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diff --git a/services/sensorservice/legacy/SecondOrderLowPassFilter.h b/services/sensorservice/legacy/SecondOrderLowPassFilter.h
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+/*
+ * Copyright (C) 2010 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef ANDROID_SECOND_ORDER_LOW_PASS_FILTER_H
+#define ANDROID_SECOND_ORDER_LOW_PASS_FILTER_H
+
+#include <stdint.h>
+#include <sys/types.h>
+
+// ---------------------------------------------------------------------------
+
+namespace android {
+// ---------------------------------------------------------------------------
+
+class BiquadFilter;
+
+/*
+ * State of a 2nd order low-pass IIR filter
+ */
+class SecondOrderLowPassFilter {
+    friend class BiquadFilter;
+    float iQ, fc;
+    float K, iD;
+    float a0, a1;
+    float b1, b2;
+public:
+    SecondOrderLowPassFilter(float Q, float fc);
+    void setSamplingPeriod(float dT);
+};
+
+/*
+ * Implements a Biquad IIR filter
+ */
+class BiquadFilter {
+    float x1, x2;
+    float y1, y2;
+    const SecondOrderLowPassFilter& s;
+public:
+    BiquadFilter(const SecondOrderLowPassFilter& s);
+    float init(float in);
+    float operator()(float in);
+};
+
+/*
+ * Two cascaded biquad IIR filters
+ * (4-poles IIR)
+ */
+class CascadedBiquadFilter {
+    BiquadFilter mA;
+    BiquadFilter mB;
+public:
+    CascadedBiquadFilter(const SecondOrderLowPassFilter& s);
+    float init(float in);
+    float operator()(float in);
+};
+
+// ---------------------------------------------------------------------------
+}; // namespace android
+
+#endif // ANDROID_SECOND_ORDER_LOW_PASS_FILTER_H