Wino-Mail/Wino.Calendar/Controls/WinoCalendarPanel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using CommunityToolkit.WinUI;
using Itenso.TimePeriod;
using Windows.Foundation;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Wino.Calendar.Models;
using Wino.Calendar.ViewModels.Data;
using Wino.Core.Domain.Interfaces;

namespace Wino.Calendar.Controls;

public partial class WinoCalendarPanel : Panel
{
    private const double LastItemRightExtraMargin = 12d;

    // Store each ICalendarItem measurements by their Id.
    private readonly Dictionary<ICalendarItem, CalendarItemMeasurement> _measurements = new Dictionary<ICalendarItem, CalendarItemMeasurement>();

    public static readonly DependencyProperty EventItemMarginProperty = DependencyProperty.Register(nameof(EventItemMargin), typeof(Thickness), typeof(WinoCalendarPanel), new PropertyMetadata(new Thickness(0, 0, 0, 0)));
    public static readonly DependencyProperty HourHeightProperty = DependencyProperty.Register(nameof(HourHeight), typeof(double), typeof(WinoCalendarPanel), new PropertyMetadata(0d));
    public static readonly DependencyProperty PeriodProperty = DependencyProperty.Register(nameof(Period), typeof(ITimePeriod), typeof(WinoCalendarPanel), new PropertyMetadata(null));

    public ITimePeriod Period
    {
        get { return (ITimePeriod)GetValue(PeriodProperty); }
        set { SetValue(PeriodProperty, value); }
    }

    public double HourHeight
    {
        get { return (double)GetValue(HourHeightProperty); }
        set { SetValue(HourHeightProperty, value); }
    }

    public Thickness EventItemMargin
    {
        get { return (Thickness)GetValue(EventItemMarginProperty); }
        set { SetValue(EventItemMarginProperty, value); }
    }

    private void ResetMeasurements() => _measurements.Clear();

    private double GetChildTopMargin(ICalendarItem calendarItemViewModel, double availableHeight)
    {
        var childStart = calendarItemViewModel.StartDate;

        if (childStart <= Period.Start)
        {
            // Event started before or exactly at the periods tart. This might be a multi-day event.
            // We can simply consider event must not have a top margin.

            return 0d;
        }

        double minutesFromStart = (childStart - Period.Start).TotalMinutes;
        return (minutesFromStart / 1440) * availableHeight;
    }

    private double GetChildWidth(CalendarItemMeasurement calendarItemMeasurement, double availableWidth)
    {
        return (calendarItemMeasurement.Right - calendarItemMeasurement.Left) * availableWidth;
    }

    private double GetChildLeftMargin(CalendarItemMeasurement calendarItemMeasurement, double availableWidth)
        => availableWidth * calendarItemMeasurement.Left;

    private double GetChildHeight(ICalendarItem child)
    {
        // All day events are not measured.
        if (child.IsAllDayEvent) return 0;

        double childDurationInMinutes = 0d;
        double availableHeight = HourHeight * 24;

        var periodRelation = child.Period.GetRelation(Period);

        // Debug.WriteLine($"Render relation of {child.Title} ({child.Period.Start} - {child.Period.End}) is {periodRelation} with {Period.Start.Day}");

        if (!child.IsMultiDayEvent)
        {
            childDurationInMinutes = child.Period.Duration.TotalMinutes;
        }
        else
        {
            // Multi-day event.
            // Check how many of the event falls into the current period.
            childDurationInMinutes = (child.Period.End - Period.Start).TotalMinutes;
        }

        return (childDurationInMinutes / 1440) * availableHeight;
    }

    protected override Size MeasureOverride(Size availableSize)
    {
        ResetMeasurements();
        return base.MeasureOverride(availableSize);
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        if (Period == null || HourHeight == 0d) return finalSize;

        // Measure/arrange each child height and width.
        // This is a vertical calendar. Therefore the height of each child is the duration of the event.
        // Children weights for left and right will be saved if they don't exist.
        // This is important because we don't want to measure the weights again.
        // They don't change until new event is added or removed.
        // Width of the each child may depend on the rectangle packing algorithm.
        // Children are first categorized into columns. Then each column is shifted to the left until
        // no overlap occurs. The width of each child is calculated based on the number of columns it spans.

        double availableHeight = finalSize.Height;
        double availableWidth = finalSize.Width;

        var calendarControls = Children.Cast<ContentPresenter>();

        if (!calendarControls.Any()) return base.ArrangeOverride(finalSize);

        var events = calendarControls.Select(a => a.Content as CalendarItemViewModel);

        LayoutEvents(events);

        foreach (var control in calendarControls)
        {
            // We can't arrange this child.
            if (!(control.Content is ICalendarItem child)) continue;

            bool isHorizontallyLastItem = false;

            double childWidth = 0,
                childHeight = Math.Max(0, GetChildHeight(child)),
                childTop = Math.Max(0, GetChildTopMargin(child, availableHeight)),
                childLeft = 0;

            // No need to measure anything here.
            if (childHeight == 0) continue;

            if (!_measurements.ContainsKey(child))
            {
                // Multi-day event.

                childLeft = 0;
                childWidth = availableWidth;
            }
            else
            {
                var childMeasurement = _measurements[child];

                childWidth = Math.Max(0, GetChildWidth(childMeasurement, finalSize.Width));
                childLeft = Math.Max(0, GetChildLeftMargin(childMeasurement, availableWidth));

                isHorizontallyLastItem = childMeasurement.Right == 1;
            }

            // Add additional right margin to items that falls on the right edge of the panel.
            double extraRightMargin = 0;

            // Multi-day events don't have any margin and their hit test is disabled.
            if (!child.IsMultiDayEvent)
            {
                // Max of 5% of the width or 20px max.
                extraRightMargin = isHorizontallyLastItem ? Math.Max(LastItemRightExtraMargin, finalSize.Width * 5 / 100) : 0;
            }

            if (childWidth < 0) childWidth = 1;

            // Regular events must have 2px margin
            if (!child.IsMultiDayEvent && !child.IsAllDayEvent)
            {
                childLeft += 2;
                childTop += 2;
                childHeight -= 2;
                childWidth -= 2;
            }

            var arrangementRect = new Rect(childLeft + EventItemMargin.Left, childTop + EventItemMargin.Top, Math.Max(childWidth - extraRightMargin, 1), childHeight);

            // Make sure measured size will fit in the arranged box.
            var measureSize = arrangementRect.ToSize();
            control.Measure(measureSize);
            control.Arrange(arrangementRect);

            //Debug.WriteLine($"{child.Title}, Measured: {measureSize}, Arranged: {arrangementRect}");
        }


        return finalSize;
    }

    #region ColumSpanning and Packing Algorithm

    private void AddOrUpdateMeasurement(ICalendarItem calendarItem, CalendarItemMeasurement measurement)
    {
        if (_measurements.ContainsKey(calendarItem))
        {
            _measurements[calendarItem] = measurement;
        }
        else
        {
            _measurements.Add(calendarItem, measurement);
        }
    }

    // Pick the left and right positions of each event, such that there are no overlap.
    private void LayoutEvents(IEnumerable<ICalendarItem> events)
    {
        var columns = new List<List<ICalendarItem>>();
        DateTime? lastEventEnding = null;

        foreach (var ev in events.OrderBy(ev => ev.StartDate).ThenBy(ev => ev.EndDate))
        {
            // Multi-day events are not measured.
            if (ev.IsMultiDayEvent) continue;

            if (ev.Period.Start >= lastEventEnding)
            {
                PackEvents(columns);
                columns.Clear();
                lastEventEnding = null;
            }

            bool placed = false;

            foreach (var col in columns)
            {
                if (!col.Last().Period.OverlapsWith(ev.Period))
                {
                    col.Add(ev);
                    placed = true;
                    break;
                }
            }
            if (!placed)
            {
                columns.Add(new List<ICalendarItem> { ev });
            }
            if (lastEventEnding == null || ev.Period.End > lastEventEnding.Value)
            {
                lastEventEnding = ev.Period.End;
            }
        }
        if (columns.Count > 0)
        {
            PackEvents(columns);
        }
    }

    // Set the left and right positions for each event in the connected group.
    private void PackEvents(List<List<ICalendarItem>> columns)
    {
        float numColumns = columns.Count;
        int iColumn = 0;

        foreach (var col in columns)
        {
            foreach (var ev in col)
            {
                int colSpan = ExpandEvent(ev, iColumn, columns);

                var leftWeight = iColumn / numColumns;
                var rightWeight = (iColumn + colSpan) / numColumns;

                AddOrUpdateMeasurement(ev, new CalendarItemMeasurement(leftWeight, rightWeight));
            }

            iColumn++;
        }
    }

    // Checks how many columns the event can expand into, without colliding with other events.
    private int ExpandEvent(ICalendarItem ev, int iColumn, List<List<ICalendarItem>> columns)
    {
        int colSpan = 1;

        foreach (var col in columns.Skip(iColumn + 1))
        {
            foreach (var ev1 in col)
            {
                if (ev1.Period.OverlapsWith(ev.Period)) return colSpan;
            }

            colSpan++;
        }

        return colSpan;
    }

    #endregion
}