Check in new file for improved time handling

* time.c: New file.

time.c

@@ -0,0 +1,94 @@
+/* Wayland compositor running on top of an X server.
+
+Copyright (C) 2022 to various contributors.
+
+This file is part of 12to11.
+
+12to11 is free software: you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation, either version 3 of the License, or (at your
+option) any later version.
+
+12to11 is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with 12to11.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include "compositor.h"
+
+/* The latest known time.  */
+static Timestamp current_time;
+
+/* Half a month; used as a threshold in various places.  */
+#define HalfMonth (1U << 31)
+
+/* The protocol translator can run for more than 48 days.  That makes
+   normal X timestamp handling unsafe, as the X server wraps around
+   timestamps after that much time.  This function creates a Timestamp
+   from the server time while handling overflow.  */
+
+Timestamp
+TimestampFromServerTime (Time time)
+{
+  if (time >= current_time.milliseconds)
+    {
+      /* No overflow happened.  */
+      current_time.milliseconds = time;
+      return current_time;
+    }
+
+  /* The server time wrapped around.  */
+  current_time.months++;
+  current_time.milliseconds = time;
+
+  return current_time;
+}
+
+Timestamp
+TimestampFromClientTime (Time time)
+{
+  Timestamp timestamp;
+
+  timestamp.months = current_time.months;
+  timestamp.milliseconds = time;
+
+  /* Create a timestamp from a Time that may lie in the past.
+     Compensate for wraparound if the difference between the
+     timestamps is more than half a month.  */
+
+  if (time < current_time.milliseconds
+      && (current_time.milliseconds - time) >= HalfMonth)
+    timestamp.months += 1;
+  else if (time > current_time.milliseconds
+	   && (time - current_time.milliseconds) >= HalfMonth)
+    timestamp.months -= 1;
+
+  return timestamp;
+}
+
+TimestampDifference
+CompareTimestamps (Timestamp a, Timestamp b)
+{
+  if (a.months < b.months)
+    return Earlier;
+
+  if (a.months > b.months)
+    return Later;
+
+  if (a.milliseconds < b.milliseconds)
+    return Earlier;
+
+  if (a.milliseconds > b.milliseconds)
+    return Later;
+
+  return Same;
+}
+
+TimestampDifference
+CompareTimeWith (Time a, Timestamp b)
+{
+  return CompareTimestamps (TimestampFromClientTime (a), b);
+}