Proof of concept rate limiting filter for Serilog

PerMinuteInExcessOf.cs

using System;
using Serilog;
using Serilog.Configuration;

namespace ConsoleApplication8
{
    static class Program
    {
        static void Main()
        {
            Log.Logger = new LoggerConfiguration()
                .Filter.PerMinuteInExcessOf(1000)
                .WriteTo.Console()
                .CreateLogger();

            Log.Information("Hello!");
        }

        static LoggerConfiguration PerMinuteInExcessOf(this LoggerFilterConfiguration filterConfiguration, int maximumRate)
        {
            var counter = new RateCounter();
            return filterConfiguration.ByExcluding(e =>
            {
                counter.Measure(1);
                return counter.RatePerMinute > maximumRate;
            });
        }
    }

    class RateCounter
    {
        // For simplicity only per-minute is implemented
        const int WindowSeconds = 60;

        readonly object _sync = new object();

        // Each bucket counts events in a one-second interval
        readonly int[] _buckets;

        // A running sum of all buckets
        int _sum;

        // The index of the current ('latest') bucket
        int _currentBucket;

        // The date/time, rounded down to the second, corresponding
        // with the latest bucket.
        DateTime _currentBucketSecondUtc;

        public RateCounter()
        {
            _buckets = new int[WindowSeconds];
            _currentBucketSecondUtc = DateTime.UtcNow;
            _currentBucket = 0;
        }

        public void Measure(int count)
        {
            lock (_sync)
            {
                var eventBucket = SlideWindow();

                _buckets[eventBucket] += count;
                _sum += count;
            }
        }

        public int RatePerMinute
        {
            get
            {
                lock (_sync)
                {
                    SlideWindow();
                    return _sum;
                }
            }
        }

        int SlideWindow()
        {
            var bucketDateTimeUtc = DateTime.UtcNow;
            var eventSeconds = SecondsFloor(bucketDateTimeUtc);
            var distance = (int)(eventSeconds - _currentBucketSecondUtc).TotalSeconds;

            if (distance < 0)
            {
                if (distance <= -_buckets.Length)
                {
                    // Log.Error("A clock change of {Distance} sec. has invalidated a rate counter; metrics will be inflated until the counter stabilizes", distance);
                    return _currentBucket;
                }

                return (_currentBucket + distance) % _buckets.Length;
            }

            if (distance > 0)
            {
                var newCurrent = (_currentBucket + distance) % _buckets.Length;
                var firstReused = (_currentBucket + 1) % _buckets.Length;
                if (distance >= _buckets.Length)
                {
                    _sum = 0;
                    for (var i = 0; i < _buckets.Length; i++)
                    {
                        _buckets[i] = 0;
                    }
                }
                else if (newCurrent >= firstReused)
                {
                    for (var i = firstReused; i <= newCurrent; i++)
                    {
                        _sum -= _buckets[i];
                        _buckets[i] = 0;
                    }
                }
                else
                {
                    for (var i = firstReused; i < _buckets.Length; i++)
                    {
                        _sum -= _buckets[i];
                        _buckets[i] = 0;
                    }

                    for (var i = 0; i <= newCurrent; i++)
                    {
                        _sum -= _buckets[i];
                        _buckets[i] = 0;
                    }
                }

                _currentBucket = newCurrent;
                _currentBucketSecondUtc = eventSeconds;
            }

            return _currentBucket;
        }

        // Returns the provided date time rounded down to the nearest second
        static DateTime SecondsFloor(DateTime dateTime)
        {
            return dateTime - TimeSpan.FromTicks(dateTime.Ticks % TimeSpan.TicksPerSecond);
        }
    }
}