为什么用于计算哈希值code XOR运算符？ [英] Why is the xor operator used in computing hash code?

问题描述

在此MSDN文章 http://msdn.microsoft.com/en-us/library/ms132123.aspx 论述类Equalitycomparer并具有example.In这个例子中大约有此类比较箱 -

In this MSDN article http://msdn.microsoft.com/en-us/library/ms132123.aspx it discusses the Class Equalitycomparer and has an example.In this example about comparing boxes it has this class -

class BoxSameDimensions : EqualityComparer<Box>
{
    public override bool Equals(Box b1, Box b2)
    {
        if (b1.Height == b2.Height & b1.Length == b2.Length
            & b1.Width == b2.Width)
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    public override int GetHashCode(Box bx)
    {
        int hCode = bx.Height ^ bx.Length ^ bx.Width;
        return hCode.GetHashCode();
    }
}

我不明白，行int H code = bx.Height ^ bx.Length ^ bx.Width;

I don't understand the line int hCode = bx.Height ^ bx.Length ^ bx.Width;

有人能解释吗？为什么XOR？

Could someone explain please? Why the xor?

推荐答案

在 ^ 运算符是的按位异或操作。

在这种情况下，它是被用来作为一种方便的方式来生成从三个整数的散列code。 （我不认为这是一个很好的方式，但是这是一个不同的问题...）

In this case it's being used as a convenient way to generate a hash code from three integers. (I don't think it's a very good way, but that's a different issue...)

古怪，构建一个散列code后，他们使用 GetHash code（）上一遍，这是毫无意义的一个int，因为它会只返回INT本身 - 所以这是一个空操作。

Weirdly, after constructing a hash code, they use GetHashCode() on it again, which is utterly pointless for an int because it will just return the int itself - so it's a no-op.

这是他们应该如何写它：

This is how they should have written it:

public override int GetHashCode(Box bx)
{
    return bx.Height ^ bx.Length ^ bx.Width;
}

这SO的回答解释了为什么XOR工作得很好，有时：<一href="http://stackoverflow.com/questions/2334218/why-are-xor-often-used-in-java-hash$c$c-but-another-bitwise-operators-are-used">Why在XOR通常用于Java的哈希code（），但另一个位操作很少被使用？

This SO answer explains why XOR works quite well sometimes: Why are XOR often used in java hashCode() but another bitwise operators are used rarely?

注：我不喜欢使用异或哈希code三个整数这样的原因是：

Note: The reason I don't like using xor for a hash code for three ints like that is because:

a ^ b ^ a == b

在换句话说，如果在第一和最后一个整数有助于散列code相同，只要不向最终散列code在所有 - 它们互相抵消，其结果总是中间的int。

In other words if the first and last ints contributing to the hash code are the same, they do not contribute to the final hash code at all - they cancel each other out and the result is always the middle int.

它甚至更糟，如果你只使用两个整数，因为：

It's even worse if you are only using two ints because:

a ^ a == 0

因此​​，对于两个整数来，因为它们是相同的所有情况下的散列code将为零。

So for two ints, for all cases where they are the same the hash code will be zero.

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