Windowed rate-counter

RateCounter.cs

/// <summary>
///     A simple windowed rate counter.
/// </summary>
/// <remarks>
///     Tracks an event count over a specific (moving) period of time. Locking omitted for clarity.
/// </remarks>
public class RateCounter
{
    /// <summary>
    ///     Timestamps representing each event tracked by the counter.
    /// </summary>
    readonly Queue<DateTime> _timestamps = new Queue<DateTime>();

    /// <summary>
    ///     The maximum rate permitted by the counter.
    /// </summary>
    readonly int _maxRate;

    /// <summary>
    ///     The period of time over which the rate is evaluated.
    /// </summary>
    readonly TimeSpan _period;

    /// <summary>
    ///     Create a new <see cref="RateCounter"/>.
    /// </summary>
    /// <param name="maxRate">
    ///     The maximum rate permitted by the counter.
    /// </param>
    /// <param name="period">
    ///     The period of time over which the rate is evaluated.
    /// </param>
    public RateCounter(int maxRate, TimeSpan period)
    {
        if (maxRate < 1)
            throw new ArgumentOutOfRangeException(nameof(maxRate), maxRate, "Maximum rate cannot be less than 1.");

        if (period <= TimeSpan.Zero)
            throw new ArgumentOutOfRangeException(nameof(period), period, "Period must be a positive time span.");

        _maxRate = maxRate;
        _period = period;
    }

    /// <summary>
    ///     The period of time (window) over which the counter is evaluated.
    /// </summary>
    public TimeSpan Period => _period;

    /// <summary>
    ///     The current rate measured by the counter.
    /// </summary>
    public int Rate => _timestamps.Count;

    /// <summary>
    ///     The maximum rate permitted by the counter.
    /// </summary>
    public int MaxRate => _maxRate;

    /// <summary>
    ///     Attempt to increment the rate by 1.
    /// </summary>
    /// <returns>
    ///     <c>false</c>, if the rate was not incremented because it is already at its maximum; otherwise, <c>true</c>.
    /// </returns>
    public bool Increment()
    {
        DateTime now = DateTime.Now;

        TrimToWindow(now);

        // We only increment if there is "room" for the new value.
        // Since this rate counter is used to reject requests that are above tenant-level quotas,
        // we don't want to register a request if we'd just wind up rejecting it.
        if (Rate < MaxRate)
        { 
            _timestamps.Enqueue(now);

            return true;
        }

        return false;
    }

    /// <summary>
    ///     Trim the current set of timestamps, removing any that now lie outside the window represented by <see cref="Period"/>.
    /// </summary>
    /// <param name="now">
    ///     A <see cref="DateTime"/> representing the current timestamp.
    /// </param>
    void TrimToWindow(DateTime now)
    {
        while (_timestamps.TryPeek(out DateTime oldestTimestamp) && (now - oldestTimestamp) > Period)
            _timestamps.Dequeue();
    }
}