/// <summary> /// "A common feature of the above techniques—indeed, the key technique that /// allows us to track the decayed weights efficiently—is that they maintain /// counts and other quantities based on g(ti − L), and only scale by g(t − L) /// at query time. But while g(ti −L)/g(t−L) is guaranteed to lie between zero /// and one, the intermediate values of g(ti − L) could become very large. For /// polynomial functions, these values should not grow too large, and should be /// effectively represented in practice by floating point values without loss of /// precision. For exponential functions, these values could grow quite large as /// new values of (ti − L) become large, and potentially exceed the capacity of /// common floating point types. However, since the values stored by the /// algorithms are linear combinations of g values (scaled sums), they can be /// rescaled relative to a new landmark. That is, by the analysis of exponential /// decay in Section III-A, the choice of L does not affect the final result. We /// can therefore multiply each value based on L by a factor of exp(−α(L′ − L)), /// and obtain the correct value as if we had instead computed relative to a new /// landmark L′ (and then use this new L′ at query time). This can be done with /// a linear pass over whatever data structure is being used." /// </summary> /// <param name="now"></param> /// <param name="next"></param> private void Rescale(long now, long next) { if (_nextScaleTime.CompareAndSet(next, now + RESCALE_THRESHOLD)) { lockForRescale(); try { var oldStartTime = _startTime; _startTime = CurrentTimeInSeconds(); double scalingFactor = Math.Exp(-_alpha * (_startTime - oldStartTime)); var keys = new List <double>(_values.Keys); foreach (double key in keys) { WeightedSample sample = null; if (_values.TryRemove(key, out sample)) { WeightedSample newSample = new WeightedSample(sample.value, sample.weight * scalingFactor); _values.AddOrUpdate(key * scalingFactor, newSample, (k, v) => v); } } } finally { unlockForRescale(); } } }
/// <summary> /// Adds an old value with a fixed timestamp to the reservoir. /// </summary> /// <param name="value">the value to be added</param> /// <param name="timestamp">the epoch timestamp of value in seconds</param> public void Update(long value, long timestamp) { rescaleIfNeeded(); lockForRegularUsage(); _lock.EnterReadLock(); try { var itemWeight = Weight(timestamp - _startTime); WeightedSample sample = new WeightedSample(value, itemWeight); var random = ThreadLocalRandom.NextNonzeroDouble(); var priority = itemWeight / random; var newCount = _count.IncrementAndGet(); if (newCount <= _size) { _values.AddOrUpdate(priority, sample, (p, v) => v); } else { var first = _values.Keys.Min(); if (first < priority) { _values.AddOrUpdate(priority, sample, (p, v) => v); WeightedSample removed; while (!_values.TryRemove(first, out removed)) { first = _values.Keys.First(); } } } } finally { unlockForRegularUsage(); _lock.ExitReadLock(); } }