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+/**
+ * Copyright (c) 2014, Timothy Stack
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * * Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer.
+ * * Redistributions in binary form must reproduce the above copyright notice,
+ * this list of conditions and the following disclaimer in the documentation
+ * and/or otherlist materials provided with the distribution.
+ * * Neither the name of Timothy Stack 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 REGENTS 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 REGENTS 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 log_accel.hh
+ */
+
+#ifndef log_accel_h
+#define log_accel_h
+
+#include <cmath>
+
+#include <stdint.h>
+
+#include "base/lnav_log.hh"
+
+/**
+ * Helper class for figuring out changes in the log message rate.
+ */
+class log_accel {
+public:
+    /*< The direction of the message rate: steady, accelerating, or decelerating
+     */
+    enum direction_t {
+        A_STEADY,
+        A_DECEL,
+        A_ACCEL,
+    };
+
+    /**
+     * Add a time point that will be used to compute velocity and then
+     * acceleration.  Points should be added in reverse order, from most
+     * recent to oldest.
+     *
+     * @param  point The point in time.
+     * @return       True if more points can be added.
+     */
+    bool add_point(int64_t point);
+
+    /**
+     * Get the average acceleration based on the time points we've received.
+     *
+     * @return The average message acceleration.
+     */
+    double get_avg_accel() const;
+
+    /**
+     * Compute the message rate direction.  If the average acceleration is less
+     * than a certain threshold, then we consider the rate to be steady.
+     * Otherwise, the message rate is increasing or decreasing.
+     *
+     * @return The direction of the message rate.
+     */
+    direction_t get_direction() const;
+
+private:
+    /**
+     * The amount of historical data to include in the average acceleration
+     * computation.
+     */
+    static const int HISTORY_SIZE = 8;
+    /**
+     * The minimum range of velocities seen.  This value should limit false-
+     * positives for small millisecond level fluctuations.
+     */
+    static const double MIN_RANGE;
+    static const double THRESHOLD;
+
+    int64_t la_last_point{0};
+    bool la_last_point_set{false};
+    int64_t la_min_velocity{INT64_MAX};
+    int64_t la_max_velocity{INT64_MIN};
+    int64_t la_velocity[HISTORY_SIZE];
+    int la_velocity_size{0};
+};
+
+#endif