// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
// See the LICENCE file in the repository root for full licence text.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using osu.Framework.Bindables;
using osu.Framework.Extensions.TypeExtensions;
using osu.Framework.Graphics;
using osu.Framework.Graphics.Containers;
using osu.Framework.Graphics.Pooling;
using osu.Framework.Graphics.Shapes;
using osu.Framework.Logging;
using osu.Game.Graphics.Containers;
using osuTK;
using osuTK.Graphics;

namespace osu.Game.Screens.SelectV2
{
    /// <summary>
    /// A highly efficient vertical list display that is used primarily for the song select screen,
    /// but flexible enough to be used for other use cases.
    /// </summary>
    public abstract partial class Carousel<T> : CompositeDrawable
    {
        /// <summary>
        /// A collection of filters which should be run each time a <see cref="QueueFilter"/> is executed.
        /// </summary>
        protected IEnumerable<ICarouselFilter> Filters { get; init; } = Enumerable.Empty<ICarouselFilter>();

        /// <summary>
        /// Height of the area above the carousel that should be treated as visible due to transparency of elements in front of it.
        /// </summary>
        public float BleedTop { get; set; } = 0;

        /// <summary>
        /// Height of the area below the carousel that should be treated as visible due to transparency of elements in front of it.
        /// </summary>
        public float BleedBottom { get; set; } = 0;

        /// <summary>
        /// The number of pixels outside the carousel's vertical bounds to manifest drawables.
        /// This allows preloading content before it scrolls into view.
        /// </summary>
        public float DistanceOffscreenToPreload { get; set; } = 0;

        /// <summary>
        /// When a new request arrives to change filtering, the number of milliseconds to wait before performing the filter.
        /// Regardless of any external debouncing, this is a safety measure to avoid triggering too many threaded operations.
        /// </summary>
        public int DebounceDelay { get; set; } = 0;

        /// <summary>
        /// Whether an asynchronous filter / group operation is currently underway.
        /// </summary>
        public bool IsFiltering => !filterTask.IsCompleted;

        /// <summary>
        /// The number of displayable items currently being tracked (before filtering).
        /// </summary>
        public int ItemsTracked => Items.Count;

        /// <summary>
        /// The number of carousel items currently in rotation for display.
        /// </summary>
        public int DisplayableItems => displayedCarouselItems?.Count ?? 0;

        /// <summary>
        /// The number of items currently actualised into drawables.
        /// </summary>
        public int VisibleItems => scroll.Panels.Count;

        /// <summary>
        /// All items which are to be considered for display in this carousel.
        /// Mutating this list will automatically queue a <see cref="QueueFilter"/>.
        /// </summary>
        /// <remarks>
        /// Note that an <see cref="ICarouselFilter"/> may add new items which are displayed but not tracked in this list.
        /// </remarks>
        protected readonly BindableList<T> Items = new BindableList<T>();

        /// <summary>
        /// The currently selected model.
        /// </summary>
        /// <remarks>
        /// Setting this will ensure <see cref="CarouselItem.Selected"/> is set to <c>true</c> only on the matching <see cref="CarouselItem"/>.
        /// Of note, if no matching item exists all items will be deselected while waiting for potential new item which matches.
        /// </remarks>
        public virtual object? CurrentSelection
        {
            get => currentSelection;
            set
            {
                if (currentSelectionCarouselItem != null)
                    currentSelectionCarouselItem.Selected.Value = false;

                currentSelection = value;

                currentSelectionCarouselItem = null;
                currentSelectionYPosition = null;
                updateSelection();
            }
        }

        private List<CarouselItem>? displayedCarouselItems;

        private readonly DoublePrecisionScroll scroll;

        protected Carousel()
        {
            InternalChildren = new Drawable[]
            {
                new Box
                {
                    Colour = Color4.Black,
                    RelativeSizeAxes = Axes.Both,
                },
                scroll = new DoublePrecisionScroll
                {
                    RelativeSizeAxes = Axes.Both,
                    Masking = false,
                }
            };

            Items.BindCollectionChanged((_, _) => QueueFilter());
        }

        /// <summary>
        /// Queue an asynchronous filter operation.
        /// </summary>
        public void QueueFilter() => Scheduler.AddOnce(() => filterTask = performFilter());

        /// <summary>
        /// Create a drawable for the given carousel item so it can be displayed.
        /// </summary>
        /// <remarks>
        /// For efficiency, it is recommended the drawables are retrieved from a <see cref="DrawablePool{T}"/>.
        /// </remarks>
        /// <param name="item">The item which should be represented by the returned drawable.</param>
        /// <returns>The manifested drawable.</returns>
        protected abstract Drawable GetDrawableForDisplay(CarouselItem item);

        /// <summary>
        /// Create an internal carousel representation for the provided model object.
        /// </summary>
        /// <param name="model">The model.</param>
        /// <returns>A <see cref="CarouselItem"/> representing the model.</returns>
        protected abstract CarouselItem CreateCarouselItemForModel(T model);

        #region Filtering and display preparation

        private Task filterTask = Task.CompletedTask;
        private CancellationTokenSource cancellationSource = new CancellationTokenSource();

        private async Task performFilter()
        {
            Debug.Assert(SynchronizationContext.Current != null);

            var cts = new CancellationTokenSource();

            lock (this)
            {
                cancellationSource.Cancel();
                cancellationSource = cts;
            }

            Stopwatch stopwatch = Stopwatch.StartNew();
            IEnumerable<CarouselItem> items = new List<CarouselItem>(Items.Select(CreateCarouselItemForModel));

            await Task.Run(async () =>
            {
                try
                {
                    if (DebounceDelay > 0)
                    {
                        log($"Filter operation queued, waiting for {DebounceDelay} ms debounce");
                        await Task.Delay(DebounceDelay, cts.Token).ConfigureAwait(false);
                    }

                    foreach (var filter in Filters)
                    {
                        log($"Performing {filter.GetType().ReadableName()}");
                        items = await filter.Run(items, cts.Token).ConfigureAwait(false);
                    }

                    log("Updating Y positions");
                    await updateYPositions(items, cts.Token).ConfigureAwait(false);
                }
                catch (OperationCanceledException)
                {
                    log("Cancelled due to newer request arriving");
                }
            }, cts.Token).ConfigureAwait(true);

            if (cts.Token.IsCancellationRequested)
                return;

            log("Items ready for display");
            displayedCarouselItems = items.ToList();
            displayedRange = null;

            updateSelection();

            void log(string text) => Logger.Log($"Carousel[op {cts.GetHashCode().ToString().Substring(0, 5)}] {stopwatch.ElapsedMilliseconds} ms: {text}");
        }

        private async Task updateYPositions(IEnumerable<CarouselItem> carouselItems, CancellationToken cancellationToken) => await Task.Run(() =>
        {
            const float spacing = 10;
            float yPos = 0;

            foreach (var item in carouselItems)
            {
                item.CarouselYPosition = yPos;
                yPos += item.DrawHeight + spacing;
            }
        }, cancellationToken).ConfigureAwait(false);

        #endregion

        #region Selection handling

        private object? currentSelection;
        private CarouselItem? currentSelectionCarouselItem;
        private double? currentSelectionYPosition;

        private void updateSelection()
        {
            currentSelectionCarouselItem = null;

            if (displayedCarouselItems == null) return;

            foreach (var item in displayedCarouselItems)
            {
                bool isSelected = item.Model == currentSelection;

                if (isSelected)
                {
                    currentSelectionCarouselItem = item;

                    if (currentSelectionYPosition != item.CarouselYPosition)
                    {
                        if (currentSelectionYPosition != null)
                        {
                            float adjustment = (float)(item.CarouselYPosition - currentSelectionYPosition.Value);
                            scroll.OffsetScrollPosition(adjustment);
                        }

                        currentSelectionYPosition = item.CarouselYPosition;
                    }
                }

                item.Selected.Value = isSelected;
            }
        }

        #endregion

        #region Display handling

        private DisplayRange? displayedRange;

        private readonly CarouselItem carouselBoundsItem = new BoundsCarouselItem();

        /// <summary>
        /// The position of the lower visible bound with respect to the current scroll position.
        /// </summary>
        private float visibleBottomBound => (float)(scroll.Current + DrawHeight + BleedBottom);

        /// <summary>
        /// The position of the upper visible bound with respect to the current scroll position.
        /// </summary>
        private float visibleUpperBound => (float)(scroll.Current - BleedTop);

        protected override void Update()
        {
            base.Update();

            if (displayedCarouselItems == null)
                return;

            var range = getDisplayRange();

            if (range != displayedRange)
            {
                Logger.Log($"Updating displayed range of carousel from {displayedRange} to {range}");
                displayedRange = range;

                updateDisplayedRange(range);
            }
        }

        private DisplayRange getDisplayRange()
        {
            Debug.Assert(displayedCarouselItems != null);

            // Find index range of all items that should be on-screen
            carouselBoundsItem.CarouselYPosition = visibleUpperBound - DistanceOffscreenToPreload;
            int firstIndex = displayedCarouselItems.BinarySearch(carouselBoundsItem);
            if (firstIndex < 0) firstIndex = ~firstIndex;

            carouselBoundsItem.CarouselYPosition = visibleBottomBound + DistanceOffscreenToPreload;
            int lastIndex = displayedCarouselItems.BinarySearch(carouselBoundsItem);
            if (lastIndex < 0) lastIndex = ~lastIndex;

            firstIndex = Math.Max(0, firstIndex - 1);
            lastIndex = Math.Max(0, lastIndex - 1);

            return new DisplayRange(firstIndex, lastIndex);
        }

        private void updateDisplayedRange(DisplayRange range)
        {
            Debug.Assert(displayedCarouselItems != null);

            List<CarouselItem> toDisplay = range.Last - range.First == 0
                ? new List<CarouselItem>()
                : displayedCarouselItems.GetRange(range.First, range.Last - range.First + 1);

            // Iterate over all panels which are already displayed and figure which need to be displayed / removed.
            foreach (var panel in scroll.Panels)
            {
                var carouselPanel = (ICarouselPanel)panel;

                // The case where we're intending to display this panel, but it's already displayed.
                // Note that we **must compare the model here** as the CarouselItems may be fresh instances due to a filter operation.
                var existing = toDisplay.FirstOrDefault(i => i.Model == carouselPanel.Item!.Model);

                if (existing != null)
                {
                    carouselPanel.Item = existing;
                    toDisplay.Remove(existing);
                    continue;
                }

                // If the new display range doesn't contain the panel, it's no longer required for display.
                expirePanelImmediately(panel);
            }

            // Add any new items which need to be displayed and haven't yet.
            foreach (var item in toDisplay)
            {
                var drawable = GetDrawableForDisplay(item);

                if (drawable is not ICarouselPanel carouselPanel)
                    throw new InvalidOperationException($"Carousel panel drawables must implement {typeof(ICarouselPanel)}");

                carouselPanel.Item = item;
                scroll.Add(drawable);
            }

            // Update the total height of all items (to make the scroll container scrollable through the full height even though
            // most items are not displayed / loaded).
            if (displayedCarouselItems.Count > 0)
            {
                var lastItem = displayedCarouselItems[^1];
                scroll.SetLayoutHeight((float)(lastItem.CarouselYPosition + lastItem.DrawHeight));
            }
            else
                scroll.SetLayoutHeight(0);
        }

        private static void expirePanelImmediately(Drawable panel)
        {
            panel.FinishTransforms();
            panel.Expire();
        }

        #endregion

        #region Internal helper classes

        private record DisplayRange(int First, int Last);

        /// <summary>
        /// Implementation of scroll container which handles very large vertical lists by internally using <c>double</c> precision
        /// for pre-display Y values.
        /// </summary>
        private partial class DoublePrecisionScroll : OsuScrollContainer
        {
            public readonly Container Panels;

            public void SetLayoutHeight(float height) => Panels.Height = height;

            public DoublePrecisionScroll()
            {
                // Managing our own custom layout within ScrollContent causes feedback with public ScrollContainer calculations,
                // so we must maintain one level of separation from ScrollContent.
                base.Add(Panels = new Container
                {
                    Name = "Layout content",
                    RelativeSizeAxes = Axes.X,
                });
            }

            public override void Clear(bool disposeChildren)
            {
                Panels.Height = 0;
                Panels.Clear(disposeChildren);
            }

            public override void Add(Drawable drawable)
            {
                if (drawable is not ICarouselPanel)
                    throw new InvalidOperationException($"Carousel panel drawables must implement {typeof(ICarouselPanel)}");

                Panels.Add(drawable);
            }

            public override double GetChildPosInContent(Drawable d, Vector2 offset)
            {
                if (d is not ICarouselPanel panel)
                    return base.GetChildPosInContent(d, offset);

                return panel.YPosition + offset.X;
            }

            protected override void ApplyCurrentToContent()
            {
                Debug.Assert(ScrollDirection == Direction.Vertical);

                double scrollableExtent = -Current + ScrollableExtent * ScrollContent.RelativeAnchorPosition.Y;

                foreach (var d in Panels)
                    d.Y = (float)(((ICarouselPanel)d).YPosition + scrollableExtent);
            }
        }

        private class BoundsCarouselItem : CarouselItem
        {
            public override float DrawHeight => 0;

            public BoundsCarouselItem()
                : base(new object())
            {
            }
        }

        #endregion
    }
}