Questions tagged [hash]

Mathematical function that maps arbitrarily-sized data to fixed-size integers, often used as keys in hash tables or to help ensure data integrity

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Choosing an independent hash function, given hash function value

Supposed we have a function $h:U\to [m_1]$. Given this hash function, can we generate without using randomization or a universal hash collection another hash $h':U \to [m_2]$, which depends on $h$ ...
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Pairwise hash functions that are independent from each other

Is there there a way to build a collection of universal hash functions $H=\{h| h:U\to D \}$ where the values of two hash functions are independent one from another? i.e., $\Pr_{h_1,h_2\in H}(h_1(x)=y ...
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Computing hash of a compound key

Why is the initial value of hash 17 and not 0? ...
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42 views

Hash function to return only positive number from integer

What would be a good hash function that will return a positive integer value, even if the key is an negative integer value? How do I pick a hash function? So what I would want is to associate negative ...
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Are hash functions one-way?

I have heard that we can convert any text to hash code , but hash code can't be converted back to text without brute force. Suppose we consider the text "mal". The hash codes of the individual ...
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Creating a specific hash setup

While solving a practice exam, this is the question I could not answer. Any help is appreciated. I am new to hashing and have no idea how to solve this question. Let $H$ be a $(0.15, 0.85, 0.9, 0.1)$-...
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How can I create 10-character, unique codes with no collisions, but without being predictable?

If we are using numbers and letters, there are $36^{10}$ unique combinations. Collision is already unlikely, but I need it to be impossible, so using hashing is out of the picture(?). The use-case is ...
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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 ...
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String to small integer mapping without collision

Is there any good approach to devise a mapping of limited number of strings $N_1 << 2^{15}$ to integers less than $2^{15}$ without conflicts? Strings are quite often of the form of prefix + ...
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vector hashing function having collisions for permutations

let's consider vectors in space dim=3 and values {0,1,2,...,99} on each dimension I would like to create hash function but with special trait: collisions only for ...
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Why did Wegman-Carter call their class of hash functions as universal on their seminal paper

Wegman-Carter on their seminal Universal classes of hash functions uses the term universal for their definition. They gave this definition as: Let $H$ be a class of functions from $A$ to $B$. We ...
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How can I hash an attributed graph?

I am interested in solving the following problem using a learning to hash algorithm: I've got several attibuted graphs (a graph with node and edge labels - you can think about that as a description ...
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How can I choose good hash function for PCY algorithm

As far as I understand from PCY (Park, Chen, and Yu) algo is that the algo uses hashing during the first pass to reduce the number of CANDIDATE pairs that are considered in the second pass. I have 2 ...
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When to apply a probe sequence when inserting an element to a hash table

So, if we start out with an empty table with M elements. Do we always need to have a specified hash function say $h(x) = x \% 8$ as well as a specific probing sequence to use when inserting a ...
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What does “prime” mean in this context?

In a lecture video the instructor introduced the quadratic probing method for hash tables. The formula he gave was the following: $h(k,i) = (h'(k) + c_1 + c_2i^2)$ $\% M$ where $h'$ was h "prime". ...
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Can data be compressed through this hash function technique?

I'd like to know if this data compression scheme would work or not, and why: Suppose we have a file. If we treat the bits that make up the file as the binary representation of a number n, we have n (...
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A tree-like data structure with rights delegation for distributed computing

Every actor can create a root node and delegate a right to add a child node. Every node contains name of its’ creator or who added it, and value S. Sum of all values S at the same level of the tree ...
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232 views

Simple Uniform Hashing Assumption and worst-case complexity for hash tables

Is the Simple Uniform Hashing Assumption (SUHA) sufficient to show that the worst-case time complexity of hash table lookups is O(1)? It says in the Wikipedia article that this assumption implies ...
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Universal hashing function probability

Can somebody explain the following: U is a universe of keys, and H is a finite collection of hash functions mapping U to {0, 1, … , m-1}. I do not understand definition 2, and thus why amount of ...
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Is there a good way to hash abstract binding trees?

The hash function should be invariant under alpha-renaming. Using de Bruijn notation seems to be possible, but it requires alpha-converting the whole tree when a binding is created, and has the ...
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Construction of hash function with a given distribution

Two questions about the construction of a hash function: Let $U = \{u_1,...,u_n\}$ be a set of size $n$, and suppose that one is interested in a function $h\colon U \rightarrow [0,1]$ such that $h$ "...
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Fast hash function for set equality

I'm searching an hash function for integer set equality that must be fast. It must support update (adding an element already in the set must not change the hash) and union. MinHash has these 2 ...
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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 ...
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Time complexity of website access analysis using Hash Tables

I would like to calculate the time complexity of an algorithm which returns the number of accesses of a website and the number of different users who entered. Each user is represented by a unique ...
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89 views

is modulo of hash function is evenly distributed?

if I take the result of a 32bit hash function(the param is random string) and apply module N on the result - will the values be evenly distributed? so if I have a histogram of values [0,N-1] will the ...
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Prove the probability of which a hash function is collision-free

Suppose $H = \{h_1, ..., h_T\}$ be a family of pairwise independent hash functions mapping $\{0, 1\}^n$ to $\{0, 1\}^{n/2}$. Let $M = \frac{2^{n/4}}{10}$ and let $x_1, ..., x_M$ be any M distinct ...
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are they any implications if i reduce a sha256 result into 248 bits?

i am working a prototype using some cryptographic properties. the question is simple but perhaps deep enough for me to decide : sha256 is a strong hash function, but i have space for 248 bits only. ...
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The expectation of the total number of pairs of keys in a hash table that collide using universal hashing

I am reading CLRS relating to perfect hashing. When computing the $$ \mathbb{E}[\sum_{j=0}^{m-1}{n_j\choose{2}}] $$ where $m$ is the number of slots in the hash table, and $n_j$ is the number of keys ...
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Indexing Edge Permutations for the Rubik's Cube

I'm working on a Rubik's Cube solver that implements Korf's algorithm, as published in his 1997 paper, Finding Optimal Solutions to Rubik's Cube Using Pattern Databases. His method involves creating ...
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134 views

Compute hash value according to multiplication method

In "Introduction to Algorithms" by C. E. Leiserson, R. L. Rivest and C. Stein (ISBN: 978-0262033848), p. 264 they state this: I get everything but the last part stating $h(k) = 67$ ...
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57 views

Reversible Merge of Integer Hash Values

Context: I am working with a tree-like data structure. I would like every node in the tree to have an integer hash value that is the result of combining the integer hash values for the node's ...
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Hash function orders ships from closest to furthest from the origin

Suppose we have a circular radar that scans for ships in an area enclosed by $x^2+y^2 \leq z^2$ (a circle). We wish to design a hash function $h$ such that, we can order the $n$ ships from closest to ...
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O(B) algorithm to find positions of all permutations of smaller string in a bigger string with length B - how is this possible?

Context: I've been working through Cracking the Code Interview and on page 70 the book asserts that there is a O(B) solution to this problem. If s = little string and S = len(s) b = big string and ...
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Search operation in hash table

Suppose that we have a hash table of some size $s$ and we have a set of keys $K$. We decide to hash the keys using chaining. Now assume that we randomly select $k \in \mathbb{N}$ keys from $K$ and ...
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Small space hash functions that are weakly but not strongly universal

This is a follow up to this this question about weakly universal hash functions A family of hash functions $H_w$ is said to be weakly universal if for all $x \ne y$ : $$P_{h \in H_w}(h(x) = h(y)) \...
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What is an example of a weakly universal hash function that is not pairwise independent?

A family of hash functions $H_w$ is said to be weakly universal if for all $x \ne y$ : $$P_{h \in H_w}(h(x) = h(y)) \leq 1/m$$ Here the function $h:U \rightarrow [m]$ is chosen uniformly from the ...
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What is a minimal, pseudo-random hash function?

I want to generate pseudo-random hashes of inputs in a way that is optimally time and space efficient. I'm not at all concerned about security. The output should be evenly distributed and appear ...
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Hash table open addressing without dummy

If a hash table is using open addressing as the collision resolution strategy, then during deletion, we mark that slot as dummy. I understand the reason behind ...
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My professor claims that inserting the same key into a hash table again will lead to a collision … – Is he really right?

My whole life I thought a collision is a situation that occurs when two distinct pieces of data have the same hash value. Everything looks a little bit different now: A few days ago we had to ...
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Fixed point of hash

Are hashing algorithms constructed to guarantee that no fixed point exists? My assumption is not, because I don’t see what utility that would have. (Please correct me if I’m wrong.) As such, purely ...
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Adaptive radix tree for storing Hash vs HAMT

Is there any disadvantage to using Adaptive radix tree to store hash of key like it is done in HAMT and Ctrie Would it be slower? Would it use more memory?
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probability of collision

my data's range is from 1 to 9 and I have two subsets of integers from this range. the hash function takes each of this subsets and calculate product of these three integers and maps this set to the ...
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What is the point in hashing a value?

I apologise if this is not the right place for this question... I didn't want to ask on Stackoverflow or CodeReview as it will closed in minutes as "too broad". A client of mine is writing an API ...
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Most efficient way to check if key exist on a table

Imagine a person called Albert. Albert lives in Europe and would love to move to America. One day he got his wish come true. A person called Margareta gave him a key. The value of the key is the same ...
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Finding duplicates in a stream of numbers

This is an interview question. Say you have a function foo() which returns some integer. You need to write an algorithm that does the following: (...
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In most locality sensitive hashing implemensions of SimHash, why is the cosine distance used and not the euclidean distance?

In Chapter 3 of Mining of Massive Datasets, the basis of locality sensitive hashing is explained. They notably mention simhash for the cosine distance, where random hyperplanes are generated, and for ...
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Are hash functions really quantum resistant for commitment schemes?

It is commonly stated that hash functions remain secure in a post quantum world, the justification being that a quantum computer only has the advantage of Grover's search to give it a quadratic ...
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Do not understand a concept in analysis of open-address hashing

I am reading the "Introduction to Algorithms" by Thomas Cormen et al. Particularly the theorem which says that given an open-address hash table with load factor α=n/m<1, the expected number of ...
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623 views

How is this the expected number of of probes in open-address hashing?

I am reading the "Introduction to Algorithms" by Thomas Cormen et al. Particularly the theorem which says that given an open-address hash table with load factor $\alpha = n/m < 1$, the expected ...
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Why can't hash coding be reversed?

Why is it impossible to reverse a hash code? There could be some way to crack this coding?