universal-hashing | 易学教程

Pairwise independent hash functions in Java

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问题 I'm looking for a quick and easy way to use a (universal) family of pairwise independent hash functions in my Java projects. Ideally, I would have some object UniversalFamily (representing the Family) which would return me objects with a method hash() which hashes integers. Example usage: // use this object to generate pairwise independent hash functions UniversalFamily family = new UniversalFamily(); // these objects represent the pairwise independent hash functions HashF hashF1 = fam

Pairwise independent hash functions in Java

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Is Universal family of hash functions only to prevent enemy attack?

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问题 If my intention is only to have a good hash function that spreads data evenly into all of the buckets, then I need not come up with a family of hash functions, I could just do with one good hash function, is that correct? The purpose of having a family of hash functions is only to make it harder for the enemy to build a pathological data set as when we pick a hash function randomly, he/she has no information about which hash function is employed. Is my understanding right? EDIT: Since someone

How to generate 64 bit random numbers?

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问题 I'm implementing universal hashing and using the following universal hash function : h(k)=((A*k)mod 2^64) rsh 64-r where A is a random number between 2^61 and 2^62. The rand() function in C++ has return type integer and it can't generate that big numbers. So how can i generate random numbers in this range? (numbers should be very random i.e. every number should have equal probability to be selected) Note: long long int random=rand(); doesn't work as the number returned by rand is int . 回答1:

Hashtable- Rehashing

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问题 I have been told that Hashtable in .NET uses rehashing in order to reduce/avoid collision. Ie. “Rehasing works as follows: assume we have a set of hash different functions, H1 ... Hn, and when inserting or retrieving an item from the hash table, initially the H1 hash function is used. If this leads to a collision, H2 is tried instead and onwards up to Hn to avoid collision in Hashtable.” Assumption: We have a hashtable with n (where n < Infinity) element where asymptotic time complexity is o

Hashtable- Rehashing

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I have been told that Hashtable in .NET uses rehashing in order to reduce/avoid collision. Ie. “Rehasing works as follows: assume we have a set of hash different functions, H1 ... Hn, and when inserting or retrieving an item from the hash table, initially the H1 hash function is used. If this leads to a collision, H2 is tried instead and onwards up to Hn to avoid collision in Hashtable.” Assumption: We have a hashtable with n (where n < Infinity) element where asymptotic time complexity is o(1); we (CLR) have achieved this while applying some hashing function ( Hn-1 hash function where n>1).

Basics in Universal Hashing, how to ensure accessibility

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问题 to my current understanding Universal Hashing is a method whereby the hash function is chosen randomly at runtime in order to guarantee reasonable performance for any kind of input. I understand we may do this in order to prevent manipulation by somebody choosing malicious input deliberately (a possibility of a deterministic hash function is know). My Question is the following: Is it not true, that we still need to guarantee that a key will be mapped to the same address every time we hash it

How to generate 64 bit random numbers?

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I'm implementing universal hashing and using the following universal hash function : h(k)=((A*k)mod 2^64) rsh 64-r where A is a random number between 2^61 and 2^62. The rand() function in C++ has return type integer and it can't generate that big numbers. So how can i generate random numbers in this range? (numbers should be very random i.e. every number should have equal probability to be selected) Note: long long int random=rand(); doesn't work as the number returned by rand is int . In C++11 you can use the random header and std::uniform_int_distribution along with a 64-bit instance of std:

Hash a Range of Values

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问题 I know that I can hash singular values as keys in a dict . For example, I can hash 5 as one of the keys in a dict . I am currently facing a problem that requires me to hash a range of values. Basically, I need a faster way to to do this: if 0 <= x <= 0.1: # f(A) elif 0.1 <= x <= 0.2: # f(B) elif 0.2 <= x <= 0.3: # f(C) elif 0.3 <= x <= 0.4: # f(D) elif 0.4 <= x <= 0.5: # f(E) elif 0.5 <= x <= 0.6: # f(F) where x is some float parameter of arbitrary precision. The fastest way I can think of is