#if NET20 || NET30 || !NET_4_6

using System.Collections;
using System.Collections.Generic;
using LinqInternal.Collections.Specialized;
using LinqInternal.Core;

namespace System.Linq.Reimplement
{
    public static partial class Enumerable
    {
        public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            return GroupBy(source, keySelector, null);
        }

        public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
        {
            return new GroupedEnumerable<TSource, TKey, TSource>(source, keySelector, FuncHelper.GetIdentityFunc<TSource>(), comparer);
        }

        public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
        {
            return GroupBy(source, keySelector, elementSelector, null);
        }

        public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
        {
            LinqCheck.SourceAndKeyElementSelectors(source, keySelector, elementSelector);

            return CreateGroupByIterator(source, keySelector, elementSelector, comparer);
        }

        public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector)
        {
            return GroupBy(source, keySelector, elementSelector, resultSelector, null);
        }

        public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey> comparer)
        {
            LinqCheck.GroupBySelectors(source, keySelector, elementSelector, resultSelector);

            return CreateGroupByIterator(source, keySelector, elementSelector, resultSelector, comparer);
        }

        public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IEnumerable<TSource>, TResult> resultSelector)
        {
            return GroupBy(source, keySelector, resultSelector, null);
        }

        public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IEnumerable<TSource>, TResult> resultSelector, IEqualityComparer<TKey> comparer)
        {
            return new GroupedEnumerable<TSource, TKey, TSource, TResult>(source, keySelector, FuncHelper.GetIdentityFunc<TSource>(), resultSelector, comparer);
        }

        private static IEnumerable<IGrouping<TKey, TElement>> CreateGroupByIterator<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
        {
            return new GroupedEnumerable<TSource, TKey, TElement>(source, keySelector, elementSelector, comparer)/* as IEnumerable<IGrouping<TKey, TElement>>*/;
        }

        private static IEnumerable<TResult> CreateGroupByIterator<TSource, TKey, TElement, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey> comparer)
        {
            return new GroupedEnumerable<TSource, TKey, TElement, TResult>(source, keySelector, elementSelector, resultSelector, comparer)/* as IEnumerable<TResult>*/;
        }

        internal class GroupedEnumerable<TSource, TKey, TElement, TResult> : IEnumerable<TResult>
        {
            private readonly IEqualityComparer<TKey> _comparer;
            private readonly Func<TSource, TElement> _elementSelector;
            private readonly Func<TSource, TKey> _keySelector;
            private readonly Func<TKey, IEnumerable<TElement>, TResult> _resultSelector;
            private readonly IEnumerable<TSource> _source;

            public GroupedEnumerable(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey> comparer)
            {
                if (source == null)
                {
                    throw new ArgumentNullException("source");
                }
                if (keySelector == null)
                {
                    throw new ArgumentNullException("keySelector");
                }
                if (elementSelector == null)
                {
                    throw new ArgumentNullException("elementSelector");
                }
                if (resultSelector == null)
                {
                    throw new ArgumentNullException("resultSelector");
                }
                _source = source;
                _keySelector = keySelector;
                _elementSelector = elementSelector;
                _resultSelector = resultSelector;
                _comparer = comparer;
            }

            public IEnumerator<TResult> GetEnumerator()
            {
                var groupings = new NullAwareDictionary<TKey, Lookup<TKey, TElement>.Grouping>(_comparer);
                foreach (var item in _source)
                {
                    var key = _keySelector(item);
                    Lookup<TKey, TElement>.Grouping grouping;
                    if (!groupings.TryGetValue(key, out grouping))
                    {
                        grouping = new Lookup<TKey, TElement>.Grouping(key);
                        groupings.Add(key, grouping);
                    }
                    grouping.Items.Add(_elementSelector(item));
                }
                return Enumerator(groupings);
            }

            IEnumerator IEnumerable.GetEnumerator()
            {
                return GetEnumerator();
            }

            private IEnumerator<TResult> Enumerator(IDictionary<TKey, Lookup<TKey, TElement>.Grouping> groupings)
            {
                foreach (var grouping in groupings.Values)
                {
                    yield return _resultSelector(grouping.Key, grouping.Items);
                }
            }
        }

        internal class GroupedEnumerable<TSource, TKey, TElement> : IEnumerable<IGrouping<TKey, TElement>>
        {
            private readonly IEqualityComparer<TKey> _comparer;
            private readonly Func<TSource, TElement> _elementSelector;
            private readonly Func<TSource, TKey> _keySelector;
            private readonly IEnumerable<TSource> _source;

            public GroupedEnumerable(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
            {
                if (source == null)
                {
                    throw new ArgumentNullException("source");
                }
                if (keySelector == null)
                {
                    throw new ArgumentNullException("keySelector");
                }
                if (elementSelector == null)
                {
                    throw new ArgumentNullException("elementSelector");
                }
                _source = source;
                _keySelector = keySelector;
                _elementSelector = elementSelector;
                _comparer = comparer;
            }

            public IEnumerator<IGrouping<TKey, TElement>> GetEnumerator()
            {
                var groupings = new NullAwareDictionary<TKey, Lookup<TKey, TElement>.Grouping>(_comparer);
                foreach (var item in _source)
                {
                    var key = _keySelector(item);
                    Lookup<TKey, TElement>.Grouping grouping;
                    if (!groupings.TryGetValue(key, out grouping))
                    {
                        grouping = new Lookup<TKey, TElement>.Grouping(key);
                        groupings.Add(key, grouping);
                    }
                    grouping.Items.Add(_elementSelector(item));
                }
                return Enumerator(groupings);
            }

            IEnumerator IEnumerable.GetEnumerator()
            {
                return GetEnumerator();
            }

            private static IEnumerator<IGrouping<TKey, TElement>> Enumerator(IDictionary<TKey, Lookup<TKey, TElement>.Grouping> groupings)
            {
                foreach (var grouping in groupings.Values)
                {
                    yield return grouping;
                }
            }
        }
    }
}

#endif