# 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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154 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? -- ...
2k 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 ...
119 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, ...
260 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 ...
922 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 ...
51 views

### 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 ...
301 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 ...
35 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. ...
13k 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 ...
27 views

### 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 ...
321 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, ...
563 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 ...
105 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], <...
899 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 ...
56 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 ...
1k views

### 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 ...
153 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 ...
500 views

### 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 ...
112 views

### 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, ...
4k 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 ...
122 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)...
56 views

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 ...
229 views

### 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. ...
415 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. ...
760 views

### Knuth's proof of O(1) for linear probing

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 ...
553 views

### Universal family of hash functions

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 ...
88 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, ...
118 views

### Are there any hash functions/stateless RNGs that do not use XOR, but produce good quality visual randomness?

I'm looking for a small function from integers to integers - in a language that only has floats - that can act as a visual RNG. Normally I would use a function such as the one described here: ...
244 views

### "Hash" Probing?

While there are many types of probing in hash tables, such as linear probing, quadratic probing, and more, I haven't encountered a so-called "hash-probing" (maybe this method which I describe below ...
158 views

### Does this problem offer any insight into $P$ vs $NP$

What is the input of a given hash? The problem can be verified in polynomial time (using a hash that executed in polynomial time), and I suspect that it may be possible to prove that there is ...
448 views

### What is the reasoning behind magic constancs in hash code calculations found in programming practice?

In real-world programming, we frequently need to compute hash codes for complicated objects. The main desired properties are that the values should be deterministic and have few collisions. Let's ...
28 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 ...
26 views

### Prove you computed hash^r(input) for some cryptographic hash function [closed]

What is the most efficient way to prove that a person computed r rounds of some cryptographic hash function (ex. sha256) on an input?. The trivial solution seems to be to show all r hashes, where <...
41 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 ...
3k views

### Hash function floating point inputs for genetic algorithm

I am implementing a genetic algorithm to use as an optimisation algorithm to evolve robots. The robots have certain parameters (represented as floats) which can lie anywhere within a certain range ...
555 views

### What is the complexity class of solving hash decision problems?

With $hash_n$, I mean a standard cryptographic hash like sha256, scaled up to have arbitrary length $n$ of its output with the same underlying principles. What is the time complexity class of the ...
85 views

### What hash algorithm is it?

I've found a simple multiply-with-add hash function in an old Usenet post. Can someone identify what hash algorithm is it? An algorithm name or any attribution? ...
22 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? ...
74 views

### Can we recognize the difference of two strings of $2^n$ length using polynomial size strings?

Is it possible to transform binary strings of length $2^n$ to $n^c$ binary strings of sized $n^d$ such that $$\forall s_1,s_2 \; \exists i \in \{1,\cdots,n^c\} \; f_i(s_1)\neq f_i(s_2),$$ Where two ...
77 views

### Hashing methods for validating dowloaded files

The standard algorithm to generate hashes of files which are downloaded is MD5. For example, when ISO files of Linux distributions are offered most of the time they also give the MD5 sum so that you ...
266 views

### Using random projections for locally sensitive hashing

I recently came to see that this library sparselsh uses random projection to perform locally sensitive hashing of documents. The proof is such that based on cosine similarity to the vector. In other ...
121 views

### How to extend a hash function to manipulate longer integers?

Carter and Wegman introduced in the paper Universal Classes of Hash Functions the $H_{1}$ universal class of hash function. This is essentially the function $h_{a,b}(x) = ((a\cdot x+b)\mod p)\mod m$. ...
3k views

### Check-digit algorithm that includes characters?

I have a data set that uses a very simple modulo 10 checksum algorithm which ignores alphabetic characters entirely. Which wasn't a big deal, as the few alphabetic characters present weren't ...
106 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 ...
3k views

### How would you prove a family of functions is universal?

A set of functions from a universe U of keys to n buckets is universal if for every pair of keys in U, say x and y, such that x != y, the probability of h(x) = h(y) is less than or equal to 1/n, for a ...
630 views

### Understanding of hash tables

I am currently studying hash tables in an introductory course to computer science. I was taught that hash table is a data structure that associates a key to an index (a hash table) and then to the ...
59 views

### Repeated fingerprinting after array updates

I've got a microprocessor and want to quickly identify the settings of my application (stored in some eeprom regions) via a fingerprint instead of having to dump the entire memory every time. So I ...
450 views

### Given a string, is it possible to determine which hashing algorithm has produced it, if any?

Given a string, is it possible to determine which hashing algorithm has produced it, if any? For example, the MD5 hash of "string" is b45cffe084dd3d20d928bee85e7b0f21. Is it possible to determine ...
Let UID denote a unique identifier. UID's are represented as 6-digit positive integers. I want to insert a collection of UID's in a hash table with $M$ buckets, where $M$ is a prime number (for ...