# 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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### 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 ...
63 views

### 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 ...
479 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 ...
239 views

### 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 ...
2k views

### 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 ...
45 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 ...
328 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: (...
318 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 ...
54 views

### 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 ...
1k 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 ...
53 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?
854 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 ...
72 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 ...
123 views

### 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 ...
152 views

### 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: ...
941 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 ...
111 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 ...
711 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 ...
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 ...
358 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 $H$. Given a hash ...
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### 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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### 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?
137 views

### 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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### 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 ...
105 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, ...
221 views

### 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 ...
807 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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### 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 ...
283 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. ...
12k 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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### 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 ...
246 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, ...
513 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 ...
53 views

### Is there a heuristic or function to determine if two arrays of integers are alike or similar

What I am trying to do is determine "closeness" or how similar are arrays of integers (or byte arrays, doesn't matter). For example, let's say a = [0, 1, 2, 3, 4], <...
861 views

### Double Hashing and Variations for Bloom Filters

I am reading a few papers on Bloom Filters – Bloom Filters in Probabilistic Verification (Dillinger and Manolios) suggests the following allocations for double and triple hashing respectively ...
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### Double Hashing Collision

Less Hashing Same Performance: Building A Better Bloom Filter (Kirsch and Mitzenmacher) mentioned that we can use $g_i(x) = (h_1 (x)+ih_2 (x))\pmod{p}$, where $h_1(x)$ and $h_2(x)$ are two ...
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### Probability of hash collision in the case of two parallel hashes

I understand how to calculate the probability of a hash collision. I am designing a DB and have a potential case where a record could have the inherited hash of its parent plus its own hash, meaning I ...
150 views

### why does message authentication using 2-universal family of hash functions require a prime number of possible hash values?

I am self-studying the book Intro to Algorithms 3ed by CLRS. One of the problems seems to give a piece of information that is not necessary, Problem 11-4 in the book states Let H be class of hash ...
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### How similar is the Goldwasser-Sipser Set Lower Bound Protocol to the Hashcash/Bitcoin Proof-of-Work?

Given a hash function $H:\{0,1\}^*\rightarrow\{0,1\}^n$, a difficulty $d\in\mathbb{N}$, and data $D\in\{0,1\}^*$, the framework of the Hashcash/Bitcoin Proof-of-Work entails finding a nonce $c$ such ...
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### Is a powerful two-way hash function provably impossible?

A two-way hash function that could hash complex strings to a fixed length would change the world. Imagine the decreased load on wires around the world if, for example, HTML pages could be hashed, ...
3k views

### Why is Big O not defined here for a hash table?

In this cheat sheet, average time complexity for access to a hash table is listed as N/A. I'm curious as to why. Since a hash table is mostly mathematical, I would assume it would be O(1) like the ...
110 views

### Bloom filters vs storing hashes as numbers

In competitive programming there is a trick for storing a set of strings(or objects really) to reduce memory - you only keep the hashes of the strings in a hash-table (usually as 32 or 64 bit integers)...
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Given a database with usernames and the first six bytes of their hashed passwords. A standard hashing function has been used for hashing. admin 827ccb0eea8a user c3981fa8d26e operator 5f4dcc3b5aa7 ...
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### range / interval query algorithm

I've an hash (base 32 for what it's worth): hash = 'ab352eghjhngd4' And I've subscribers that want to listen to new hashes in a range. ...
398 views

### Can proofs-of-work be probabilistically checkable?

I have been lurking for a while; this is my first post here. I’m sorry if my question is ill-formed or formatted poorly. This question came out of some ideas in another question from a sister site. ...
I'm currently reading Knuth's proof about O(1) number of probes in linear probing. I have a small question on the page 536 (Volume 3, 2nd Edition). Knuth says Let $f(M, N)$ be the number of hash ...
How to prove that a $k$-universal family of hash functions is $(k-1)$-universal family? I tried to prove it by definition of k-universal family of hash functions but I didn't know how to use the ...