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
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22 views
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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1answer
11 views
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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15 views
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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29 views
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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0answers
12 views
Internal vs External Hashing in dbms context
I've studied the external hashing in the context of a dbms and it seems clear to me. What i've understood is that, in the external hashing the hash table is called a directory and it composed of pages ...
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0answers
27 views
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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12 views
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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2answers
52 views
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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2answers
48 views
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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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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2answers
95 views
Simple Uniform Hashing Assumption and worst-case complexity for hash tables
My question: 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 ...
2
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2answers
36 views
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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0answers
20 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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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 $h$ "...
3
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1answer
63 views
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 ...
2
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1answer
24 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 ...
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0answers
18 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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1answer
70 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 ...
4
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1answer
62 views
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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1answer
65 views
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.
...
3
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1answer
43 views
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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1answer
78 views
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 ...
2
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1answer
129 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 $h(k) = 67$
...
3
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1answer
54 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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2answers
56 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 ...
2
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2answers
53 views
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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0answers
27 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 ...
3
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1answer
63 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 $x \ne y$ :
$$P_{h \in H_w}(h(x) = h(y)) \...
3
votes
1answer
285 views
What is an example of a weakly universal hash function that is not pairwise independent?
A family of hash functions $H_w$ is said to be weakly universal if for all $x \ne y$ :
$$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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0answers
30 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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1answer
116 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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1answer
75 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 ...
4
votes
1answer
132 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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0answers
38 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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2answers
100 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 ...
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3answers
1k 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 ...
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1answer
40 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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1answer
119 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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1answer
162 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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1answer
47 views
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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1answer
23 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 ...
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1answer
484 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
50 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
459 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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1answer
59 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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1answer
249 views
How to get all in constant time?
We are planning to design a system where following operations are supported.
...
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1answer
50 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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3answers
83 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 ...
2
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1answer
99 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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1answer
167 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 $S$ ...
