Does perfect hashing exist?
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.
Is SHA256 a perfect hash function?
A regular hash function turns a key (a string or a number) into an integer. Most people will know them as either the cryptographic hash functions (MD5, SHA1, SHA256, etc) or their smaller non-cryptographic counterparts frequently encountered in hash tables (the map keyword in Go).
What is a perfect hash table?
(algorithm) Definition: A hash function that maps each different key to a distinct integer. Usually all possible keys must be known beforehand. A hash table that uses a perfect hash has no collisions.
How do you write a good hash function?
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.
What is perfect hashing and how does it work?
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”.
What is Gperf used for?
Gperf is a perfect hash-function generator that constructs perfect hash functions from a user-supplied list of keywords. Gperf translates n element list of user-supplied keywords into source code containing k element lookup table and two functions: hash : This routine uniquely maps keywords to the range 0 ..
Is sha256 Injective?
SHA-256 is almost certainly not injective on 256-bit inputs, so it is almost certainly not a bijection or a surjection onto 256-bit outputs either.
What is space complexity for perfect hashing?
The hash function is computed modulo the size of a reference vector that is much smaller than the hash function range. Because this value is fixed, it is not considered in the space complexity computation. Consequently, the space complexity of every reasonable hash table is O(n).
Which hash function is best?
If you just want to have a good hash function, and cannot wait, djb2 is one of the best string hash functions i know. it has excellent distribution and speed on many different sets of keys and table sizes. you are not likely to do better with one of the “well known” functions such as PJW, K&R[1], etc.
What are the 3 main properties of hash function?
It achieves the three basic properties required from a cryptographic hash function: collision (Coll), second preimage (Sec) and preimage (Pre) security.
What is two level hashing?
The double hashing technique uses one hash value as an index into the table and then repeatedly steps forward an interval until the desired value is located, an empty location is reached, or the entire table has been searched; but this interval is set by a second, independent hash function.
What is Gperf file?