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Questions tagged [hashing]

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45k views

What exactly (and precisely) is “hash?”

I have heard the word "hash" being used in different contexts (all within the world of computing) with different meanings. For example, in the book Learn Python the Hard Way, in the chapter on ...
15k views

Two-way Hash Functions

While I'm aware most (good) hash functions are one-way (or at least mostly so), I'm wondering if there's any construct (not necessarily called a hash function) which behaves in many ways like a hash ...
223 views

Can we remove duplicates faster than we can sort?

The problem is (integer) duplicate removal, which can also be perceived as producing the image of an evaluated function (of integers): Given a sequence $S_\text{in}$ of $n$ integers, produce a ...
195 views

Two definitions of universal hash functions

I have seen two definitions of universal hash functions in the literature. For any $i \geqslant 2$ let $[i]=\{1,\ldots,i\}$. Definition 1: A family $\mathcal H$ of hash functions from $[n]$ to ...
170 views

How do you find a hash function that respects a custom equality function?

I've been tasked with hashing arbitrary types in C++, with the caveat that A == B implies hash(A) == hash(B) even if equality of ...
53 views

Finding a (minimal?) program that maps $M$ items to indices $[0,M)$

Let's say there are $M$ strings that we are trying to create a perfect hash for such that we get as output of the hash $[0,M)$ with no collisions, when hashing those $M$ items. I know that there are ...
126 views

Lemma 2 in Knuth's “Notes on Open Addressing”

I'm trying to read Knuth's Notes on Open Addressing, and I don't quite follow the proof of Lemma 2. The set-up We're thinking about hashing $k - 1$ keys into a size $N$ array, with collision ...
459 views

Is FKS hashing really linear space?

In FKS hashing, I wonder if the size of the table $G[ 1..n]$ (used to record the functions $g_i$ which is chosen randomly; one entry per bucket) is really strictly $O(n)$. Given that the probability ...
281 views

What is the complexity class of solving hash decision problems?

With $hash_n$, I mean a standard cryptographic hash like sha256, scaled up to have arbitrary length $n$ of its output with the same underlying principles. What is the time complexity class of the ...
64 views

Expected search times with quadratic vs linear probing

Why exactly does quadratic probing lead to a shorter avg. search time than linear probing? I fully get that linear probing leads to a higher concentration of used slots in the hash table (i.e. ...
324 views

74 views

Given a family of hash functions in table form, how can I know whether it's universal?

I've been given the following two families of hash functions: H and G Each family has three functions $\{0,1,2,3,4\} \to \{0,1,2\}$ that can be seen in the tables above. For each family I need to ...
78 views

Avoiding correlated values and reduced collision resistance in multiple hash states

I need to design a hash function that fits this criteria: Limited to 32-bit integer operations (for compatibility with JavaScript bitwise operations, my target system) Produces an n-bit hash digest, ...
63 views

General name for linked lists based on hashes

I am thinking of a particular datastructure, but don't know the name of it. A sequence of elements may be modeled by a collection of some X, where each X consists of: The element, serialized as a ...
347 views

Heuristic analysis of Bloom filters

I am currently watching a lecture on Bloom filters, and the professor is doing a heuristic analysis of Bloom filters. It's all based on the following assumption: All $h_{i}(x)$'s are uniformly ...
44 views

Is there a way to theoretically compare hash functions?

Say I am given two hash functions f1 and f2 is there anyway that I can prove one hash function will produce fewer collisions ...
164 views

Confuse on using bucket number to replace hashing multiple times in hyperloglog explanation

In this blog the author states that the following So we now have a rather poor estimate of the number of values in the dataset based on bit patterns. How can we improve on it? One ...
92 views

How to extend a hash function to manipulate longer integers?

Carter and Wegman introduced in the paper Universal Classes of Hash Functions the $H_{1}$ universal class of hash function. This is essentially the function $h_{a,b}(x) = ((a\cdot x+b)\mod p)\mod m$. ...
30 views

In Universal Hashing- probability of poor performance is small and is the same for any set of keys of the same size

I was going through the text Introduction to Algorithms by Cormen et. al. where I came across the following claim: Universal Hashing uses randomization.For the example of a compiler's symbol table, ...
1k views

Open hashing vs closed hashing

What are advantages of closed hashing over open hashing? I know the difference between those two but can't figure out why would closed hashing be better in any way. Thanks.
105 views

Approximate Similarity Search

I am implementing an approximate similarity search using multi-index hashing. I have a set (T) of millions of strings (of same length) and I have a query string(P) (or set of strings) that needs to ...
194 views

Using two hash functions for increased password security?

Forgive me for my brief knowledge on hash functions as I am not from a computer science background, however I am researching password security for my thesis and have been looking into hashing ...
406 views

Hash function which is invariant under small changes

I am looking for a hash function which is invariant under small changes. E.g., if I have two strings MyString and MySttring ...
11k views

Understanding hashtable performance in the worst-case

Under assumption that the hash function is uniform, we have worst-case performance for the search operation in a separate-chaining (e.g. java.util.HashMap) ...
2k views

How does Rabin fingerprint finds breakpoints (chunk boundary)?

I was reading up on chunking and found Rabin fingerprinting is also used to break files into chunks. After reading about this algorithm, I understood how rolling hash is computed using Rabin ...
64 views

Check whether all elements are different

Assume we have $n$ double elements $a_1 \dots a_n$. We want to find out if two of the elements of the array are identical. And we have a hash function $h(x)$ which assigns each double value an integer ...