libc: minimal: provide gmtime implementation

Implement the conversion from UNIX time to broken-down civil time per
the gmtime() and gmtime_r() functions.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

lib/libc/minimal/CMakeLists.txt

@@ -17,4 +17,5 @@ zephyr_library_sources(
   source/stdout/stdout_console.c
   source/stdout/sprintf.c
   source/stdout/fprintf.c
+  source/time/gmtime.c
 )

lib/libc/minimal/include/time.h

@@ -9,6 +9,7 @@
 #define ZEPHYR_LIB_LIBC_MINIMAL_INCLUDE_TIME_H_
 
 #include <stdint.h>
+#include <bits/restrict.h>
 
 /* Minimal time.h to fulfill the requirements of certain libraries
  * like mbedTLS and to support time APIs.
@@ -38,6 +39,15 @@ struct timespec {
 	long tv_nsec;
 };
 
+/*
+ * Conversion between civil time and UNIX time.  The companion
+ * localtime() and inverse mktime() are not provided here since they
+ * require access to time zone information.
+ */
+struct tm *gmtime(const time_t *timep);
+struct tm *gmtime_r(const time_t *_MLIBC_RESTRICT timep,
+		    struct tm *_MLIBC_RESTRICT result);
+
 #ifdef __cplusplus
 }
 #endif

lib/libc/minimal/source/time/gmtime.c

@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2019 Peter Bigot Consulting, LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * The time_civil_from_days function is derived directly from public
+ * domain content written by Howard Hinnant and available at:
+ * http://howardhinnant.github.io/date_algorithms.html#civil_from_days
+ */
+
+#include <time.h>
+
+/* A signed type with the representation of time_t without its
+ * impliciations.
+ */
+typedef time_t bigint_type;
+
+/** Convert a UNIX time to civil time.
+ *
+ * This converts integral seconds since (before) 1970-01-01T00:00:00
+ * to the POSIX standard civil time representation.  Any adjustments
+ * due to time zone, leap seconds, or a different epoch must be
+ * applied to @p time before invoking this function.
+ *
+ * @param time the time represented as seconds.
+ *
+ * @return the time information for corresponding to the provided
+ * instant.
+ *
+ * @see http://howardhinnant.github.io/date_algorithms.html#civil_from_days
+ */
+static void time_civil_from_days(bigint_type z,
+				 struct tm *_MLIBC_RESTRICT tp)
+{
+	tp->tm_wday = (z >= -4) ? ((z + 4) % 7) : ((z + 5) % 7 + 6);
+	z += 719468;
+
+	bigint_type era = ((z >= 0) ? z : (z - 146096)) / 146097;
+	unsigned int doe = (z - era * (bigint_type)146097);
+	unsigned int yoe = (doe - doe / 1460U + doe / 36524U - doe / 146096U)
+		/ 365U;
+	bigint_type y = (time_t)yoe + era * 400;
+	unsigned int doy = doe - (365U * yoe + yoe / 4U - yoe / 100U);
+	unsigned int mp = (5U * doy + 2U) / 153U;
+	unsigned int d = doy - (153U * mp + 2U) / 5U + 1U;
+	unsigned int m = mp + ((mp < 10) ? 3 : -9);
+
+	tp->tm_year = y + (m <= 2) - 1900;
+	tp->tm_mon = m - 1;
+	tp->tm_mday = d;
+
+	/* Everything above is explained on the referenced page, but
+	 * doy is relative to --03-01 and we need it relative to
+	 * --01-01.
+	 *
+	 * doy=306 corresponds to --01-01, doy=364 to --02-28, and
+	 * doy=365 to --02-29.  So we can just subtract 306 to handle
+	 * January and February.
+	 *
+	 * For doy<306 we have to add the number of days before
+	 * --03-01, which is 59 in a common year and 60 in a leap
+	 * year.  Note that the first year in the era is a leap year.
+	 */
+	if (doy >= 306U) {
+		tp->tm_yday = doy - 306U;
+	} else {
+		tp->tm_yday = doy + 59U + (((yoe % 4U == 0U) && (yoe % 100U != 0U)) || (yoe == 0U));
+	}
+}
+
+/* Convert a UNIX time to civil time.
+ *
+ * This converts integral seconds since (before) 1970-01-01T00:00:00
+ * to the POSIX standard civil time representation.  Any adjustments
+ * due to time zone, leap seconds, or a different epoch must be
+ * applied to @p time before invoking this function.
+ */
+struct tm *gmtime_r(const time_t *_MLIBC_RESTRICT timep,
+		    struct tm *_MLIBC_RESTRICT tp)
+{
+	time_t z = *timep;
+	bigint_type days = (z >= 0 ? z : z - 86399) / 86400;
+	unsigned int rem = z - days * 86400;
+
+	time_civil_from_days(days, tp);
+
+	tp->tm_hour = rem / 60U / 60U;
+	rem -= tp->tm_hour * 60 * 60;
+	tp->tm_min = rem / 60;
+	tp->tm_sec = rem - tp->tm_min * 60;
+
+	return tp;
+}
+
+struct tm *gmtime(const time_t *timep)
+{
+	static struct tm shared;
+
+	return gmtime_r(timep, &shared);
+}