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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questions with no upvoted or accepted answers
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1answer
154 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 $S$ ...
6
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0answers
656 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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0answers
224 views
Mean and variance of number of buckets of length $i$ in hashing with chaining
Consider a hash table with $m$ buckets, with chaining as collision resolution policy. Given the set $S$ that will be stored in the hash table, let $X_i$ be the number of buckets whose chain length is $...
4
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1answer
383 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 ...
3
votes
1answer
51 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 ...
3
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0answers
84 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 ...
3
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0answers
459 views
Why is exact nearest neighbor search hard in high dimensional spaces?
I started research on nearest neighbor search in IR a couple of weeks ago.
I am still very new to this field, but what I discovered so far from literature is:
1) For the exact nearest neighbor ...
3
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0answers
112 views
Algorithm that hashes a collection of sets following their disjointness relation
I wonder whether there is a simple algorithm to do this work:
Say I have a collection of objects $C$, and a binary relation $R: C\times C$ that is auto-reflective ($\forall c\in C: c R c$) , and ...
2
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0answers
76 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 ...
2
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0answers
37 views
Altering the size of a Hash
Does removing the leading(or trailing) n bits of any given hash have any negative effects other then increasing the likeliness of collisions? Does appending 2 hashes of the same object with different ...
2
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0answers
563 views
Distance preserving projection for Euclidean distance
Suppose I have two vectors $V_1, V_2 \in R^l$. Can they be converted into bit vectors $B_1,B_2 \in \{0,1\}^l $ such that if $V_1, V_2$ is close in Euclidean distance, $B_1,B_2$ is close in hamming ...
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0answers
93 views
Collisions of a compression function
I tried posting this in the math forum but I didn't get any responses. I was hoping someone could give me some advice for how to approach the following problem.
If $n$ is a positive integer, let $...
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0answers
65 views
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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0answers
18 views
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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0answers
41 views
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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41 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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0answers
94 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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0answers
51 views
What is the difference between O(n^2) and O(N)[N*O(1)]?
I was reading this article on gperf.
In it they claim that the use of nested if statements for parsing command line input of $N$ options ends up making $O(N^2)$ ...
1
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0answers
131 views
What needs to be done after deleting an item in a hash table built under opening addressing with linear probing
In some textbook problems, the problem asks me to insert a bunch of elements and remove a bunch of elements.
Insertion is done using linear probing, i.e. h = XmodR + I, when there is a collision, I ...
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0answers
66 views
Min/max of hash function (Whirlpool)
The Whirlpool hash function generates a 128-digit hexadecimal number. Could that number ever be ...
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0answers
30 views
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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17 views
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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2answers
52 views
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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0answers
25 views
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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0answers
28 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 ...
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0answers
33 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?
0
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1answer
196 views
How to get all in constant time?
We are planning to design a system where following operations are supported.
...
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0answers
100 views
Is this PJW hash function incorrect?
I came across this implementation of PJW and it seems incorrect:
...
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0answers
200 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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0answers
998 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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0answers
312 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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0answers
26 views
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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0answers
81 views
Existence of perfect hash function
In their original paper, Storing a sparse table with O(1) worst case access time (Fredman, Kolmos and Szemeredi, Proc. FOCS '82, IEEE, 1982), the authors show that a perfect hash function must exist, ...
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27 views
Hash function for searching, is that feasible?
I have a set of sorted values $\left\{a_j \right\}_{1 \leq j \leq n}$, suppose now a number $x$ is given and we would like to find out the index $j$ such that $a_j \leq x < a_{j+1}$ (you can assume ...
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0answers
20 views
are these functions for open address hashing, proper?
I want to use these two functions in two hash-tables with open-addressing. Is there any problems with these functions? If yes, why they are not appropriate?
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0answers
88 views
Perfect hashing function algorithm
Is there a good (perfect) hashing algorithm for the following problem?
We have $n \ll m$ (say $n=150$ and $m=5\times 10^{12}$) and we want a hash table to store integers up to $m$ whose prime ...
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0answers
25 views
How to ensure that no manipulation to data records has been made
Let's say I have a transaction history, like this:
Transaction #1: Add 5
Transaction #2: Add 8
Transaction #3: Remove 2
Transaction #4: Cancel Transaction #2
Is ...
0
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0answers
693 views
Hash Table: How to Calculate Max Load of a Bucket in Practice
My question is related to this question I posted in math forum:
https://math.stackexchange.com/questions/1512644/balls-and-bins-hash-table-a-concrete-example
but I could not get an answer that I ...
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0answers
79 views
How to design Hash Functions for the problem of Set Intersection?
This is a specific question about [1].
Before the question, I must explain and organize some topics:
1) In section 3.2, during the "Pre-processing Stage", a hash function maps an element to a bit-...
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0answers
199 views
How does hashing achieve sketching?
Given a sequence $x \in \{ 1,2,3...,\vert \Sigma \vert \}^*$ one wants to create a sketch of it say $s(x)$ of size $\frac{2c}{3}k (ln^2 k)$ bits. And that seems to be achieved as follows,
pick at ...
