Core源码（二） Linq的Distinct扩展

 先贴源码地址

https://github.com/dotnet/corefx/tree/master/src/System.Linq/src

.NET CORE很大一个好处就是代码的开源，你可以详细的查看你使用类的源代码，并学习微软的写法和实现思路。我们这个系列熟悉基本类库是一个目的，另一个目的就是学习微软的实现思路和编程方法。

       今天我们就单独讨论的问题是linq中的distinct方法是如何实现。最后还会有我们实际编程时候对distinct方法的扩展。

System.Linq

linq中Distinct方法在Enumerable类中

Enumerable

public static partial class Enumerable

内部去重方法实现有2个重载

1 

public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source) => Distinct(source, null);

2

public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
{
    if (source == null)
    {
        throw Error.ArgumentNull(nameof(source));
    }
    return new DistinctIterator<TSource>(source, comparer);
}

去重迭代器DistinctIterator

private sealed class DistinctIterator

去重迭代器，先把元素都加到Set<TSource> _set;中，然后用set的UnionWith去重

这里的set是内部实现的一个轻量级的hash set 具体代码下一部分介绍

/// <summary>
/// An iterator that yields the distinct values in an <see cref="IEnumerable{TSource}"/>.
/// </summary>
/// <typeparam name="TSource">The type of the source enumerable.</typeparam>
private sealed class DistinctIterator<TSource> : Iterator<TSource>, IIListProvider<TSource>
{
    private readonly IEnumerable<TSource> _source;
    private readonly IEqualityComparer<TSource> _comparer;
    private Set<TSource> _set;
private IEnumerator<TSource> _enumerator;

    public DistinctIterator(IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
    {
        Debug.Assert(source != null);
        _source = source;
        _comparer = comparer;
    }
public override Iterator<TSource> Clone() => new DistinctIterator<TSource>(_source, _comparer);

    public override bool MoveNext()
    {
        switch (_state)
        {
            case 1:
                _enumerator = _source.GetEnumerator();
                if (!_enumerator.MoveNext())
                {
                    Dispose();
                    return false;
                }
                TSource element = _enumerator.Current;
                _set = new Set<TSource>(_comparer);
                _set.Add(element);
                _current = element;
                _state = 2;
                return true;
            case 2:
                while (_enumerator.MoveNext())
                {
                    element = _enumerator.Current;
                    if (_set.Add(element))
                    {
                        _current = element;
                        return true;
                    }
                }
                break;
        }
        Dispose();
        return false;
    }
    public override void Dispose()
    {
        if (_enumerator != null)
        {
            _enumerator.Dispose();
            _enumerator = null;
            _set = null;
        }
        base.Dispose();
    }
    private Set<TSource> FillSet()
    {
        Set<TSource> set = new Set<TSource>(_comparer);
        set.UnionWith(_source);
        return set;
    }

    public TSource[] ToArray() => FillSet().ToArray();
    public List<TSource> ToList() => FillSet().ToList();
    public int GetCount(bool onlyIfCheap) => onlyIfCheap ? -1 : FillSet().Count;
}

内部密封的哈希Set

这部分其实是distinct实现的重点，所以内容较多。

在添加元素的时候会对数据进行过滤，如果相同返回false，不同的才会被加入哈希链表中。

/// <summary>
/// A lightweight hash set.
/// </summary>
/// <typeparam name="TElement">The type of the set's items.</typeparam>
internal sealed class Set<TElement>
{
变量
    /// <summary>
    /// The comparer used to hash and compare items in the set.
    /// </summary>
    private readonly IEqualityComparer<TElement> _comparer;

    /// <summary>
    /// The hash buckets, which are used to index into the slots.
    /// </summary>
    private int[] _buckets;

    /// <summary>
    /// The slots, each of which store an item and its hash code.
    /// </summary>
    private Slot[] _slots;
    /// <summary>
    /// An entry in the hash set.
/// </summary>

    private struct Slot
    {
        /// <summary>
        /// The hash code of the item.
        /// </summary>
        internal int _hashCode;

        /// <summary>
        /// In the case of a hash collision（碰撞）, the index of the next slot to probe（查看）.
        /// </summary>
        internal int _next;

        /// <summary>
        /// The item held by this slot.
        /// </summary>
        internal TElement _value;
    }

    /// <summary>
    /// The number of items in this set.
    /// </summary>
    private int _count;
构造函数
    /// <summary>
    /// Constructs a set that compares items with the specified comparer.
    /// </summary>
    /// <param name="comparer">
    /// The comparer. If this is <c>null</c>, it defaults to <see cref="EqualityComparer{TElement}.Default"/>.
    /// </param>
    public Set(IEqualityComparer<TElement> comparer)
    {
        _comparer = comparer ?? EqualityComparer<TElement>.Default;
        _buckets = new int[7];
        _slots = new Slot[7];
    }
新增方法
    /// <summary>
    /// Attempts to add an item to this set.
    /// </summary>
    /// <param name="value">The item to add.</param>
    /// <returns>
    /// <c>true</c> if the item was not in the set; otherwise, <c>false</c>.
    /// </returns>
    public bool Add(TElement value)
    {
//根据值获取哈希值   最终调用的是_comparer.GetHashCode(value)
        int hashCode = InternalGetHashCode(value);
//遍历对比值    直接找到对应的桶，遍历桶中的元素  _slots[i]._next最后一个值会是-1，所以会跳出循环
        for (int i = _buckets[hashCode % _buckets.Length] - 1; i >= 0; i = _slots[i]._next)
        {
            if (_slots[i]._hashCode == hashCode && _comparer.Equals(_slots[i]._value, value))
            {
                return false;
            }
        }
//如果超出长度，就扩展  乘以2加1
        if (_count == _slots.Length)
        {
            Resize();
        }

        int index = _count;
        _count++;
        int bucket = hashCode % _buckets.Length;//这里具体桶的位置需要除以总体长度，这样空间利用率更好
        _slots[index]._hashCode = hashCode;
        _slots[index]._value = value;
        _slots[index]._next = _buckets[bucket] - 1;//桶中前一个元素的位置索引
        _buckets[bucket] = index + 1;
        return true;
    }
去除方法
    /// <summary>
    /// Attempts to remove an item from this set.
    /// </summary>
    /// <param name="value">The item to remove.</param>
    /// <returns>
    /// <c>true</c> if the item was in the set; otherwise, <c>false</c>.
    /// </returns>
    public bool Remove(TElement value)
    {
        int hashCode = InternalGetHashCode(value);
        int bucket = hashCode % _buckets.Length;
        int last = -1;
        for (int i = _buckets[bucket] - 1; i >= 0; last = i, i = _slots[i]._next)
        {
            if (_slots[i]._hashCode == hashCode && _comparer.Equals(_slots[i]._value, value))
            {
                if (last < 0)
                {
                    _buckets[bucket] = _slots[i]._next + 1;
                }
                else
                {
                    _slots[last]._next = _slots[i]._next;
                }

                _slots[i]._hashCode = -1;
                _slots[i]._value = default(TElement);
                _slots[i]._next = -1;
                return true;
            }
        }

        return false;
    }
扩展set 
    /// <summary>
    /// Expands the capacity of this set to double the current capacity, plus one.
    /// </summary>
    private void Resize()
    {
        int newSize = checked((_count * 2) + 1);//这个要检测是否超出int长度限制
        int[] newBuckets = new int[newSize];
        Slot[] newSlots = new Slot[newSize];
        Array.Copy(_slots, 0, newSlots, 0, _count);//赋值newSlots数组
        for (int i = 0; i < _count; i++)
        {
            int bucket = newSlots[i]._hashCode % newSize;
            newSlots[i]._next = newBuckets[bucket] - 1;//重新记录桶位置
            newBuckets[bucket] = i + 1;
        }

        _buckets = newBuckets;
        _slots = newSlots;
    }

    /// <summary>
    /// Creates an array from the items in this set.
    /// </summary>
    /// <returns>An array of the items in this set.</returns>
    public TElement[] ToArray()
    {
        TElement[] array = new TElement[_count];
        for (int i = 0; i != array.Length; ++i)
        {
            array[i] = _slots[i]._value;
        }

        return array;
    }

    /// <summary>
    /// Creates a list from the items in this set.
    /// </summary>
    /// <returns>A list of the items in this set.</returns>
    public List<TElement> ToList()
    {
        int count = _count;
        List<TElement> list = new List<TElement>(count);
        for (int i = 0; i != count; ++i)
        {
            list.Add(_slots[i]._value);
        }

        return list;
    }
UnionWith方法，实际是执行add
    /// <summary>
    /// The number of items in this set.
    /// </summary>
    public int Count => _count;

    /// <summary>
    /// Unions this set with an enumerable.
    /// </summary>
    /// <param name="other">The enumerable.</param>
    public void UnionWith(IEnumerable<TElement> other)
    {
        Debug.Assert(other != null);

        foreach (TElement item in other)
        {
            Add(item);
        }
    }
内部哈希方法
    /// <summary>
    /// Gets the hash code of the provided value with its sign bit zeroed out, so that modulo has a positive result.
    /// </summary>
    /// <param name="value">The value to hash.</param>
    /// <returns>The lower 31 bits of the value's hash code.</returns>
    private int InternalGetHashCode(TElement value) => value == null ? 0 : _comparer.GetHashCode(value) & 0x7FFFFFFF;
}

扩展distinct的关键

实现IEqualityComparer接口

public interface IEqualityComparer<in T>
{
    // true if the specified objects are equal; otherwise, false.
    bool Equals(T x, T y);
    //     Returns a hash code for the specified object.
    // 异常:
    //   T:System.ArgumentNullException:
    //     The type of obj is a reference type and obj is null.
    int GetHashCode(T obj);
}

distinct扩展方法

使用params，支持多字段。

public static class ComparerHelper
{
    /// <summary>
    ///     自定义Distinct扩展方法
    /// </summary>
    /// <typeparam name="T">要去重的对象类</typeparam>
    /// <param name="source">要去重的对象</param>
    /// <param name="getfield">获取自定义去重字段的委托</param>
    /// <returns></returns>
    public static IEnumerable<T> DistinctEx<T>(this IEnumerable<T> source, params Func<T, object>[] getfield)
    {
        return source.Distinct(new CompareEntityFields<T>(getfield));
    }
}
public class CompareEntityFields<T> : IEqualityComparer<T>
{
    private readonly Func<T, object>[] _compareFields;

    /// <summary>
    ///     可以根据字段比对数据
    /// </summary>
    /// <param name="compareFields">比对字段引用</param>
    public CompareEntityFields(params Func<T, object>[] compareFields)
    {
        _compareFields = compareFields;
    }

    /// <summary>Determines whether the specified objects are equal.</summary>
    /// <param name="x">The first object of type T to compare.</param>
    /// <param name="y">The second object of type T to compare.</param>
    /// <returns>true if the specified objects are equal; otherwise, false.</returns>
    bool IEqualityComparer<T>.Equals(T x, T y)
    {
        if (_compareFields == null || _compareFields.Length <= 0)
        {
            return EqualityComparer<T>.Default.Equals(x, y);
        }

        bool result = true;
        foreach (var func in _compareFields)
        {
            var xv = func(x);
            var yv = func(y);
            result = xv == null && yv == null || Equals(xv, yv);
            if (!result) break;
        }

        return result;
    }

    /// <summary>Returns a hash code for the specified object.</summary>
    /// <param name="obj">The <see cref="T:System.Object"></see> for which a hash code is to be returned.</param>
    /// <returns>A hash code for the specified object.</returns>
    /// <exception cref="T:System.ArgumentNullException">
    ///     The type of <paramref name="obj">obj</paramref> is a reference type
    ///     and <paramref name="obj">obj</paramref> is null.
    /// </exception>
    int IEqualityComparer<T>.GetHashCode(T obj)
    {
        return obj.ToString().GetHashCode();
    }
}