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//
// FrequencyStatistics.cs
//
// Copyright (C) 2004 Novell, Inc.
//
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
//
using System;
using System.Collections;
using System.Threading;
namespace Beagle.Util {
public class FrequencyStatistics {
// In this code, all time spans represented
// by doubles are measured in seconds.
// OK, this is crack. We fix a very large
// constant that is returned as the gap before
// the second event comes in. This is a hack
// that lets us pretty much ignore the special
// case of the initial state in our backoff code.
private const double first_gap = 1.0e+8;
// decay is between 0.0 and 1.0.
// A lower number implies a longer memory.
private const double decay = 0.317732; // a magic constant
private int count = 0;
private double expma_gap;
private DateTime previous_time;
public int Count {
get { return count; }
}
public double TimeSinceLastEvent {
get {
if (count == 0)
return first_gap;
else
return (DateTime.Now - previous_time).TotalSeconds;
}
}
public double EstimatedFrequency {
get {
if (count == 0)
return 1 / first_gap;
// expma_gap will always be > 0
// It might be very small, though.
return 1 / expma_gap;
}
}
public double ImpliedFrequency {
get {
return ComputeExpMaGap (DateTime.Now, expma_gap);
}
}
// How many seconds would be have to be idle for our exmpa
// frequency to reach the required level.
public double TimeToReachFrequency (double target_frequency)
{
if (target_frequency < 0.001)
return 0;
double target_gap = 1 / target_frequency;
if (count < 2 || (target_gap < expma_gap))
return 0;
// Compute the size of gap that would get us to the
// target level.
double t;
t = (target_gap - (1 - decay) * expma_gap) / decay;
// Adjust by the amount of time that has actually passed.
t -= (DateTime.Now - previous_time).TotalSeconds;
return Math.Max (t, 0);
}
private double ComputeExpMaGap (DateTime now, double prev)
{
if (count <= 1)
return first_gap;
double gap = (now - previous_time).TotalSeconds;
if (count == 2)
return gap;
return decay * gap + (1 - decay) * prev;
}
public void AddEvent ()
{
DateTime now = DateTime.Now;
expma_gap = ComputeExpMaGap (now, expma_gap);
previous_time = now;
++count;
}
}
}
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