What is a good hash function for integers?
A good hash function to use with integer key values is the mid-square method. The mid-square method squares the key value, and then takes out the middle r bits of the result, giving a value in the range 0 to 2r−1. This works well because most or all bits of the key value contribute to the result.
How do I find the best hash function?
Choosing a good hashing function, h(k), is essential for hash-table based searching. h should distribute the elements of our collection as uniformly as possible to the “slots” of the hash table. The key criterion is that there should be a minimum number of collisions. will provide uniform hashing.
How do you int hash?
The most commonly used method for hashing integers is called modular hashing: we choose the array size M to be prime, and, for any positive integer key k, compute the remainder when dividing k by M. This function is very easy to compute (k % M, in Java), and is effective in dispersing the keys evenly between 0 and M-1.
What is the simplest hash function?
Simple hash functions h(k) = k mod m. Works badly for many types of patterns in the input data. Knuth Variant on Division h(k) = k(k+3) mod m. Supposedly works much better than the raw division method. Multiplication Method (Cormen).
What is perfect hash table?
Perfect hashing is defined as a model of hashing in which any set of n elements can be stored in a hash table of equal size and can have lookups performed in constant time. It was specifically invented and discussed by Fredman, Komlos and Szemeredi (1984) and has therefore been nicknamed as “FKS Hashing”.
How do you create a hash value?
Hashing involves applying a hashing algorithm to a data item, known as the hashing key, to create a hash value. Hashing algorithms take a large range of values (such as all possible strings or all possible files) and map them onto a smaller set of values (such as a 128 bit number).
What is the formula of hash function?
With modular hashing, the hash function is simply h(k) = k mod m for some m (usually, the number of buckets). The value k is an integer hash code generated from the key. If m is a power of two (i.e., m=2p), then h(k) is just the p lowest-order bits of k.
How do I choose between Hashtable and BST?
If the input size is not known to you in advance, then use the Hash Table. Range Search: If you want to perform range search i.e. searching some key in between some keys, then you should go with Binary Search Tree because, in Binary Search Tree, you ignore that subtree which is impossible to have the answer.
What is a perfect hash function?
In computer science, a perfect hash function h for a set S is a hash function that maps distinct elements in S to a set of m integers, with no collisions. In mathematical terms, it is an injective function . Perfect hash functions may be used to implement a lookup table with constant worst-case access time.
What integer hash function are good that accepts an integer key?
What integer hash function are good that accepts an integer hash key? In general, you should pick a multiplier that is in the order of your hash size ( 2^32 in the example) and has no common factors with it. This way the hash function covers all your hash space uniformly.
What is the minimum number of bits needed for perfect hashing?
It has been proven that a general purpose minimal perfect hash scheme requires at least 1.44 bits/key. The best currently known minimal perfect hashing schemes can be represented using less than 1.56 bits/key if given enough time. A hash function is k -perfect if at most k elements from S are mapped onto the same value in the range.
What is the range of minimal perfect hash function?
Then h is a minimal perfect hash function if and only if h(j) = h(k) implies j = k ( injectivity) and there exists an integer a such that the range of h is a..a + |S| − 1. It has been proven that a general purpose minimal perfect hash scheme requires at least 1.44 bits/key.