1 files changed, 98 insertions, 0 deletions
diff --git a/subsurface-core/time.c b/subsurface-core/time.c
new file mode 100644
index 000000000..b658954bc
--- /dev/null
+++ b/subsurface-core/time.c
@@ -0,0 +1,98 @@
+#include <string.h>
+#include "dive.h"
+
+/*
+ * Convert 64-bit timestamp to 'struct tm' in UTC.
+ *
+ * On 32-bit machines, only do 64-bit arithmetic for the seconds
+ * part, after that we do everything in 'long'. 64-bit divides
+ * are unnecessary once you're counting minutes (32-bit minutes:
+ * 8000+ years).
+ */
+void utc_mkdate(timestamp_t timestamp, struct tm *tm)
+{
+	static const int mdays[] = {
+		31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
+	};
+	static const int mdays_leap[] = {
+		31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
+	};
+	unsigned long val;
+	unsigned int leapyears;
+	int m;
+	const int *mp;
+
+	memset(tm, 0, sizeof(*tm));
+
+	/* seconds since 1970 -> minutes since 1970 */
+	tm->tm_sec = timestamp % 60;
+	val = timestamp /= 60;
+
+	/* Do the simple stuff */
+	tm->tm_min = val % 60;
+	val /= 60;
+	tm->tm_hour = val % 24;
+	val /= 24;
+
+	/* Jan 1, 1970 was a Thursday (tm_wday=4) */
+	tm->tm_wday = (val + 4) % 7;
+
+	/*
+	 * Now we're in "days since Jan 1, 1970". To make things easier,
+	 * let's make it "days since Jan 1, 1968", since that's a leap-year
+	 */
+	val += 365 + 366;
+
+	/* This only works up until 2099 (2100 isn't a leap-year) */
+	leapyears = val / (365 * 4 + 1);
+	val %= (365 * 4 + 1);
+	tm->tm_year = 68 + leapyears * 4;
+
+	/* Handle the leap-year itself */
+	mp = mdays_leap;
+	if (val > 365) {
+		tm->tm_year++;
+		val -= 366;
+		tm->tm_year += val / 365;
+		val %= 365;
+		mp = mdays;
+	}
+
+	for (m = 0; m < 12; m++) {
+		if (val < *mp)
+			break;
+		val -= *mp++;
+	}
+	tm->tm_mday = val + 1;
+	tm->tm_mon = m;
+}
+
+timestamp_t utc_mktime(struct tm *tm)
+{
+	static const int mdays[] = {
+		0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
+	};
+	int year = tm->tm_year;
+	int month = tm->tm_mon;
+	int day = tm->tm_mday;
+
+	/* First normalize relative to 1900 */
+	if (year < 70)
+		year += 100;
+	else if (year > 1900)
+		year -= 1900;
+
+	/* Normalized to Jan 1, 1970: unix time */
+	year -= 70;
+
+	if (year < 0 || year > 129) /* algo only works for 1970-2099 */
+		return -1;
+	if (month < 0 || month > 11) /* array bounds */
+		return -1;
+	if (month < 2 || (year + 2) % 4)
+		day--;
+	if (tm->tm_hour < 0 || tm->tm_min < 0 || tm->tm_sec < 0)
+		return -1;
+	return (year * 365 + (year + 1) / 4 + mdays[month] + day) * 24 * 60 * 60UL +
+	       tm->tm_hour * 60 * 60 + tm->tm_min * 60 + tm->tm_sec;
+}