# 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

200 questions
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### Efficient representation of a given surjective function $\{1 \ldots N\} \rightarrow \{1 \ldots M\}$ when $N \gg M$

More precisely: The input is set of $M$ sets (most likely stored sequentially on disk) that contain partitions of the set $\{1..N\}$. I want to efficiently (as far as memory and time complexity goes) ...
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### Find the approximate number of messages (n) that need to be tried

Find the approximate number of messages (n) that need to be tried before finding two that had the same message digest (size k) with probability 0.8. You need to find n as a function of k . What is n ...
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### How to explain the steps in the implementation of hashing functions like SHA1

I was reading the article about the SHA1 hashing function (I know it is not secure anymore) and I've found a pseudocode implementation on Wikipedia. I can see that there are a lot of mathematical ...
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### How to avoid cascading resizes when resizing hash tables?

With conventional collision resolution methods like separate chaining and linear/quadratic probing, the probe sequence for a key can be arbitrarily long - it is simply kept short with high probability ...
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### Term for binary search tree using hashes?

I was looking for a way to easily store and access a symbol table using the least memory and code as possible and I went with a BST. Symbols, however, tend to be defined in order as in foo0, foo1, ...
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### Finding hash of a substring $[i, j]$ in $O(1)$ using $O(|S|)$ pre computation

Given a string $S$ of length $n$ characters, is it possible to calculate the hash of its substring $[i, j]$ (from index $i$ to $j$, both inclusive) in $O(1)$ using some form of precomputation? Can we ...
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### Find longest common substring using a rolling hash

The longest common substring (LCS) of two input strings $s,t$ is a common substring (in both of them) of maximum length. We can relax the constraints to generalize the problem: find a common substring ...
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### Efficient set data structure supporting insert and set equal

What's the best way to represent sets that support the following two operations: Insert(s, i) - adds nonnegative integer i to set s Equal(s1, s2) - Tests if s1 and s2 are the same set. In addition, ...
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### Find $k$ subsets containing a particular element quickly

Suppose there are $n$ subsets of $U$. I want to quickly (in terms of average-case) find k $(< n)$ subsets that contain $e \in U$ (call this Extraction(e)). Elements are integers. To that effect, ...
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### Is there an anti-Bloom filter?

A Bloom filter makes it possible to efficiently keep track of whether various values have already been encountered during processing. When there are many data items then a Bloom filter can result in ...
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### 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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### Why does appending permutations of servers at the end of hash table avoid bottlenecks?

I was reading the following FDS paper: https://www.usenix.org/system/files/conference/osdi12/osdi12-final-75.pdf The paper has a TLT (tract locator table) for identifying where to write in each ...
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### Why does the following function distribute things in a binomial distribution?

I was reading the following FDS paper: https://www.usenix.org/system/files/conference/osdi12/osdi12-final-75.pdf and it says that the the following hash function does not distribute things uniformly ...
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### Proving that collision is less likely if the table size is prime in case modulo arithmetic is used

If suppose your hashCode function results in the following hashCodes among others {x , 2x, 3x, 4x, 5x, 6x...}, then all these are going to be clustered in just m number of buckets, ...
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### Why is the initial state of Zobrist hashing random?

When generating a board for Zobrist hashing, why are the initial elements random? How can you detect changes to elements later if the initial values are random?
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### Help in understanding exactly how lattices used as one way functions for hashing

(This question is related to homework) I am doing a cryptography course via long distance and we have been given an assignment which is based on lattice-based cryptography. I have spent the majority ...
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### Sequential numbers to unique-looking numbers

I'm not sure how to word this because I'm not familiar with this, but I'm sure a process like this is rather common. Basically, I've got members signing up for our website, and each one is assigned a ...
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### Transform values of hash functions like MD5 into numbers

I have read about the hash function MD5. I found an online calculator which counts the value of MD5 for different data. But the MD5 function returns as result strings like ...
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### Fast hashing: combination of different techniques to identify changes in a file?

I want to create a fast way to detect whether a file might or might not be the same. For almost 100% sureness I would use an existing hash algorithm, e.g. SHA256. However, the files are expected to be ...
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### Hashing a Specific Range Of a Character Array [closed]

I need to process queries to Hash various ranges of a character array. I am currently using the Arrays.hashCode from the standard java library. But the problem is that this method is too slow. Also my ...
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### Find similar vector by Locality Sensitive Hashing

I have many vectors in my database. They are in high dimensions such as: $v_1$ : $\langle 23, 23, 1, 33, 103, 219, \dots \rangle$ $v_2$ : $\langle 92, 83, 1, 33, 239, 192, \dots \rangle$ ... I will ...
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### Retrieving data from hash table ordered by insertion time

By default, a hash table is unordered. I have a simple question on the retrieval of elements in a hash table Can we retrieve elements from a hash table in the same order as they are put inside?
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### Complexity of a hash tables with balanced trees in the buckets

If I use a balanced tree instead of lists in a hash table implementation, and also after initializing my table I don't enlarge nor reduce the size of the table, what would be the worst case complexity?...
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### Bloom filter and perfect hashing

A Bloom filter uses a hash function to test membership in a given set $S$, by checking if an item is present of not at the specified position. To mitigate the effect of hash collision, multiple ...
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### Are those definitions of universal hash family equivalent?

I've seen two definitions of a universal hash family, and my questions is if those are equivalent, i think they are and will explain why but i'm not sure if it is. Definition 1: $H$ is a universal ...
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### How do the following Hash Functions compare?

Consider the two hash functions used to map IP addresses. $x_i$ represents a octave (or "bit field") of the address. Hash Function 1: $$h_a(x_1, x_2, x_3, x_4) = \sum^{4}_{i=1} a_ix_i \bmod n$$ ...
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### Modulo hash function and multiples of three

This is a textbook based question. In The Art of Computer Programming Volume 3, Knuth says that for a hash function $h(k) = k \bmod M$, $M$ should not be a multiple of $3$. The explanation given is: ...
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### Collisions of prefixes of MD5 hashes in some fixed interval

I was wondering, if there is an MD5 hash collision in the UNSIGNED MEDIUMINT Range (0 - 16777215). Sadly, I'm unable to run a script to check this myself, due to Memory (RAM) limitations. The answer ...
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### 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 ...
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### A hash function with predicted collisions

As far as I know, the more collision-resistant a hash function is, the better. But is there any way to define a hash function with predicted collisions? In other words, a hash function that collides ...
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### Weak hashing function for memorable IPv6 addresses

IPv6 addresses in the form of 862A:7373:3386:BF1F:8D77:D3D2:220F:D7E0 are much harder to memorize or even transcribe than the 4 octets of IPv4. There have been ...
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### Is open adressing with prime steps bijective?

Who can help me with this topic: Probing with a step width that is a prime number. I am struggling with this question about defining a hashing function $h(k, i)$ for open addressing on a table of ...
I am revising for a computer security course and I am stuck on one of the past questions. Here is it: Alice ($A$) wants to send a short message $M$ to Bob ($B$) using a shared secret $S_{ab}$ to ...