sys: timeutil: add module

Add a generic API to provide the inverse operation for gmtime and as a
home for future generic time-related functions that are not in POSIX.

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

include/sys/timeutil.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2019 Peter Bigot Consulting, LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @file
+ * @brief Utilities supporting operation on time data structures.
+ *
+ * POSIX defines gmtime() to convert from time_t to struct tm, but all
+ * inverse transformations are non-standard or require access to time
+ * zone information.  timeutil_timegm() implements the functionality
+ * of the GNU extension timegm() function.
+ */
+
+#ifndef ZEPHYR_INCLUDE_SYS_TIMEUTIL_H_
+#define ZEPHYR_INCLUDE_SYS_TIMEUTIL_H_
+
+#include <time.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Convert broken-down time to a POSIX epoch offset in seconds.
+ *
+ * @param tm pointer to broken down time.
+ *
+ * @return the corresponding time in the POSIX epoch time scale.
+ *
+ * @see http://man7.org/linux/man-pages/man3/timegm.3.html
+ */
+time_t timeutil_timegm(struct tm *tm);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ZEPHYR_INCLUDE_SYS_TIMEUTIL_H_ */

lib/os/CMakeLists.txt

@@ -12,6 +12,7 @@ zephyr_sources(
   mempool.c
   rb.c
   thread_entry.c
+  timeutil.c
   work_q.c
   )
 

lib/os/timeutil.c

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2019 Peter Bigot Consulting, LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * The time_days_from_civil function is derived directly from public
+ * domain content written by Howard Hinnant and available at:
+ * http://howardhinnant.github.io/date_algorithms.html#days_from_civil
+ */
+
+#include <zephyr/types.h>
+#include <sys/timeutil.h>
+
+/** Convert a civil (proleptic Gregorian) date to days relative to
+ * 1970-01-01.
+ *
+ * @param y the calendar year
+ * @param m the calendar month, in the range [1, 12]
+ * @param d the day of the month, in the range [1, last_day_of_month(y, m)]
+ *
+ * @return the signed number of days between the specified day and
+ * 1970-01-01
+ *
+ * @see http://howardhinnant.github.io/date_algorithms.html#days_from_civil
+ */
+static s64_t time_days_from_civil(s64_t y,
+				  unsigned int m,
+				  unsigned int d)
+{
+	y -= m <= 2;
+
+	s64_t era = (y >= 0 ? y : y - 399) / 400;
+	unsigned int yoe = y - era * 400;
+	unsigned int doy = (153U * (m + (m > 2 ? -3 : 9)) + 2U) / 5U + d;
+	unsigned int doe = yoe * 365U + yoe / 4U - yoe / 100U + doy;
+
+	return era * 146097 + (time_t)doe - 719468;
+}
+
+/** Convert civil time to UNIX time.
+ *
+ * @param tvp pointer to a civil time structure.  `tm_year`, `tm_mon`,
+ * `tm_mday`, `tm_hour`, `tm_min`, and `tm_sec` must be valid.  All
+ * other fields are ignored.
+ *
+ * @return the signed number of seconds between 1970-01-01T00:00:00
+ * and the specified time ignoring leap seconds and DST offsets.
+ */
+time_t timeutil_timegm(struct tm *tm)
+{
+	s64_t y = 1900 + (s64_t)tm->tm_year;
+	unsigned int m = tm->tm_mon + 1;
+	unsigned int d = tm->tm_mday - 1;
+	s64_t ndays = time_days_from_civil(y, m, d);
+
+	return (time_t)tm->tm_sec
+	       + 60 * (tm->tm_min + 60 * tm->tm_hour)
+	       + 86400 * ndays;
+}