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diff --git a/engine/lowpassstream.h b/engine/lowpassstream.h
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+++ b/engine/lowpassstream.h
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+//========= Copyright Valve Corporation, All rights reserved. ============//
+//
+// Purpose: 
+//
+// $NoKeywords: $
+//=============================================================================//
+
+#ifndef LOWPASSSTREAM_H
+#define LOWPASSSTREAM_H
+#ifdef _WIN32
+#pragma once
+#endif
+
+
+// ------------------------------------------------------------------ //
+// Default implementations of the operators used for averaging.
+// ------------------------------------------------------------------ //
+template< class DataType >
+inline void SetToZero( DataType &data )
+{
+	data = 0;
+}
+
+template< class DataType >
+inline void AddSamples( DataType const &d1, DataType const &d2, DataType &out )
+{
+	out = d1 + d2;
+}
+
+template< class DataType >
+inline DataType DivideSample( DataType const &data, int factor )
+{
+	return data / factor;
+}
+
+
+// This class does a simple low pass filter on data that you pass through it.
+// You tell it the number of samples to hang onto and it will average that number
+// of samples as you feed it data.
+template< class DataType, int nSamples >
+class CLowPassStream
+{
+public:
+				CLowPassStream();
+
+	// Add a sample to the list.
+	void		AddSample( DataType const &data );
+
+	// Get the current average.
+	DataType	GetCurrentAverage();
+
+private:
+	
+	DataType	m_Samples[nSamples];
+	int			m_iOutSample;
+	int			m_nSamplesGotten;	// incremented until it reaches nSamples.
+									// helps avoid the startup transient.
+};
+
+
+template< class DataType, int nSamples >
+inline CLowPassStream<DataType, nSamples>::CLowPassStream()
+{
+	for( int i=0; i < nSamples; i++ )
+		SetToZero( m_Samples[i] );
+
+	m_iOutSample = 0;
+	m_nSamplesGotten = 0;
+}
+
+template< class DataType, int nSamples >
+inline void CLowPassStream<DataType, nSamples>::AddSample( DataType const &data )
+{
+	m_Samples[m_iOutSample] = data;
+	
+	++m_nSamplesGotten;
+	if( m_nSamplesGotten >= nSamples )
+		m_nSamplesGotten = nSamples;
+
+	++m_iOutSample;
+	if( m_iOutSample >= nSamples )
+		m_iOutSample = 0;
+}
+
+template< class DataType, int nSamples >
+inline DataType CLowPassStream<DataType, nSamples>::GetCurrentAverage()
+{
+	DataType data;
+
+	SetToZero( data );
+	for( int i=0; i < nSamples; i++ )
+		AddSamples( data, m_Samples[i], data );
+
+	return DivideSample( data, m_nSamplesGotten );
+}
+
+
+#endif // LOWPASSSTREAM_H