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+// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+
+/****************************************************************************
+ *
+ *   Copyright (C) 2012 PX4 Development Team. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/// @file	LowPassFilter.h
+/// @brief	A class to implement a second order low pass filter 
+/// Author: Leonard Hall <LeonardTHall@gmail.com>
+/// Adapted for PX4 by Andrew Tridgell
+
+#pragma once
+
+namespace math
+{
+class __EXPORT LowPassFilter2p
+{
+public:
+    // constructor
+    LowPassFilter2p(float sample_freq, float cutoff_freq) {
+        // set initial parameters
+        set_cutoff_frequency(sample_freq, cutoff_freq);
+        _delay_element_1 = _delay_element_2 = 0;
+    }
+
+    // change parameters
+    void set_cutoff_frequency(float sample_freq, float cutoff_freq);
+
+    // apply - Add a new raw value to the filter 
+    // and retrieve the filtered result
+    float apply(float sample);
+
+    // return the cutoff frequency
+    float get_cutoff_freq(void) const {
+        return _cutoff_freq;
+    }
+
+private:
+    float           _cutoff_freq; 
+    float           _a1;
+    float           _a2;
+    float           _b0;
+    float           _b1;
+    float           _b2;
+    float           _delay_element_1;        // buffered sample -1
+    float           _delay_element_2;        // buffered sample -2
+};
+
+} // namespace math