Questions tagged [hashing]
158 questions
7
votes
2 answers
simple uniform hashing: unclear definition of probability
I am trying to understand the assumption of Simple Uniform Hashing (SUHA) as e.g., in CLRS textbook; or other courses about hashing.
The usual description given to SUHA is (cf. CLRS):
"we shall assume that any given element [i.e., key] is equally…
Jack
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4
votes
1 answer
Lemma 2 in Knuth's "Notes on Open Addressing"
I'm trying to read Knuth's Notes on Open Addressing, and I don't quite follow the proof of Lemma 2.
The set-up
We're thinking about hashing $k - 1$ keys into a size $N$ array, with collision resolution via linear probing. In particular, we're…
CruiskeenLawn
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4
votes
2 answers
Merkle tree collision probability
Say I use a perfect 128-bit hash function to construct a merkle tree.
By perfect I mean that any of the values in the $0$–$2^{128}$ range has an equal probability to be an outcome of the function, over a large enough domain of hashed entities.
Does…
Jake
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4
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0 answers
Expected search times with quadratic vs linear probing
Why exactly does quadratic probing lead to a shorter avg. search time than linear probing?
I fully get that linear probing leads to a higher concentration of used slots in the hash table (i.e. higher "clustering" of used consecutive indices).…
Amelio Vazquez-Reina
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3
votes
1 answer
Difference between properties of good hash function: uniformity and randomness
I did go through Korth's book of DBMS and I got these definitions:
Uniformity:
Each bucket is assigned the same number of search-key values from the set of all possible values.
Randomness:
Each bucket will have the same number of records assigned to…
Maharaj
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3
votes
1 answer
Ketama hash explanation
(I originally posted this on stackoverflow but thought it would be a better fit here)
I'm trying to understand the Ketama hash code used in consistent hashing.
link and snippet below:
public static Long md5HashingAlg(String key) {
MessageDigest…
Shobit
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2
votes
3 answers
Building a perfect hashing table
My understanding is that one way to build a perfect hash, as per CLRS, is to use two levels of hashing, with universal hashing functions at each level.
More specifically, CLRS shows that assuming $n$ is the total number of keys, and $n_j$ the…
Amelio Vazquez-Reina
- 457
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2
votes
1 answer
Bit representation of the hashing multiplication method
In the picture below from CLRS, I fail to understand why exactly $h(k)$ = the $p$ highest-order bits of the lower w-bit half of the product.
For context, this is supposed to compute
$h(k) = \lfloor m (k A \; \text{mod} 1) \rfloor $
For further…
Josh
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1
vote
2 answers
Why does Locality Sensitive Hashing use multiple sets of hash tables? How does it guarantee similarity?
With locality sensitive hashing (specifically multi-probe hashing http://www.cs.princeton.edu/cass/papers/mplsh_vldb07.pdf) how are the guarantee of similarity returns made? Why are there multiple tables involved in the hash indexing? Wouldn't that…
nobody
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1
vote
1 answer
Unique ID from date and rotating offset
Hopefully this is the right board for my question, We are facing an issue creating a unique ID from the following inputs :
datetime (can be present more than once in the same set, cannot not be present in 2 different sets)
offset ( an integer…
noob 86466
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1
vote
0 answers
Checking whether a given Hashing Function fulfills Uniform Hashing Condition
I am learning about Hashing and have struggles with the following assignment:
$U$ is a universe of keys with $Z(m)=\{0,\ldots, m-1 \}$ the amount of possible hash-values.
Given $U= \mathbb{Z}_{55} = \{0, \ldots, 54 \}$ and $m=5$.
Does the function…
SimonAda
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1
vote
2 answers
Algorithm to convert a large number (output of a hashing algorithm) into something easily compared visually by humans?
I'm looking for a way to help my users more easily compare the output of a fingerprinting algorithm (which is a very large number/byte array).
So instead of showing something like "8c0db1f40b6e4c1e268a5f3e2405278f" as the fingerprint, I'd rather…
Tobias Gierke
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0
votes
0 answers
Hashing routine explanation
As the title states, I would like to know what a hashing routine is. I found online about hashing algorithms but I heard it, being used in a different context.
hal
- 101
0
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0 answers
How does exactly consistent hashing is solving the problem of overloaded nodes?
I am taking a course on Eductive on System Design and I read about the ways the data can be partitioned amongst DB servers. See the snippet below:
b. Hash-Based Partitioning: In this scheme, we take a hash of the object we are storing. We then…
Ihor M.
- 101
0
votes
1 answer
How the out-of-sample problem is solved in the article "One Loss for All Deep Hashing with a Single Cosine Similarity based Learning Objective"?
My question is about the article One Loss for All Deep Hashing with a Single Cosine Similarity based Learning Objective.
I would like to know how to solve the out-of-sample problem: given a new code how to find its corresponding orthogonal…
R. S.
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