[英]what am i doing wrong in this bloom filter implementation?
I have this bit table for a segmented bloom filter. 我有这个位表用于分段布隆过滤器。 Here every column is managed by a single hash function.
这里每列都由一个哈希函数管理。
unsigned char bit_table_[ROWS][COLUMNS];//bit_table now have 8*ROWS*COLUMNS bits
unsigned char bit_mask[bits_per_char] = { 0x01,0x02,0x04,0x08,
0x10,0x20,0x40,0x80};
There are ROWS number of hash functions each of which handles the setting and checking of COLUMNS*8 bits. 有ROWS个哈希函数,每个哈希函数处理COLUMNS * 8位的设置和检查。
Elements are hashed and bit_index and bit are calculated as 元素经过哈希处理, bit_index和bit计算为
compute_indices(unsigned int hash)
{
bit_index=hash%COLUMNS;
bit=bit_index%8;
}
Now insetion is done as 现在就完成了摄制
for (std::size_t i = 0; i < ROWS; ++i)
{
hash=compute_hash(i,set_element);
compute_indices(hash);
bit_table_[i][bit_index ] |= bit_mask[bit];
}
And the query is 而查询是
for (std::size_t i = 0; i < ROWS; ++i)
{
hash=compute_hash(i,set_element);
compute_indices(hash);
if (((bit_table_[i][bit_index])& bit_mask[bit]) != bit_mask[bit])
{
return false;
}
}
My problem is the bloom filter gets full too soon and I suspect that i am not using the individual bits of the characters correctly. 我的问题是布隆过滤器太快就满了,我怀疑我没有正确使用字符的各个位。 For example i suppose i should have something like:
例如,我想我应该有类似的东西:
bit_table_[i][bit_index][bit]|=bit_mask[bit]; bit_table_ [I] [bit_index] [比特] | = BIT_MASK [比特];
for insertion but, since the bit_table is declared as two dimensional array i am not allowed to do this. 插入但是,由于bit_table被声明为二维数组,我不允许这样做。
What should i do to make use of the individual bits of the char array? 我该怎么做才能利用char数组的各个位?
English is my second language, so you might have trouble understanding my question. 英语是我的第二语言,所以你可能无法理解我的问题。 I would be happy to explain my points more if requested.
如果有要求,我会很乐意解释我的观点。
EDIT: compute_hash(i,set_elemnt) uses predefined salt values to compute hash value of the element to be inserted or queried. 编辑: compute_hash(i,set_elemnt)使用预定义的salt值来计算要插入或查询的元素的哈希值。
There is an error in your compute_indices method. 您的compute_indices方法中存在错误。
You are computing a column index and then apply a modulo 8 on this column index. 您正在计算列索引,然后在此列索引上应用模8。 At the end you will always use the same bit in a column.
最后,您将始终在列中使用相同的位。 For example for the column 10, you will always use the bit 2.
例如,对于列10,您将始终使用位2。
You should have : 你应该有 :
compute_indices(unsigned int hash)
{
int bitIndex = hash % (COLUMNS * 8);
bit_index= bitIndex / 8;
bit = bitIndex % 8;
}
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