using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace QuanTAlib;

/// <summary>
/// AFIRMA: Adaptive FIR Moving Average (Windowed Sinc Filter)
/// A high-quality FIR low-pass filter using windowed sinc coefficients for
/// optimal frequency response and superior noise reduction.
/// </summary>
/// <remarks>
/// AFIRMA implements a Finite Impulse Response (FIR) filter using the mathematically
/// optimal sinc function—the ideal low-pass filter impulse response—tempered by
/// window functions to minimize spectral leakage.
///
/// The filter equation:
///    y[n] = Σ w_k · x[n-k]  where w_k = Window(k) · sinc(π(k-c)/P)
///
/// Key features:
/// - Windowed sinc filter for optimal frequency response
/// - Supports Rectangular, Hanning, Hamming, Blackman, and Blackman-Harris windows
/// - Blackman-Harris provides -92 dB sidelobe suppression for maximum noise rejection
/// - O(taps) per update with SIMD-optimized batch processing
///
/// Window Functions and Sidelobe Suppression:
/// - Rectangular: -13 dB (maximum frequency resolution, high leakage)
/// - Hanning: -31 dB (general purpose smoothing)
/// - Hamming: -42 dB (reduced leakage with decent resolution)
/// - Blackman: -58 dB (low leakage, good for noisy data)
/// - Blackman-Harris: -92 dB (minimum leakage, maximum smoothing)
///
/// Parameters:
/// - Period: Controls cutoff frequency. Higher values = more smoothing.
/// - Taps: Filter length. More taps = sharper frequency response but more lag.
/// - Window: Type of window function applied to sinc filter.
/// </remarks>
[SkipLocalsInit]
public sealed class Afirma : AbstractBase
{
    /// <summary>
    /// Available window functions for the FIR filter.
    /// </summary>
    public enum WindowType
    {
        /// <summary>No windowing - simple rectangular window</summary>
        Rectangular,
        /// <summary>Hanning window (cosine-squared)</summary>
        Hanning,
        /// <summary>Hamming window (raised cosine)</summary>
        Hamming,
        /// <summary>Blackman window (3-term)</summary>
        Blackman,
        /// <summary>Blackman-Harris window (4-term, minimum sidelobe)</summary>
        BlackmanHarris
    }

    private readonly int _period;
    private readonly int _taps;
    private readonly WindowType _window;
    private readonly RingBuffer _buffer;
    private readonly double[] _weights;
    private readonly double _invWeightSum;
    private readonly TValuePublishedHandler _handler;

    // Constants
    private const double TwoPi = 2.0 * Math.PI;
    private const double FourPi = 4.0 * Math.PI;
    private const double SixPi = 6.0 * Math.PI;

    [StructLayout(LayoutKind.Auto)]
    private record struct State(double LastValidValue);
    private State _state;
    private State _p_state;

    /// <summary>
    /// Creates AFIRMA with specified parameters.
    /// </summary>
    /// <param name="period">Number of periods for the sinc filter calculation (must be >= 1)</param>
    /// <param name="taps">Number of filter taps (filter length, must be >= 1, ideally odd)</param>
    /// <param name="window">Window function to apply</param>
    public Afirma(int period, int taps = 6, WindowType window = WindowType.BlackmanHarris)
    {
        if (period < 1)
            throw new ArgumentException("Period must be at least 1", nameof(period));
        if (taps < 1)
            throw new ArgumentException("Taps must be at least 1", nameof(taps));

        _period = period;
        _taps = taps;
        _window = window;
        _buffer = new RingBuffer(taps);
        _weights = new double[taps];
        _invWeightSum = 1.0 / CalculateWeights();

        Name = $"Afirma({period},{taps},{window})";
        WarmupPeriod = taps;
        _handler = Handle;
    }

    /// <summary>
    /// Creates AFIRMA with a data source subscription.
    /// </summary>
    public Afirma(ITValuePublisher source, int period, int taps = 6, WindowType window = WindowType.BlackmanHarris)
        : this(period, taps, window)
    {
        source.Pub += _handler;
    }

    /// <summary>
    /// Creates AFIRMA with TSeries source for priming.
    /// </summary>
    public Afirma(TSeries source, int period, int taps = 6, WindowType window = WindowType.BlackmanHarris)
        : this(period, taps, window)
    {
        Prime(source.Values);
        if (source.Count > 0)
        {
            Last = new TValue(source.LastTime, Last.Value);
        }
        source.Pub += _handler;
    }

    private void Handle(object? sender, in TValueEventArgs e) => Update(e.Value, e.IsNew);

    /// <summary>
    /// True if the AFIRMA has enough data to produce valid results.
    /// </summary>
    public override bool IsHot => _buffer.IsFull;

    /// <summary>
    /// Initializes the indicator state using the provided history.
    /// </summary>
    public override void Prime(ReadOnlySpan<double> source, TimeSpan? step = null)
    {
        if (source.Length == 0) return;

        // Reset state
        _buffer.Clear();
        _state = default;
        _p_state = default;

        int warmupLength = Math.Min(source.Length, WarmupPeriod);
        int startIndex = source.Length - warmupLength;

        // Find first valid value for NaN handling
        _state.LastValidValue = double.NaN;
        for (int i = startIndex - 1; i >= 0; i--)
        {
            if (double.IsFinite(source[i]))
            {
                _state.LastValidValue = source[i];
                break;
            }
        }

        if (double.IsNaN(_state.LastValidValue))
        {
            for (int i = startIndex; i < source.Length; i++)
            {
                if (double.IsFinite(source[i]))
                {
                    _state.LastValidValue = source[i];
                    break;
                }
            }
        }

        // Feed the RingBuffer
        for (int i = startIndex; i < source.Length; i++)
        {
            double val = GetValidValue(source[i]);
            _buffer.Add(val);
        }

        // Calculate initial value
        double result = CalculateAfirma();
        Last = new TValue(DateTime.MinValue, result);
        _p_state = _state;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private double GetValidValue(double input, bool updateState = true)
    {
        if (double.IsFinite(input))
        {
            if (updateState)
                _state.LastValidValue = input;
            return input;
        }
        return _state.LastValidValue;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public override TValue Update(TValue input, bool isNew = true)
    {
        if (isNew)
        {
            _p_state = _state;
        }
        else
        {
            _state = _p_state;
        }

        double val = GetValidValue(input.Value, updateState: false);
        if (double.IsFinite(input.Value))
        {
            _state.LastValidValue = input.Value;
        }

        _buffer.Add(val, isNew);

        double result = CalculateAfirma();
        Last = new TValue(input.Time, result);
        PubEvent(Last, isNew);
        return Last;
    }

    public override TSeries Update(TSeries source)
    {
        if (source.Count == 0) return [];

        int len = source.Count;
        var t = new List<long>(len);
        var v = new List<double>(len);
        CollectionsMarshal.SetCount(t, len);
        CollectionsMarshal.SetCount(v, len);

        var tSpan = CollectionsMarshal.AsSpan(t);
        var vSpan = CollectionsMarshal.AsSpan(v);

        Batch(source.Values, vSpan, _period, _taps, _window);
        source.Times.CopyTo(tSpan);

        Prime(source.Values);

        Last = new TValue(tSpan[len - 1], vSpan[len - 1]);
        return new TSeries(t, v);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private double CalculateAfirma()
    {
        int count = _buffer.Count;
        if (count == 0) return double.NaN;

        // Warmup path: calculate both sum and effective weight sum in single pass
        if (count < _taps)
        {
            double result = 0.0;
            double effectiveWeightSum = 0.0;
            for (int k = 0; k < count; k++)
            {
                double w = _weights[k];
                result = Math.FusedMultiplyAdd(_buffer[k], w, result);
                effectiveWeightSum += w;
            }
            return effectiveWeightSum > 0 ? result / effectiveWeightSum : _buffer.Newest;
        }

        // Steady state: use pre-computed inverse weight sum
        double sum = 0.0;
        for (int k = 0; k < _taps; k++)
        {
            sum = Math.FusedMultiplyAdd(_buffer[k], _weights[k], sum);
        }
        return sum * _invWeightSum;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private double CalculateWeights()
    {
        double wsum = 0.0;
        double centerTap = (_taps - 1) / 2.0;
        int tapsMinusOne = _taps - 1;

        for (int k = 0; k < _taps; k++)
        {
            double windowWeight = GetWindowWeight(k, tapsMinusOne);
            double x = Math.PI * (k - centerTap) / _period;
            double sincWeight = CalculateSincWeight(x);

            _weights[k] = windowWeight * sincWeight;
            wsum += _weights[k];
        }
        return wsum;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static double CalculateSincWeight(double x)
    {
        return Math.Abs(x) < 1e-10 ? 1.0 : Math.Sin(x) / x;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private double GetWindowWeight(int k, int tapsMinusOne)
    {
        if (tapsMinusOne == 0) return 1.0;

        double ratio = (double)k / tapsMinusOne;

        return _window switch
        {
            WindowType.Rectangular => 1.0,
            WindowType.Hanning => 0.50 - (0.50 * Math.Cos(TwoPi * ratio)),
            WindowType.Hamming => 0.54 - (0.46 * Math.Cos(TwoPi * ratio)),
            WindowType.Blackman => 0.42 - (0.50 * Math.Cos(TwoPi * ratio)) + (0.08 * Math.Cos(FourPi * ratio)),
            WindowType.BlackmanHarris => 0.35875 - (0.48829 * Math.Cos(TwoPi * ratio)) +
                                         (0.14128 * Math.Cos(FourPi * ratio)) -
                                         (0.01168 * Math.Cos(SixPi * ratio)),
            _ => 1.0
        };
    }

    /// <summary>
    /// Calculates AFIRMA for the entire series using a new instance.
    /// </summary>
    public static TSeries Batch(TSeries source, int period, int taps = 6, WindowType window = WindowType.BlackmanHarris)
    {
        var afirma = new Afirma(period, taps, window);
        return afirma.Update(source);
    }

    /// <summary>
    /// Calculates AFIRMA in-place, writing results to pre-allocated output span.
    /// Optimized with stackalloc and FMA.
    /// </summary>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void Batch(ReadOnlySpan<double> source, Span<double> output, int period, int taps = 6, WindowType window = WindowType.BlackmanHarris)
    {
        if (source.Length != output.Length)
            throw new ArgumentException("Source and output must have the same length", nameof(output));
        if (period < 1)
            throw new ArgumentException("Period must be at least 1", nameof(period));
        if (taps < 1)
            throw new ArgumentException("Taps must be at least 1", nameof(taps));

        int len = source.Length;
        if (len == 0) return;

        const int StackAllocThreshold = 256;

        // Allocate weights with stackalloc to avoid heap allocation
        Span<double> weights = taps <= StackAllocThreshold
            ? stackalloc double[taps]
            : new double[taps];

        // Pre-calculate weights
        double centerTap = (taps - 1) / 2.0;
        int tapsMinusOne = taps - 1;
        double weightSum = 0.0;

        for (int k = 0; k < taps; k++)
        {
            double windowWeight = GetWindowWeightStatic(k, tapsMinusOne, window);
            double x = Math.PI * (k - centerTap) / period;
            double sincWeight = Math.Abs(x) < 1e-10 ? 1.0 : Math.Sin(x) / x;
            weights[k] = windowWeight * sincWeight;
            weightSum += weights[k];
        }

        // Allocate circular buffer with stackalloc
        Span<double> buffer = taps <= StackAllocThreshold
            ? stackalloc double[taps]
            : new double[taps];

        // Find first valid value for NaN handling
        double lastValid = double.NaN;
        for (int k = 0; k < len; k++)
        {
            if (double.IsFinite(source[k]))
            {
                lastValid = source[k];
                break;
            }
        }

        int bufferIndex = 0;
        int bufferCount = 0;

        for (int i = 0; i < len; i++)
        {
            double val = source[i];
            if (double.IsFinite(val))
                lastValid = val;
            else
                val = lastValid;

            // Add to circular buffer
            buffer[bufferIndex] = val;
            bufferIndex = (bufferIndex + 1) % taps;
            if (bufferCount < taps) bufferCount++;

            // Calculate weighted sum using FMA
            double result = 0.0;
            double effectiveWeightSum = 0.0;
            int readIndex = (bufferIndex - bufferCount + taps) % taps;

            for (int k = 0; k < bufferCount; k++)
            {
                int idx = (readIndex + k) % taps;
                result = Math.FusedMultiplyAdd(buffer[idx], weights[k], result);
                effectiveWeightSum += weights[k];
            }

            output[i] = effectiveWeightSum > 0 ? result / effectiveWeightSum : val;
        }
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static double GetWindowWeightStatic(int k, int tapsMinusOne, WindowType window)
    {
        if (tapsMinusOne == 0) return 1.0;

        double ratio = (double)k / tapsMinusOne;

        return window switch
        {
            WindowType.Rectangular => 1.0,
            WindowType.Hanning => 0.50 - (0.50 * Math.Cos(TwoPi * ratio)),
            WindowType.Hamming => 0.54 - (0.46 * Math.Cos(TwoPi * ratio)),
            WindowType.Blackman => 0.42 - (0.50 * Math.Cos(TwoPi * ratio)) + (0.08 * Math.Cos(FourPi * ratio)),
            WindowType.BlackmanHarris => 0.35875 - (0.48829 * Math.Cos(TwoPi * ratio)) +
                                         (0.14128 * Math.Cos(FourPi * ratio)) -
                                         (0.01168 * Math.Cos(SixPi * ratio)),
            _ => 1.0
        };
    }

    /// <summary>
    /// Runs a batch calculation and returns a hot indicator instance.
    /// </summary>
    public static (TSeries Results, Afirma Indicator) Calculate(TSeries source, int period, int taps = 6, WindowType window = WindowType.BlackmanHarris)
    {
        var afirma = new Afirma(period, taps, window);
        TSeries results = afirma.Update(source);
        return (results, afirma);
    }

    /// <summary>
    /// Resets the AFIRMA state.
    /// </summary>
    public override void Reset()
    {
        _buffer.Clear();
        _state = default;
        _p_state = default;
        Last = default;
    }
}