS
     libc.include.time
     libc.src.__support.CPP.limits
+    libc.src.__support.CPP.string_view
+    libc.src.__support.CPP.optional
     libc.src.errno.errno
     .time_constants
     libc.hdr.types.time_t
diff --git a/libc/src/time/mktime.cpp b/libc/src/time/mktime.cpp
@@ -14,100 +14,8 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-// Returns number of years from (1, year).
-static constexpr int64_t get_num_of_leap_years_before(int64_t year) {
-  return (year / 4) - (year / 100) + (year / 400);
-}
-
-// Returns True if year is a leap year.
-static constexpr bool is_leap_year(const int64_t year) {
-  return (((year) % 4) == 0 && (((year) % 100) != 0 || ((year) % 400) == 0));
-}
-
 LLVM_LIBC_FUNCTION(time_t, mktime, (struct tm * tm_out)) {
-  // Unlike most C Library functions, mktime doesn't just die on bad input.
-  // TODO(rtenneti); Handle leap seconds.
-  int64_t tm_year_from_base = tm_out->tm_year + time_constants::TIME_YEAR_BASE;
-
-  // 32-bit end-of-the-world is 03:14:07 UTC on 19 January 2038.
-  if (sizeof(time_t) == 4 &&
-      tm_year_from_base >= time_constants::END_OF32_BIT_EPOCH_YEAR) {
-    if (tm_year_from_base > time_constants::END_OF32_BIT_EPOCH_YEAR)
-      return time_utils::out_of_range();
-    if (tm_out->tm_mon > 0)
-      return time_utils::out_of_range();
-    if (tm_out->tm_mday > 19)
-      return time_utils::out_of_range();
-    else if (tm_out->tm_mday == 19) {
-      if (tm_out->tm_hour > 3)
-        return time_utils::out_of_range();
-      else if (tm_out->tm_hour == 3) {
-        if (tm_out->tm_min > 14)
-          return time_utils::out_of_range();
-        else if (tm_out->tm_min == 14) {
-          if (tm_out->tm_sec > 7)
-            return time_utils::out_of_range();
-        }
-      }
-    }
-  }
-
-  // Years are ints.  A 32-bit year will fit into a 64-bit time_t.
-  // A 64-bit year will not.
-  static_assert(
-      sizeof(int) == 4,
-      "ILP64 is unimplemented. This implementation requires 32-bit integers.");
-
-  // Calculate number of months and years from tm_mon.
-  int64_t month = tm_out->tm_mon;
-  if (month < 0 || month >= time_constants::MONTHS_PER_YEAR - 1) {
-    int64_t years = month / 12;
-    month %= 12;
-    if (month < 0) {
-      years--;
-      month += 12;
-    }
-    tm_year_from_base += years;
-  }
-  bool tm_year_is_leap = is_leap_year(tm_year_from_base);
-
-  // Calculate total number of days based on the month and the day (tm_mday).
-  int64_t total_days = tm_out->tm_mday - 1;
-  for (int64_t i = 0; i < month; ++i)
-    total_days += time_constants::NON_LEAP_YEAR_DAYS_IN_MONTH[i];
-  // Add one day if it is a leap year and the month is after February.
-  if (tm_year_is_leap && month > 1)
-    total_days++;
-
-  // Calculate total numbers of days based on the year.
-  total_days += (tm_year_from_base - time_constants::EPOCH_YEAR) *
-                time_constants::DAYS_PER_NON_LEAP_YEAR;
-  if (tm_year_from_base >= time_constants::EPOCH_YEAR) {
-    total_days += get_num_of_leap_years_before(tm_year_from_base - 1) -
-                  get_num_of_leap_years_before(time_constants::EPOCH_YEAR);
-  } else if (tm_year_from_base >= 1) {
-    total_days -= get_num_of_leap_years_before(time_constants::EPOCH_YEAR) -
-                  get_num_of_leap_years_before(tm_year_from_base - 1);
-  } else {
-    // Calculate number of leap years until 0th year.
-    total_days -= get_num_of_leap_years_before(time_constants::EPOCH_YEAR) -
-                  get_num_of_leap_years_before(0);
-    if (tm_year_from_base <= 0) {
-      total_days -= 1; // Subtract 1 for 0th year.
-      // Calculate number of leap years until -1 year
-      if (tm_year_from_base < 0) {
-        total_days -= get_num_of_leap_years_before(-tm_year_from_base) -
-                      get_num_of_leap_years_before(1);
-      }
-    }
-  }
-
-  // TODO: https://github.com/llvm/llvm-project/issues/121962
-  // Need to handle timezone and update of tm_isdst.
-  int64_t seconds = tm_out->tm_sec +
-                    tm_out->tm_min * time_constants::SECONDS_PER_MIN +
-                    tm_out->tm_hour * time_constants::SECONDS_PER_HOUR +
-                    total_days * time_constants::SECONDS_PER_DAY;
+  int64_t seconds = time_utils::mktime_internal(tm_out);
 
   // Update the tm structure's year, month, day, etc. from seconds.
   if (time_utils::update_from_seconds(seconds, tm_out) < 0)
diff --git a/libc/src/time/time_constants.h b/libc/src/time/time_constants.h
@@ -18,7 +18,7 @@ namespace LIBC_NAMESPACE_DECL {
 namespace time_constants {
 
 enum Month : int {
-  JANUARY,
+  JANUARY = 0,
   FEBRUARY,
   MARCH,
   APRIL,
@@ -32,14 +32,28 @@ enum Month : int {
   DECEMBER
 };
 
+enum WeekDay : int {
+  SUNDAY = 0,
+  MONDAY,
+  TUESDAY,
+  WEDNESDAY,
+  THURSDAY,
+  FRIDAY,
+  SATURDAY
+};
+
 constexpr int SECONDS_PER_MIN = 60;
 constexpr int MINUTES_PER_HOUR = 60;
 constexpr int HOURS_PER_DAY = 24;
 constexpr int DAYS_PER_WEEK = 7;
+constexpr int WEEKS_PER_YEAR = 52;
 constexpr int MONTHS_PER_YEAR = 12;
 constexpr int DAYS_PER_NON_LEAP_YEAR = 365;
 constexpr int DAYS_PER_LEAP_YEAR = 366;
 
+constexpr int LAST_DAY_OF_NON_LEAP_YEAR = DAYS_PER_NON_LEAP_YEAR - 1;
+constexpr int LAST_DAY_OF_LEAP_YEAR = DAYS_PER_LEAP_YEAR - 1;
+
 constexpr int SECONDS_PER_HOUR = SECONDS_PER_MIN * MINUTES_PER_HOUR;
 constexpr int SECONDS_PER_DAY = SECONDS_PER_HOUR * HOURS_PER_DAY;
 constexpr int NUMBER_OF_SECONDS_IN_LEAP_YEAR =
@@ -49,6 +63,8 @@ constexpr int TIME_YEAR_BASE = 1900;
 constexpr int EPOCH_YEAR = 1970;
 constexpr int EPOCH_WEEK_DAY = 4;
 
+constexpr int ISO_FIRST_DAY_OF_YEAR = 3; // the 4th day of the year, 0-indexed.
+
 // For asctime the behavior is undefined if struct tm's tm_wday or tm_mon are
 // not within the normal ranges as defined in <time.h>, or if struct tm's
 // tm_year exceeds {INT_MAX}-1990, or if the below asctime_internal algorithm
diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
@@ -12,9 +12,103 @@
 #include "src/__support/macros/config.h"
 #include "src/time/time_constants.h"
 
+#include <stdint.h>
+
 namespace LIBC_NAMESPACE_DECL {
 namespace time_utils {
 
+// TODO: clean this up in a followup patch
+int64_t mktime_internal(const tm *tm_out) {
+  // Unlike most C Library functions, mktime doesn't just die on bad input.
+  // TODO(rtenneti); Handle leap seconds.
+  int64_t tm_year_from_base = tm_out->tm_year + time_constants::TIME_YEAR_BASE;
+
+  // 32-bit end-of-the-world is 03:14:07 UTC on 19 January 2038.
+  if (sizeof(time_t) == 4 &&
+      tm_year_from_base >= time_constants::END_OF32_BIT_EPOCH_YEAR) {
+    if (tm_year_from_base > time_constants::END_OF32_BIT_EPOCH_YEAR)
+      return time_utils::out_of_range();
+    if (tm_out->tm_mon > 0)
+      return time_utils::out_of_range();
+    if (tm_out->tm_mday > 19)
+      return time_utils::out_of_range();
+    else if (tm_out->tm_mday == 19) {
+      if (tm_out->tm_hour > 3)
+        return time_utils::out_of_range();
+      else if (tm_out->tm_hour == 3) {
+        if (tm_out->tm_min > 14)
+          return time_utils::out_of_range();
+        else if (tm_out->tm_min == 14) {
+          if (tm_out->tm_sec > 7)
+            return time_utils::out_of_range();
+        }
+      }
+    }
+  }
+
+  // Years are ints.  A 32-bit year will fit into a 64-bit time_t.
+  // A 64-bit year will not.
+  static_assert(
+      sizeof(int) == 4,
+      "ILP64 is unimplemented. This implementation requires 32-bit integers.");
+
+  // Calculate number of months and years from tm_mon.
+  int64_t month = tm_out->tm_mon;
+  if (month < 0 || month >= time_constants::MONTHS_PER_YEAR - 1) {
+    int64_t years = month / 12;
+    month %= 12;
+    if (month < 0) {
+      years--;
+      month += 12;
+    }
+    tm_year_from_base += years;
+  }
+  bool tm_year_is_leap = time_utils::is_leap_year(tm_year_from_base);
+
+  // Calculate total number of days based on the month and the day (tm_mday).
+  int64_t total_days = tm_out->tm_mday - 1;
+  for (int64_t i = 0; i < month; ++i)
+    total_days += time_constants::NON_LEAP_YEAR_DAYS_IN_MONTH[i];
+  // Add one day if it is a leap year and the month is after February.
+  if (tm_year_is_leap && month > 1)
+    total_days++;
+
+  // Calculate total numbers of days based on the year.
+  total_days += (tm_year_from_base - time_constants::EPOCH_YEAR) *
+                time_constants::DAYS_PER_NON_LEAP_YEAR;
+  if (tm_year_from_base >= time_constants::EPOCH_YEAR) {
+    total_days +=
+        time_utils::get_num_of_leap_years_before(tm_year_from_base - 1) -
+        time_utils::get_num_of_leap_years_before(time_constants::EPOCH_YEAR);
+  } else if (tm_year_from_base >= 1) {
+    total_days -=
+        time_utils::get_num_of_leap_years_before(time_constants::EPOCH_YEAR) -
+        time_utils::get_num_of_leap_years_before(tm_year_from_base - 1);
+  } else {
+    // Calculate number of leap years until 0th year.
+    total_days -=
+        time_utils::get_num_of_leap_years_before(time_constants::EPOCH_YEAR) -
+        time_utils::get_num_of_leap_years_before(0);
+    if (tm_year_from_base <= 0) {
+      total_days -= 1; // Subtract 1 for 0th year.
+      // Calculate number of leap years until -1 year
+      if (tm_year_from_base < 0) {
+        total_days -=
+            time_utils::get_num_of_leap_years_before(-tm_year_from_base) -
+            time_utils::get_num_of_leap_years_before(1);
+      }
+    }
+  }
+
+  // TODO: https://github.com/llvm/llvm-project/issues/121962
+  // Need to handle timezone and update of tm_isdst.
+  int64_t seconds = tm_out->tm_sec +
+                    tm_out->tm_min * time_constants::SECONDS_PER_MIN +
+                    tm_out->tm_hour * time_constants::SECONDS_PER_HOUR +
+                    total_days * time_constants::SECONDS_PER_DAY;
+  return seconds;
+}
+
 static int64_t computeRemainingYears(int64_t daysPerYears,
                                      int64_t quotientYears,
                                      int64_t *remainingDays) {
@@ -42,7 +136,7 @@ static int64_t computeRemainingYears(int64_t daysPerYears,
 //
 // Compute the number of months from the remaining days. Finally, adjust years
 // to be 1900 and months to be from January.
-int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
+int64_t update_from_seconds(int64_t total_seconds, tm *tm) {
   // Days in month starting from March in the year 2000.
   static const char daysInMonth[] = {31 /* Mar */, 30, 31, 30, 31, 31,
                                      30,           31, 30, 31, 31, 29};
diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
