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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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What's the problem with deletion in FKS perfect hashing?

In FKS perfect hashing we construct two levels of tables. To lookup an element we first check in the first table which points us to the correct second level table that will contain the element if it ...
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44 views

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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242 views

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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22 views

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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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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59 views

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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30 views

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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9 views

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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346 views

How to find a 2-wise independent hash family that is not 3-wise independent?

I'm trying to find a family of hash functions mapping $\{1, 2, ..., 2^n\}$ to $\{0, 1\}$ that is 2-wise independent but not 3-wise independent. Any ideas on that? I know two 2-wise independent ...
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1answer
121 views

Most space-efficient lossy dictionary?

Short version: What is the most space-efficient lossy dictionary (with given false positive and false negative rates)? Long version: A lossy dictionary is a data structure $D$ that encodes a set $S$ ...
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37 views

Bounds on probe lengths and number of rehashes with universal hashing

Say I have a family of hash functions that are weakly universal, i.e. the probability of two non-identical keys $x\neq y$ are mapped to the same hash-value is bounded by $k/m$ when I have $m$ bins and ...
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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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40 views

Does this excerpt from the linear probing Wikipedia page make an assumption?

Here is the excerpt from the linear probing page at Wikipedia. To search for a given key x, the cells of T are examined, beginning with the cell at index h(x) (where h is the hash function) and ...
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120 views

How to get all in constant time?

We are planning to design a system where following operations are supported. ...
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32 views

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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35 views

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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36 views

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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51 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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2answers
47 views

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?
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1answer
125 views

Can we do 4-sum algorithm in O(n^2)?

this is related to the following question: Generalised 3SUM (k-SUM) problem? Without loss of generality, let's only consider even $k$, or just $k=4$. My question is, after summing all pairs of ...
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45 views

Efficient way to compute mod(w +1) or mod(w - 1) where w= 2^p

Knuth in his book provides a method of how to efficiently calculate mod(w +1) or mod(w-1) where w is a power of 2. I am not sure I could understand his assembly language completely. Could you explain ...
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Is the capacity of a hash table a constant value?

In this paper, page 4, it is said: "...there is always a constant expected number of elements that map to the same slot" Assume we have a set $S$ of $n$ values, and we want to insert them into a ...
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Clarification reg. jump consistent hashing

I was reading the paper on Jump consistent hashing, and I'm having some trouble understanding one particular line (page 5, paragraph 2) "Since we want P(j ≥ i) = (b+1) / i, we set P(j ≥ i) iff r ≤ (b+...
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Is this PJW hash function incorrect?

I came across this implementation of PJW and it seems incorrect: ...
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1answer
303 views

How to actually implement universal hashing?

I sort of get what it is, but I don't understand how its actually supposed to be used in algorithms. Suppose the hash function is $h_{ab}(x) = ((ax+b) \mod p ) \mod m$ where $a$ not equal to 0. If I ...
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Can I uses with same hashing-function for two hash-tables?

Given 2 hash tables , can I use only one hashing function for the two hash tables? I don't sure if the size of the hash-tables is necessarily affect about the hashing-function that this hash-table ...
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48 views

Approximate dot product between neural network output layer's parameter vector and input activations with winner-take-all hashing

In the paper Deep Networks with Large Output Spaces, Vijayanarasimhan et al. describe their approach to approximating the dot product between a neural network's output layer's parameter vector and ...
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2answers
165 views

Perfect (or almost perfect) Hash function for n bit integers with exactly k bits set

I have a a data set with 2598960 elements of of unsigned integers of (at most) 52-bit length. The data set has a property that exactly 5 bits set. This is only 1 data sets – I have similar data sets ...
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1answer
11 views

Method for eliminating spent hashes from a large collection?

Consider a DB with a large set of numbers, like hashcash tokens, where new numbers are constantly added(spent). Only numbers that has not been already added can be accepted. Is there a functional way ...
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109 views

Proving a hash function family is weakly universal

$H$ is a family of weakly universal hash functions if for two elements $x,y$: $$ P(h(x) = h(y)) \le \frac1m, $$ where $m$ is the size of the domain, and $h$ is chosen at random from $H$. ...
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48 views

Can anyone find a mapping from the set of all possible string to the natural numbers?

Can anyone find a map(injection) $h$ from the set of all possible strings $S^*$ to the natural numbers $\mathbb{N}$? $$h : S^* \rightarrow \mathbb{N} $$ Assume $S$ is finite. I would prefer an ...
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1answer
25 views

Bloom Filter which does not fit in RAM [closed]

Is it possible to efficiently create a bloom filter with 10^12 buckets on a single machine with 32GB of RAM and a hard drive. We can assume that the keys are already on the disk and are small in size?
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1answer
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Surprisingly high collision rates when hashing a short list with few buckets

I'm trying to help my daughter with her CS assignment on hashing. She has an input list of about 4000 English words, each 5 letters long. The prof has limited her to 4000 output buckets (digests? -- ...
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158 views

Average number of comparisons for hash/rehash operations

I have the below problem, which I've already been given the answer to, I'm just trying to understand why the answer is the way it is. I've modified the language of the problem significantly so it ...
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514 views

Rolling Hash calculation with Horner's method

I understood how Horner's method reduces the complexity(number of operations) while evaluating a polynomial. I have a character array derived from a string ...
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1answer
54 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, ...
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Is it possible to compute an equality hash for nodes in a *cyclic* directed graph in less than quadratic time?

Calculating hashes for nodes in an acyclic graph is well known using a Merkle tree. With some simplifying assumptions, a simple algorithm will also calculate hashes for nodes in a cyclic graph... but ...
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202 views

Why is the module of the second function in double hashing a prime number?

When using double hashing, the second hash function is defined as $$h_2(x)=A-x\mod A,$$ where $A$ is a prime number less than the capacity of the hash table. But why must $A$ be a prime number? (This ...
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1answer
41 views

Computing 'score' of string which preserves < relation

Is there a way to calculate some kind of numerical 'score' for arbitrary strings which when compared with score of some other string will preserve '<' relation? I've searched for order preserving ...
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Can you compress standard HyperLogLog sketches to a small vector?

I've come across this paper by Otmar Ertl in Github. If I understand it correctly, he points out that when you use p + q bit hashes, you put integers at most of value q into 2^p registers. Instead, he ...
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1answer
105 views

Will Java's hashCode() reduce collisions regardless of table size?

for a class project we have to count the number of collisions from inserting words from a plaintext document into a hashtable. I read that using prime numbers for table sizes can offset the collisions ...
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Is it possible to retrive the Hash from an encrypted folder?

So first of all I've a software which basically locks/encrypts a folder with a given password. Afterwards it's almost impossible to open the folder without entering the password through the software. ...
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20 views

Expected block count of linear hashing

I ran in to this problem and I can't figure it out myself. Can anyone give me a hint? A linear-hashing scheme with blocks that hold k records uses a threshold constant c, such that the current number ...
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1answer
5k views

Explain Hashed page tables in operating system

I have a difficult time understanding hashed page tables used in virtual memory management. Here is picture of the slide that I am referring to: I understand that p is hashed and then the hash is ...
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24 views

Function to generate longer bit-sequence from shorter sequence with certain properties

im not familiar with the terminology of computer-science which makes it pretty difficult to search for the problem I have. I'm looking for a function that generates a sequence of bits (B) of a given ...
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45 views

Deterministic algorithm to find number of collisions

Let $h:U \to[m]=\{0,1,\dots,m\}$ be hash function, which can calculate $h(u), \forall u\in U$ in $O(1).$ Let $D \subseteq U$ be a subset of size $n.$ I'm looking for a deterministic algorithm, ...
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246 views

Can Double Hashing return the same value 2 times?

$h(i,k)=(h^{}_{1}(k)+i * h^{}_{2}(k)) mod |T|$ as it defined in wikipedia I would like to know if it possible that Double Hashing return the same value for given $k$ when $i=1$ and $i=2$. Or from ...