From d5e1d5ad99d4a1dd05f3c72cbbd9a985ef43fcf3 Mon Sep 17 00:00:00 2001
From: Nick Mathewson <nickm@torproject.org>
Date: Tue, 17 Apr 2012 15:16:08 -0400
Subject: [PATCH] Implement a GetTickCount-based monotonic timer for Windows

---
 event-internal.h |  7 +++++++
 event.c          | 19 +++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/event-internal.h b/event-internal.h
index 6910469c..a2fc8e80 100644
--- a/event-internal.h
+++ b/event-internal.h
@@ -68,6 +68,8 @@ extern "C" {
 #define HAVE_ANY_MONOTONIC 1
 #elif defined(EVENT__HAVE_MACH_ABSOLUTE_TIME)
 #define HAVE_ANY_MONOTONIC 1
+#elif defined(_WIN32)
+#define HAVE_ANY_MONOTONIC 1
 #endif
 
 /** Structure to define the backend of a given event_base. */
@@ -262,8 +264,13 @@ struct event_base {
 #define CLOCK_IS_SELECTED
 	int monotonic_clock;
 #endif
+#ifdef _WIN32
+	DWORD last_tick_count;
+	struct timeval adjust_tick_count;
+#endif
 #if defined(HAVE_ANY_MONOTONIC)
 	/** True iff we should use our system's monotonic time implementation */
+	/* TODO: Support systems where we don't need to detct monotonic time */
 	int use_monotonic;
 	/** Difference between internal time (maybe from clock_gettime) and
 	 * gettimeofday. */
diff --git a/event.c b/event.c
index fe55e216..c1d00eb3 100644
--- a/event.c
+++ b/event.c
@@ -375,6 +375,8 @@ detect_monotonic(struct event_base *base, const struct event_config *cfg)
 			memcpy(&base->mach_timebase_units, &mi, sizeof(mi));
 		}
 	}
+#elif defined(_WIN32)
+	base->use_monotonic = 1;
 #endif
 }
 
@@ -418,6 +420,23 @@ gettime(struct event_base *base, struct timeval *tp)
 		    / (base->mach_timebase_units.denom);
 		tp->tv_sec = usec / 1000000;
 		tp->tv_usec = usec % 1000000;
+#elif defined(_WIN32)
+		/* TODO: Support GetTickCount64. */
+		/* TODO: Support alternate timer backends if the user asked
+		 * for a high-precision timer. QueryPerformanceCounter is
+		 * possibly a good idea, but it is also supposed to have
+		 * reliability issues under various circumstances. */
+		DWORD ticks = GetTickCount();
+		if (ticks < base->last_tick_count) {
+			/* The 32-bit timer rolled over. Let's assume it only
+			 * happened once.  Add 2**32 msec to adjust_tick_count. */
+			const struct timeval tv_rollover = { 4294967, 296000 };
+			evutil_timeradd(&tv_rollover, &base->adjust_tick_count, &base->adjust_tick_count);
+		}
+		base->last_tick_count = ticks;
+		tp->tv_sec = ticks / 1000;
+		tp->tv_usec = (ticks % 1000) * 1000;
+		evutil_timeradd(tp, &base->adjust_tick_count, tp);
 #else
 #error "Missing monotonic time implementation."
 #endif