如何判断TBucketList的存储桶数 [英] How to judge number of buckets for TBucketList

问题描述

我一直在使用TBucketList和TObjectBucketList满足我的所有哈希需求，但从未尝试过切换存储桶数.我隐约记得在Data Structures类中这是什么意思，但是有人可以在Delphi中详细说明该特定类的细微差别

I've been using the TBucketList and TObjectBucketList for all my hashing needs, but never experiemented with switching the number of buckets. I vaguely remember what this means from Data Structures class, but could someone elaborated on the nuances of this particular class in Delphi

The following table lists the possible values:

Value   Number of buckets

bl2   2
bl4   4
bl8   8
bl16     16
bl32     32
bl64     64
bl128   128
bl256   256

推荐答案

TBucketList和TObjectBucketList存储指针.他们使用的哈希函数只是掩盖了地址的高位.掩盖多少位取决于对象具有多少个存储桶.例如，如果使用bl2，则会屏蔽掉31位，只有地址的一位确定存储桶.使用bl256，将使用指针的整个字节.它是中间两个字节之一.权衡只是您将拥有的存储桶数.一个存储桶仅占用8个字节，因此拥有256个字节并不是很大的代价.

TBucketList and TObjectBucketList store pointers. The hash function they use simply masks out the upper bits of the address. How many bits get masked out depends on how many buckets the object has. If you use bl2, for example, 31 bits get masked out and only one bit of the address determines the bucket. With bl256, an entire byte of the pointer gets used. It's one of the middle two bytes. The trade-off is simply the number of buckets you'll have. A bucket only takes eight bytes, so having 256 of them isn't a huge cost.

除此之外，TBucketList只是一个普通的哈希表，就像您在数据结构类中学到的一样.

Aside from that, TBucketList is just an ordinary hash table like you learned about in your data-structure class.

TIntegerBucketList使用与其他函数相同的哈希函数.如果要使用更复杂的哈希函数，请编写TCustomBucketList的后代并覆盖BucketFor方法.在后代类中，您还可以分配受保护的BucketCount属性以使用除TBucketList提供的计数以外的其他内容.请注意，由于存储桶计数的变化，该类不进行任何重新分配项目的操作，因此，除非已计划自己进行重新分配，否则不要在将项目添加到列表后重新分配BucketCount.

TIntegerBucketList uses the same hash function as the others. If you want a more sophisticated hash function, write a descendant of TCustomBucketList and override the BucketFor method. In your descendant class, you can also assign the protected BucketCount property to use something other than the counts provided by TBucketList. Note that the class makes no effort to redistribute items due to a change in the bucket count, so don't re-assign BucketCount after items have already been added to the list unless you plan to do the redistribution yourself.

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