HC_APTBS/ViewModels/SingleFlowChartViewModel.cs

using System;
using System.Collections.ObjectModel;
using CommunityToolkit.Mvvm.ComponentModel;
using LiveChartsCore;
using LiveChartsCore.Defaults;
using LiveChartsCore.SkiaSharpView;
using LiveChartsCore.SkiaSharpView.Painting;
using SkiaSharp;

namespace HC_APTBS.ViewModels
{
    /// <summary>
    /// Reusable ViewModel for a single real-time scrolling line chart.
    /// Backed by LiveChartsCore with a fixed-width sample window.
    ///
    /// <para>Two modes:</para>
    /// <list type="bullet">
    ///   <item><b>Index-axis (default):</b> <see cref="LineSeries{TModel}"/> of doubles; X = array index;
    ///         axis is static. Samples shuffle through fixed X slots on each <see cref="AddValue"/>.</item>
    ///   <item><b>Smooth-scroll (opt-in):</b> <see cref="LineSeries{TModel}"/> of <see cref="ObservablePoint"/>
    ///         with X on a monotonic sample-index timeline. A host-driven per-frame hook calls
    ///         <see cref="UpdateViewport"/> to slide the X-axis window, producing true continuous
    ///         leftward motion independent of data cadence.</item>
    /// </list>
    /// </summary>
    public sealed partial class SingleFlowChartViewModel : ObservableObject
    {
        private const int DefaultMaxSamples = 200;
        private const int SmoothTrimMargin = 4;

        private readonly bool _smoothScroll;
        private readonly int _maxSamples;

        // Index-axis mode storage.
        private readonly ObservableCollection<double>? _values;

        // Smooth-scroll mode storage + timeline state.
        // The series holds N committed points at integer X positions plus one trailing
        // "endpoint" whose X/Y interpolate each render frame from the previous committed
        // point toward the pending sample. On each new sample the endpoint snaps to its
        // target (becoming committed) and a new endpoint is appended at the same spot.
        // Net effect: the line tip slides continuously instead of hopping +1 in X per sample.
        private readonly ObservableCollection<ObservablePoint>? _points;
        private long _nextCommitX;
        private double _prevCommittedValue;
        private double _pendingValue;
        private DateTime _pendingArrivalUtc;
        private int _sampleCount;
        private ObservablePoint? _endpoint;

        /// <summary>Chart title label.</summary>
        [ObservableProperty] private string _title = string.Empty;

        /// <summary>Most recent sample value, displayed as a numeric label alongside the chart.</summary>
        [ObservableProperty] private double _currentValue;

        /// <summary>Series array bound to the CartesianChart.</summary>
        public ISeries[] Series { get; }

        /// <summary>X axes for the chart (auto-scrolling, no labels).</summary>
        public Axis[] XAxes { get; }

        /// <summary>Y axes for the chart.</summary>
        public Axis[] YAxes { get; }

        /// <summary>
        /// Tolerance band sections overlaid on the chart.
        /// Updated when <see cref="SetTolerance"/> is called.
        /// </summary>
        [ObservableProperty] private RectangularSection[] _sections = Array.Empty<RectangularSection>();

        /// <summary>
        /// Creates a new chart ViewModel.
        /// </summary>
        /// <param name="title">Display title for the chart.</param>
        /// <param name="lineColor">SKColor for the line series.</param>
        /// <param name="maxSamples">Maximum number of samples before the oldest is dropped.</param>
        /// <param name="smoothScroll">
        /// When true, store samples on a monotonic X timeline and expect the host to call
        /// <see cref="UpdateViewport"/> each render frame to slide the visible window.
        /// When false (default), use the legacy index-axis behavior.
        /// </param>
        public SingleFlowChartViewModel(string title, SKColor lineColor, int maxSamples = DefaultMaxSamples, bool smoothScroll = false)
        {
            _title        = title;
            _maxSamples   = maxSamples;
            _smoothScroll = smoothScroll;

            if (smoothScroll)
            {
                _points = new ObservableCollection<ObservablePoint>();
                Series = new ISeries[]
                {
                    new LineSeries<ObservablePoint>
                    {
                        Values = _points,
                        Fill = null,
                        GeometrySize = 0,
                        Stroke = new SolidColorPaint(lineColor, 2),
                        LineSmoothness = 0,
                        AnimationsSpeed = TimeSpan.Zero
                    }
                };

                XAxes = new Axis[]
                {
                    new Axis
                    {
                        IsVisible = false,
                        AnimationsSpeed = TimeSpan.Zero,
                        MinLimit = 0,
                        MaxLimit = maxSamples
                    }
                };
            }
            else
            {
                _values = new ObservableCollection<double>();
                Series = new ISeries[]
                {
                    new LineSeries<double>
                    {
                        Values = _values,
                        Fill = null,
                        GeometrySize = 0,
                        Stroke = new SolidColorPaint(lineColor, 2),
                        LineSmoothness = 0,
                        AnimationsSpeed = TimeSpan.Zero
                    }
                };

                XAxes = new Axis[]
                {
                    new Axis
                    {
                        IsVisible = false,
                        AnimationsSpeed = TimeSpan.Zero
                    }
                };
            }

            YAxes = new Axis[]
            {
                new Axis
                {
                    AnimationsSpeed = TimeSpan.Zero,
                    MinLimit = 0,
                    SeparatorsPaint = new SolidColorPaint(new SKColor(224, 224, 224), 0.75f)
                }
            };
        }

        /// <summary>
        /// Appends a value to the chart. Drops the oldest value when the window is full.
        /// Must be called on the UI thread.
        /// </summary>
        public void AddValue(double value)
        {
            CurrentValue = value;

            if (_smoothScroll)
            {
                _sampleCount++;

                if (_sampleCount == 1)
                {
                    // First sample — commit at X=0 and anchor the viewport so the point sits
                    // at the right edge. No endpoint yet; we need two samples before we can
                    // interpolate between previous and pending.
                    _points!.Add(new ObservablePoint(0, value));
                    _nextCommitX        = 1;
                    _prevCommittedValue = value;
                    _pendingValue       = value;
                    _pendingArrivalUtc  = DateTime.UtcNow;
                    XAxes[0].MaxLimit = 0;
                    XAxes[0].MinLimit = -_maxSamples;
                }
                else if (_sampleCount == 2)
                {
                    // Second sample — create the sliding endpoint at the previous committed
                    // point. UpdateViewport will interpolate it toward the pending target.
                    _pendingValue      = value;
                    _pendingArrivalUtc = DateTime.UtcNow;
                    _endpoint = new ObservablePoint(_nextCommitX - 1, _prevCommittedValue);
                    _points!.Add(_endpoint);
                }
                else
                {
                    // Third+ sample — finalize the existing endpoint at its target (making
                    // it a committed point), advance the commit index, then append a fresh
                    // endpoint at the just-finalized position. Because the endpoint is
                    // already at X = _nextCommitX after the last render frame with
                    // fraction≈1, the viewport continues from the same position without
                    // a visible hop.
                    _endpoint!.X = _nextCommitX;
                    _endpoint.Y  = _pendingValue;
                    _prevCommittedValue = _pendingValue;
                    _nextCommitX++;

                    _pendingValue      = value;
                    _pendingArrivalUtc = DateTime.UtcNow;
                    _endpoint = new ObservablePoint(_nextCommitX - 1, _prevCommittedValue);
                    _points!.Add(_endpoint);

                    // Keep a small margin past the visible window so the leftmost point
                    // doesn't pop as the viewport advances past the trim boundary.
                    while (_points.Count > _maxSamples + SmoothTrimMargin)
                        _points.RemoveAt(0);
                }
            }
            else
            {
                _values!.Add(value);
                if (_values.Count > _maxSamples)
                    _values.RemoveAt(0);
            }
        }

        /// <summary>
        /// Smooth-scroll mode only: slides the X-axis viewport so the rightmost visible edge
        /// drifts continuously past the most recent sample, producing frame-rate smooth motion
        /// independent of data cadence. Host calls this once per render frame (e.g. from
        /// <c>CompositionTarget.Rendering</c>).
        /// </summary>
        /// <param name="nowUtc">Current time; pass <see cref="DateTime.UtcNow"/>.</param>
        /// <param name="nominalPeriodMs">
        /// Expected inter-sample period in milliseconds (e.g. <c>1000/Hz</c>). Used to interpolate
        /// fractional progress between samples.
        /// </param>
        public void UpdateViewport(DateTime nowUtc, double nominalPeriodMs)
        {
            if (!_smoothScroll || _endpoint == null || nominalPeriodMs <= 0)
                return;

            double fraction = (nowUtc - _pendingArrivalUtc).TotalMilliseconds / nominalPeriodMs;
            if (fraction < 0) fraction = 0;
            else if (fraction > 1) fraction = 1;

            double slideX = (_nextCommitX - 1) + fraction;
            double slideY = _prevCommittedValue + (_pendingValue - _prevCommittedValue) * fraction;

            _endpoint.X = slideX;
            _endpoint.Y = slideY;

            XAxes[0].MaxLimit = slideX;
            XAxes[0].MinLimit = slideX - _maxSamples;
        }

        /// <summary>
        /// Sets the tolerance band (displayed as a shaded region on the chart).
        /// </summary>
        /// <param name="target">Center value of the tolerance band.</param>
        /// <param name="tolerance">Half-width of the tolerance band.</param>
        public void SetTolerance(double target, double tolerance)
        {
            Sections = new[]
            {
                new RectangularSection
                {
                    Yi = target - tolerance,
                    Yj = target + tolerance,
                    Fill = new SolidColorPaint(SKColors.LimeGreen.WithAlpha(40))
                }
            };
        }

        /// <summary>
        /// Clears all sample data and tolerance bands. Resets the smooth-scroll timeline.
        /// </summary>
        public void Clear()
        {
            if (_smoothScroll)
            {
                _points!.Clear();
                _nextCommitX        = 0;
                _prevCommittedValue = 0;
                _pendingValue       = 0;
                _sampleCount        = 0;
                _endpoint           = null;
                XAxes[0].MinLimit = 0;
                XAxes[0].MaxLimit = _maxSamples;
            }
            else
            {
                _values!.Clear();
            }
            Sections = Array.Empty<RectangularSection>();
        }
    }
}