Lookup.cs

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using System.Collections;
using System.Collections.Generic;
 
namespace System.Linq.Parallel;
 
internal sealed class Lookup<TKey, TElement> : ILookup<TKey, TElement>, IEnumerable<IGrouping<TKey, TElement>>, IEnumerable
{
    private readonly IDictionary<TKey, IGrouping<TKey, TElement>> _dict;
 
    private readonly IEqualityComparer<TKey> _comparer;
 
    private IGrouping<TKey, TElement> _defaultKeyGrouping;
 
    public int Count
    {
        get
        {
            int num = _dict.Count;
            if (_defaultKeyGrouping != null)
            {
                num++;
            }
            return num;
        }
    }
 
    public IEnumerable<TElement> this[TKey key]
    {
        get
        {
            if (_comparer.Equals(key, default(TKey)))
            {
                if (_defaultKeyGrouping != null)
                {
                    return _defaultKeyGrouping;
                }
                return Enumerable.Empty<TElement>();
            }
            if (_dict.TryGetValue(key, out var value))
            {
                return value;
            }
            return Enumerable.Empty<TElement>();
        }
    }
 
    internal Lookup(IEqualityComparer<TKey> comparer)
    {
        _comparer = comparer;
        _dict = new Dictionary<TKey, IGrouping<TKey, TElement>>(_comparer);
    }
 
    public bool Contains(TKey key)
    {
        if (_comparer.Equals(key, default(TKey)))
        {
            return _defaultKeyGrouping != null;
        }
        return _dict.ContainsKey(key);
    }
 
    internal void Add(IGrouping<TKey, TElement> grouping)
    {
        if (_comparer.Equals(grouping.Key, default(TKey)))
        {
            _defaultKeyGrouping = grouping;
        }
        else
        {
            _dict.Add(grouping.Key, grouping);
        }
    }
 
    public IEnumerator<IGrouping<TKey, TElement>> GetEnumerator()
    {
        foreach (IGrouping<TKey, TElement> value in _dict.Values)
        {
            yield return value;
        }
        if (_defaultKeyGrouping != null)
        {
            yield return _defaultKeyGrouping;
        }
    }
 
    IEnumerator IEnumerable.GetEnumerator()
    {
        return ((IEnumerable<IGrouping<TKey, TElement>>)this).GetEnumerator();
    }
}