c# - 类似值的 GetHashCode

Question

我有以下类(class):

public class Foo
{
    int year;       
    string name;    
    int category;   
}

这是一些示例数据:

2012    Test1   1000
2012    Test2   1000
2012    Test3   1000    
2012    Test4   1000
2012    Test4   10
...

如果我覆盖 GetHashCode 所有结果都非常相似:

return year ^ name ^ category;

int hash = 13;
    hash = hash * 33 + year.GetHashCode();
    hash = hash * 33 + name.GetHashCode();
    hash = hash * 33 + category.GetHashCode();
    return hash; 

对于这种情况，什么是好的哈希函数(具有最大分布)？

编辑:也许我对哈希桶的理解是错误的。将相似的哈希值转到同一个桶？

"Test1".GetHashCode() --> -1556460260
"Test2".GetHashCode() --> -1556460257

Answer 1

我建议的一件事是检查字符串对象是否为空。

实现似乎没问题，应该是相似的，但哈希码应该不同，因为主要目标是使它们落入不同的桶中，从而有助于进一步的操作。

   public int hashCode() {    // Assuming year and category are String like name.
    int hash = 31;
    hash = hash * 331 + (this.year != null ? this.year.GethashCode() : 0);
    hash = hash * 331 + (this.name != null ? this.name.GethashCode() : 0);
    hash = hash * 331 + (this.category != null ? this.category.GethashCode() : 0);

    return hash;
}

我在重写 hashCode 时学到的几个步骤是；

1. Choose a prime hash e.g. 5, 7, 17 or 31 (prime number as hash, results in distinct hashcode for distinct object).
2. Take another prime as multiplier different than hash is good.
3. Compute hashcode for each member and add them into final hash. Repeat this for all members which participated in equals.
4. Return hash.