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

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.

1 files changed, 95 insertions, 1 deletions

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index abc93b8cb961..3ccb2dd93496 100644
--- 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};