QuanTAlib/Source/Statistics/PSDEV_Series.cs

namespace QuanTAlib;
using System;

/* <summary>
PSDEV: Population Standard Deviation
  Population Standard Deviation is the square root of the biased variance, also knons as
  Uncorrected Sample Standard Deviation

Sources:
  https://en.wikipedia.org/wiki/Standard_deviation#Uncorrected_sample_standard_deviation

Remark:
  PSDEV (Population Standard Deviation) is also known as a biased/uncorrected Standard Deviation.
  For unbiased version that uses Bessel's correction, use SDEV instead.
    
</summary> */

public class PSDEV_Series : Single_TSeries_Indicator
{
    public PSDEV_Series(TSeries source, int period, bool useNaN = false) : base(source, period, useNaN)
    {
        if (base._data.Count > 0) { base.Add(base._data); }
    }
    private readonly System.Collections.Generic.List<double> _buffer = new();

    public override void Add((System.DateTime t, double v) d, bool update)
    {
        if (update) { _buffer[_buffer.Count - 1] = d.v; }
        else { _buffer.Add(d.v); }
        if (_buffer.Count > this._p && this._p != 0) { _buffer.RemoveAt(0); }

        double _sma = 0;
        for (int i = 0; i < _buffer.Count; i++) { _sma += _buffer[i]; }
        _sma /= this._buffer.Count;

        double _pvar = 0;
        for (int i = 0; i < _buffer.Count; i++) { _pvar += (_buffer[i] - _sma) * (_buffer[i] - _sma); }
        _pvar /= this._buffer.Count;
        double _psdev = Math.Sqrt(_pvar);

        var result = (d.t, (this.Count < this._p - 1 && this._NaN) ? double.NaN : _psdev);
        base.Add(result, update);
    }
}