QuanTAlib/lib/statistics/Median.cs

namespace QuanTAlib;

/// <summary>
/// Calculates the median value over a specified period.
/// Provides a measure of central tendency that is robust to outliers.
/// </summary>
public class Median : AbstractBase
{
    private readonly int Period;
    private readonly CircularBuffer _buffer;

    /// <summary>
    /// Initializes a new instance of the Median class.
    /// </summary>
    /// <param name="period">The number of data points to consider. Must be at least 1.</param>
    /// <exception cref="ArgumentOutOfRangeException">
    /// Thrown when the period is less than 1.
    /// </exception>
    public Median(int period) : base()
    {
        if (period < 1)
        {
            throw new ArgumentOutOfRangeException(nameof(period),
                "Period must be greater than or equal to 1.");
        }
        Period = period;
        WarmupPeriod = period;
        _buffer = new CircularBuffer(period);
        Name = $"Median(period={period})";
        Init();
    }

    /// <summary>
    /// Initializes a new instance of the Median class with a data source.
    /// </summary>
    /// <param name="source">The source object that publishes data.</param>
    /// <param name="period">The number of data points to consider.</param>
    public Median(object source, int period) : this(period)
    {
        var pubEvent = source.GetType().GetEvent("Pub");
        pubEvent?.AddEventHandler(source, new ValueSignal(Sub));
    }

    /// <summary>
    /// Manages the state of the indicator.
    /// </summary>
    /// <param name="isNew">Indicates if the current data point is new.</param>
    protected override void ManageState(bool isNew)
    {
        if (isNew)
        {
            _lastValidValue = Input.Value;
            _index++;
        }
    }

    /// <summary>
    /// Performs the median calculation.
    /// </summary>
    /// <returns>
    /// The current median value of the dataset.
    /// </returns>
    /// <remarks>
    /// Uses a sorting approach to find the median. If there's not enough data,
    /// it uses the average as a temporary measure.
    /// </remarks>
    protected override double Calculation()
    {
        ManageState(Input.IsNew);
        _buffer.Add(Input.Value, Input.IsNew);

        double median;
        if (_index >= Period)
        {
            var sortedValues = _buffer.GetSpan().ToArray();
            Array.Sort(sortedValues);
            int middleIndex = sortedValues.Length / 2;

            if (sortedValues.Length % 2 == 0)
            {
                // Even number of values: average of two middle values
                median = (sortedValues[middleIndex - 1] + sortedValues[middleIndex]) / 2.0;
            }
            else
            {
                // Odd number of values: middle value
                median = sortedValues[middleIndex];
            }
        }
        else
        {
            // Not enough data, use average as temporary measure
            median = _buffer.Average();
        }

        IsHot = _index >= WarmupPeriod;
        return median;
    }
}