// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE

using OpenTK;
using osu.Framework.Graphics;
using osu.Framework.Graphics.Containers;
using osu.Game.Database;
using System;
using System.Collections.Generic;
using System.Linq;
using osu.Game.Beatmaps.Drawables;
using osu.Framework.Input;
using OpenTK.Input;
using System.Collections;
using osu.Framework.MathUtils;
using System.Diagnostics;
using System.Threading.Tasks;
using osu.Framework.Allocation;
using osu.Framework.Threading;

namespace osu.Game.Screens.Select
{
    internal class BeatmapCarousel : ScrollContainer, IEnumerable<BeatmapGroup>
    {
        public BeatmapInfo SelectedBeatmap => selectedPanel?.Beatmap;

        public Action BeatmapsChanged;

        public IEnumerable<BeatmapSetInfo> Beatmaps
        {
            get
            {
                return groups.Select(g => g.BeatmapSet);
            }

            set
            {
                scrollableContent.Clear(false);
                panels.Clear();
                groups.Clear();

                List<BeatmapGroup> newGroups = null;

                Task.Run(() =>
                {
                    newGroups = value.Select(createGroup).ToList();
                    criteria.Filter(newGroups);
                }).ContinueWith(t =>
                {
                    Schedule(() =>
                    {
                        foreach (var g in newGroups)
                            addGroup(g);

                        computeYPositions();
                        BeatmapsChanged?.Invoke();
                    });
                });
            }
        }

        private readonly List<float> yPositions = new List<float>();

        /// <summary>
        /// Required for now unfortunately.
        /// </summary>
        private BeatmapDatabase database;

        private readonly Container<Panel> scrollableContent;

        private readonly List<BeatmapGroup> groups = new List<BeatmapGroup>();

        private readonly List<Panel> panels = new List<Panel>();

        private BeatmapGroup selectedGroup;

        private BeatmapPanel selectedPanel;

        public BeatmapCarousel()
        {
            Add(scrollableContent = new Container<Panel>
            {
                RelativeSizeAxes = Axes.X,
            });
        }

        public void AddBeatmap(BeatmapSetInfo beatmapSet)
        {
            var group = createGroup(beatmapSet);

            //for the time being, let's completely load the difficulty panels in the background.
            //this likely won't scale so well, but allows us to completely async the loading flow.
            Schedule(delegate
            {
                addGroup(group);
                computeYPositions();
                if (selectedGroup == null)
                    selectGroup(group);
            });
        }

        public void SelectBeatmap(BeatmapInfo beatmap, bool animated = true)
        {
            if (beatmap == null)
            {
                SelectNext();
                return;
            }

            foreach (BeatmapGroup group in groups)
            {
                var panel = group.BeatmapPanels.FirstOrDefault(p => p.Beatmap.Equals(beatmap));
                if (panel != null)
                {
                    selectGroup(group, panel, animated);
                    return;
                }
            }
        }

        public void RemoveBeatmap(BeatmapSetInfo info) => removeGroup(groups.Find(b => b.BeatmapSet.ID == info.ID));

        public Action<BeatmapGroup, BeatmapInfo> SelectionChanged;

        public Action StartRequested;

        public void SelectNext(int direction = 1, bool skipDifficulties = true)
        {
            if (groups.Count == 0)
            {
                selectedGroup = null;
                selectedPanel = null;
                return;
            }

            if (!skipDifficulties && selectedGroup != null)
            {
                int i = selectedGroup.BeatmapPanels.IndexOf(selectedPanel) + direction;

                if (i >= 0 && i < selectedGroup.BeatmapPanels.Count)
                {
                    //changing difficulty panel, not set.
                    selectGroup(selectedGroup, selectedGroup.BeatmapPanels[i]);
                    return;
                }
            }

            int startIndex = Math.Max(0, groups.IndexOf(selectedGroup));
            int index = startIndex;

            do
            {
                index = (index + direction + groups.Count) % groups.Count;
                if (groups[index].State != BeatmapGroupState.Hidden)
                {
                    SelectBeatmap(groups[index].BeatmapPanels.First().Beatmap);
                    return;
                }
            } while (index != startIndex);
        }

        public void SelectRandom()
        {
            List<BeatmapGroup> visibleGroups = groups.Where(selectGroup => selectGroup.State != BeatmapGroupState.Hidden).ToList();
            if (visibleGroups.Count < 1)
                return;
            BeatmapGroup group = visibleGroups[RNG.Next(visibleGroups.Count)];
            BeatmapPanel panel = group?.BeatmapPanels.First();

            if (panel == null)
                return;

            selectGroup(group, panel);
        }

        private FilterCriteria criteria = new FilterCriteria();

        private ScheduledDelegate filterTask;

        public void Filter(FilterCriteria newCriteria = null, bool debounce = true)
        {
            if (newCriteria != null)
                criteria = newCriteria;

            if (!IsLoaded) return;

            Action perform = delegate
            {
                filterTask = null;

                criteria.Filter(groups);

                var filtered = new List<BeatmapGroup>(groups);

                scrollableContent.Clear(false);
                panels.Clear();
                groups.Clear();

                foreach (var g in filtered)
                    addGroup(g);

                computeYPositions();

                if (selectedGroup == null || selectedGroup.State == BeatmapGroupState.Hidden)
                    SelectNext();
                else
                    selectGroup(selectedGroup);
            };

            filterTask?.Cancel();
            if (debounce)
                filterTask = Scheduler.AddDelayed(perform, 250);
            else
                perform();
        }

        public IEnumerator<BeatmapGroup> GetEnumerator() => groups.GetEnumerator();

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

        private BeatmapGroup createGroup(BeatmapSetInfo beatmapSet)
        {
            database.GetChildren(beatmapSet);
            beatmapSet.Beatmaps.ForEach(b =>
            {
                database.GetChildren(b);
                if (b.Metadata == null)
                    b.Metadata = beatmapSet.Metadata;
            });

            return new BeatmapGroup(beatmapSet, database)
            {
                SelectionChanged = SelectionChanged,
                StartRequested = b => StartRequested?.Invoke(),
                State = BeatmapGroupState.Collapsed
            };
        }

        [BackgroundDependencyLoader(permitNulls: true)]
        private void load(BeatmapDatabase database)
        {
            this.database = database;
        }

        private void addGroup(BeatmapGroup group)
        {
            groups.Add(group);
            panels.Add(group.Header);
            panels.AddRange(group.BeatmapPanels);
        }

        private void removeGroup(BeatmapGroup group)
        {
            groups.Remove(group);
            panels.Remove(group.Header);
            foreach (var p in group.BeatmapPanels)
                panels.Remove(p);

            scrollableContent.Remove(group.Header);
            scrollableContent.Remove(group.BeatmapPanels);

            if (selectedGroup == group)
                SelectNext();

            computeYPositions();
        }

        /// <summary>
        /// Computes the target Y positions for every panel in the carousel.
        /// </summary>
        /// <returns>The Y position of the currently selected panel.</returns>
        private float computeYPositions(bool animated = true)
        {
            yPositions.Clear();

            float currentY = DrawHeight / 2;
            float selectedY = currentY;

            foreach (BeatmapGroup group in groups)
            {
                movePanel(group.Header, group.State != BeatmapGroupState.Hidden, animated, ref currentY);

                if (group.State == BeatmapGroupState.Expanded)
                {
                    group.Header.MoveToX(-100, 500, EasingTypes.OutExpo);
                    var headerY = group.Header.Position.Y;

                    foreach (BeatmapPanel panel in group.BeatmapPanels)
                    {
                        if (panel == selectedPanel)
                            selectedY = currentY + panel.DrawHeight / 2 - DrawHeight / 2;

                        panel.MoveToX(-50, 500, EasingTypes.OutExpo);

                        //on first display we want to begin hidden under our group's header.
                        if (panel.Alpha == 0)
                            panel.MoveToY(headerY);

                        movePanel(panel, true, animated, ref currentY);
                    }
                }
                else
                {
                    group.Header.MoveToX(0, 500, EasingTypes.OutExpo);

                    foreach (BeatmapPanel panel in group.BeatmapPanels)
                    {
                        panel.MoveToX(0, 500, EasingTypes.OutExpo);
                        movePanel(panel, false, animated, ref currentY);
                    }
                }
            }

            currentY += DrawHeight / 2;
            scrollableContent.Height = currentY;

            return selectedY;
        }

        private void movePanel(Panel panel, bool advance, bool animated, ref float currentY)
        {
            yPositions.Add(currentY);
            panel.MoveToY(currentY, animated ? 750 : 0, EasingTypes.OutExpo);

            if (advance)
                currentY += panel.DrawHeight + 5;
        }

        private void selectGroup(BeatmapGroup group, BeatmapPanel panel = null, bool animated = true)
        {
            if (panel == null)
                panel = group.BeatmapPanels.First();

            Trace.Assert(group.BeatmapPanels.Contains(panel), @"Selected panel must be in provided group");

            if (selectedGroup != null && selectedGroup != group && selectedGroup.State != BeatmapGroupState.Hidden)
                selectedGroup.State = BeatmapGroupState.Collapsed;

            group.State = BeatmapGroupState.Expanded;
            selectedGroup = group;
            panel.State = PanelSelectedState.Selected;
            selectedPanel = panel;

            float selectedY = computeYPositions(animated);
            ScrollTo(selectedY, animated);
        }

        protected override bool OnKeyDown(InputState state, KeyDownEventArgs args)
        {
            int direction = 0;
            bool skipDifficulties = false;

            switch (args.Key)
            {
                case Key.Up:
                    direction = -1;
                    break;
                case Key.Down:
                    direction = 1;
                    break;
                case Key.Left:
                    direction = -1;
                    skipDifficulties = true;
                    break;
                case Key.Right:
                    direction = 1;
                    skipDifficulties = true;
                    break;
            }

            if (direction == 0)
                return base.OnKeyDown(state, args);

            SelectNext(direction, skipDifficulties);
            return true;
        }

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

            float drawHeight = DrawHeight;

            // Remove all panels that should no longer be on-screen
            scrollableContent.RemoveAll(delegate (Panel p)
            {
                float panelPosY = p.Position.Y;
                bool remove = panelPosY < Current - p.DrawHeight || panelPosY > Current + drawHeight || !p.IsPresent;
                return remove;
            });

            // Find index range of all panels that should be on-screen
            Trace.Assert(panels.Count == yPositions.Count);

            int firstIndex = yPositions.BinarySearch(Current - Panel.MAX_HEIGHT);
            if (firstIndex < 0) firstIndex = ~firstIndex;
            int lastIndex = yPositions.BinarySearch(Current + drawHeight);
            if (lastIndex < 0) lastIndex = ~lastIndex;

            // Add those panels within the previously found index range that should be displayed.
            for (int i = firstIndex; i < lastIndex; ++i)
            {
                Panel panel = panels[i];
                if (panel.State == PanelSelectedState.Hidden)
                    continue;

                // Only add if we're not already part of the content.
                if (!scrollableContent.Contains(panel))
                {
                    // Makes sure headers are always _below_ panels,
                    // and depth flows downward.
                    panel.Depth = i + (panel is BeatmapSetHeader ? panels.Count : 0);
                    scrollableContent.Add(panel);
                }
            }

            // Update externally controlled state of currently visible panels
            // (e.g. x-offset and opacity).
            float halfHeight = drawHeight / 2;
            foreach (Panel p in scrollableContent.Children)
                updatePanel(p, halfHeight);
        }

        /// <summary>
        /// Computes the x-offset of currently visible panels. Makes the carousel appear round.
        /// </summary>
        /// <param name="dist">
        /// Vertical distance from the center of the carousel container
        /// ranging from -1 to 1.
        /// </param>
        /// <param name="halfHeight">Half the height of the carousel container.</param>
        private static float offsetX(float dist, float halfHeight)
        {
            // The radius of the circle the carousel moves on.
            const float circle_radius = 3;
            double discriminant = Math.Max(0, circle_radius * circle_radius - dist * dist);
            float x = (circle_radius - (float)Math.Sqrt(discriminant)) * halfHeight;

            return 125 + x;
        }

        /// <summary>
        /// Update a panel's x position and multiplicative alpha based on its y position and
        /// the current scroll position.
        /// </summary>
        /// <param name="p">The panel to be updated.</param>
        /// <param name="halfHeight">Half the draw height of the carousel container.</param>
        private void updatePanel(Panel p, float halfHeight)
        {
            var height = p.IsPresent ? p.DrawHeight : 0;

            float panelDrawY = p.Position.Y - Current + height / 2;
            float dist = Math.Abs(1f - panelDrawY / halfHeight);

            // Setting the origin position serves as an additive position on top of potential
            // local transformation we may want to apply (e.g. when a panel gets selected, we
            // may want to smoothly transform it leftwards.)
            p.OriginPosition = new Vector2(-offsetX(dist, halfHeight), 0);

            // We are applying a multiplicative alpha (which is internally done by nesting an
            // additional container and setting that container's alpha) such that we can
            // layer transformations on top, with a similar reasoning to the previous comment.
            p.SetMultiplicativeAlpha(MathHelper.Clamp(1.75f - 1.5f * dist, 0, 1));
        }
    }
}