cleanup low-level time handling: add FixupTimeIfLooped, FixupTimeIfBackwards

stepmania/src/arch/ArchHooks/ArchHooks.cpp

@@ -6,6 +6,63 @@ ArchHooks *HOOKS = NULL;
 #include "../arch_platform.h"
 ArchHooks *MakeArchHooks() { return new ARCH_HOOKS; }
 
+/*
+ * This is a helper for GetMicrosecondsSinceStart on systems with a system
+ * timer that may loop or move backwards.
+ *
+ * The time may decrease last for at least two reasons:
+ *
+ * 1. The underlying timer may be 32-bit and use millisecs internally, in which case
+ * the timer will loop every 2^32 ms.
+ *
+ * 2. The underlying clock may have moved backwards (eg. system clock and ntpd).
+ *
+ * If the system clock moves backwards, we can't just clamp the time; if it moved back
+ * an hour, we'd sit around for an hour until it catches up.
+ *
+ * Keep track of an offset: the amount of time to add to the result.  If we move back
+ * by 100ms, the offset will be increased by 100ms.  If we loop, the offset will be
+ * increased by the duration 2^32 ms.
+ *
+ * This helper only needs to be used if one or both of the above conditions can occur.
+ * If the underlying timer is reliable, this doesn't need to be used (for a small
+ * efficiency bonus).  Also, you may omit this for GetMicrosecondsSinceStart() when
+ * bAccurate == false.
+ */
+
+uint64_t ArchHooks::FixupTimeIfLooped( uint64_t usecs )
+{
+	static uint64_t last = 0;
+	static uint64_t offset_us = 0;
+
+	/* The time has wrapped if the last time was very high and the current time is very low. */
+	const uint64_t i32BitMaxMs = uint64_t(1) << 32;
+	const uint64_t i32BitMaxUs = i32BitMaxMs*1000;
+	const uint64_t one_day = uint64_t(24*60*60)*1000000;
+	if( last > (i32BitMaxUs-one_day) && usecs < one_day )
+		offset_us += i32BitMaxUs;
+
+	last = usecs;
+
+	return usecs + offset_us;
+}
+
+uint64_t ArchHooks::FixupTimeIfBackwards( uint64_t usecs )
+{
+	static uint64_t last = 0;
+	static uint64_t offset_us = 0;
+
+	if( usecs < last )
+	{
+		/* The time has moved backwards.  Increase the offset by the amount we moved. */
+		offset_us += last - usecs;
+	}
+
+	last = usecs;
+
+	return usecs + offset_us;
+}
+
 /*
  * (c) 2003-2004 Glenn Maynard, Chris Danford
  * All rights reserved.

stepmania/src/arch/ArchHooks/ArchHooks.h

@@ -48,6 +48,12 @@ public:
 	 * wrapping if possible.
 	 */
 	static int64_t GetMicrosecondsSinceStart( bool bAccurate );
+
+private:
+	/* This is a helper for GetMicrosecondsSinceStart on systems with a system
+	 * timer that may loop or move backwards. */
+	static uint64_t FixupTimeIfLooped( uint64_t usecs );
+	static uint64_t FixupTimeIfBackwards( uint64_t usecs );
 };
 
 ArchHooks *MakeArchHooks();