ersatztv/ErsatzTV.Core/Scheduling/ShuffleInOrderCollectionEnumerator.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using ErsatzTV.Core.Domain;
using ErsatzTV.Core.Interfaces.Scheduling;
using LanguageExt;
using static LanguageExt.Prelude;

namespace ErsatzTV.Core.Scheduling
{
    public class ShuffleInOrderCollectionEnumerator : IMediaCollectionEnumerator
    {
        private readonly IList<CollectionWithItems> _collections;
        private readonly int _mediaItemCount;
        private Random _random;
        private IList<MediaItem> _shuffled;

        public ShuffleInOrderCollectionEnumerator(
            IList<CollectionWithItems> collections,
            CollectionEnumeratorState state)
        {
            _collections = collections;
            _mediaItemCount = collections.Sum(c => c.MediaItems.Count);

            if (state.Index >= _mediaItemCount)
            {
                state.Index = 0;
                state.Seed = new Random(state.Seed).Next();
            }
            
            _random = new Random(state.Seed);
            _shuffled = Shuffle(_collections, _random);
            
            State = new CollectionEnumeratorState { Seed = state.Seed };
            while (State.Index < state.Index)
            {
                MoveNext();
            }
        }

        public CollectionEnumeratorState State { get; }
        
        public Option<MediaItem> Current => _shuffled.Any() ? _shuffled[State.Index % _mediaItemCount] : None;

        public void MoveNext()
        {
            if ((State.Index + 1) % _shuffled.Count == 0)
            {
                Option<MediaItem> tail = Current;

                State.Index = 0;
                do
                {
                    State.Seed = _random.Next();
                    _random = new Random(State.Seed);
                    _shuffled = Shuffle(_collections, _random);
                } while (_collections.Count > 1 && Current == tail);
            }
            else
            {
                State.Index++;
            }

            State.Index %= _shuffled.Count;
        }

        public Option<MediaItem> Peek(int offset) => throw new NotSupportedException();

        private IList<MediaItem> Shuffle(IList<CollectionWithItems> collections, Random random)
        {
            // based on https://keyj.emphy.de/balanced-shuffle/
            
            var orderedCollections = collections
                .Filter(c => c.ScheduleAsGroup)
                .Map(c => new OrderedCollection { Index = 0, Items = OrderItems(c) })
                .ToList();

            if (collections.Any(c => !c.ScheduleAsGroup))
            {
                orderedCollections.Add(
                    new OrderedCollection
                    {
                        Index = 0,
                        Items = Shuffle(
                            collections.Filter(c => !c.ScheduleAsGroup).SelectMany(c => c.MediaItems.Map(Some)),
                            random)
                    });
            }

            List<OrderedCollection> filled = Fill(orderedCollections, random);

            var result = new List<MediaItem>();
            for (var i = 0; i < filled[0].Items.Count; i++)
            {
                var batch = filled.Select(collection => collection.Items[i]).ToList();
                foreach (Option<MediaItem> maybeItem in Shuffle(batch, random))
                {
                    result.AddRange(maybeItem);
                }
            }

            return result;
        }

        private List<OrderedCollection> Fill(List<OrderedCollection> orderedCollections, Random random)
        {
            var result = new List<OrderedCollection>();
            int maxLength = orderedCollections.Max(c => c.Items.Count);

            foreach (OrderedCollection collection in orderedCollections)
            {
                var items = new Queue<Option<MediaItem>>(collection.Items);
                var spaces = new Queue<Option<MediaItem>>(
                    Range(0, maxLength - collection.Items.Count).Map(_ => Option<MediaItem>.None).ToList());

                Queue<Option<MediaItem>> smaller = collection.Items.Count < maxLength - collection.Items.Count
                    ? items
                    : spaces;
                Queue<Option<MediaItem>> larger = collection.Items.Count < maxLength - collection.Items.Count
                    ? spaces
                    : items;

                var ordered = new List<Option<MediaItem>>();
                
                int k = smaller.Count;
                while (k > 0)
                {
                    int n = maxLength - ordered.Count;
                    
                    // compute optimal length +/- 10%
                    double optimalLength = n / (double)k + (random.NextDouble() - 0.5) / 5.0;
                    int r = Math.Clamp((int)optimalLength, 1, maxLength - k + 1);
                    ordered.Add(smaller.Dequeue());
                    for (var i = 0; i < r - 1; i++)
                    {
                        ordered.Add(larger.Dequeue());
                    }

                    k--;
                }

                if (smaller.Any())
                {
                    ordered.AddRange(smaller);
                }

                if (larger.Any())
                {
                    ordered.AddRange(larger);
                }

                result.Add(new OrderedCollection { Index = 0, Items = ordered });
            }

            return result;
        }

        private static IList<Option<MediaItem>> OrderItems(CollectionWithItems collectionWithItems)
        {
            if (collectionWithItems.UseCustomOrder)
            {
                return collectionWithItems.MediaItems.Map(Some).ToList();
            }

            return collectionWithItems.MediaItems
                .OrderBy(identity, new ChronologicalMediaComparer())
                .Map(Some)
                .ToList();
        }
        
        private static IList<Option<MediaItem>> Shuffle(IEnumerable<Option<MediaItem>> list, Random random)
        {
            Option<MediaItem>[] copy = list.ToArray();

            int n = copy.Length;
            while (n > 1)
            {
                n--;
                int k = random.Next(n + 1);
                (copy[k], copy[n]) = (copy[n], copy[k]);
            }

            return copy;
        }

        private class OrderedCollection
        {
            public int Index { get; set; }
            public IList<Option<MediaItem>> Items { get; set; }
        }
    }
}