Questions tagged [hashing]

A hash function is any algorithm that maps data of arbitrary length to data of a fixed length. The values returned by a hash function are called hash values, hash codes, hash sums, checksums or simply hashes. From Wikipedia: http://en.wikipedia.org/wiki/Hash_function

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74 views

Using PostgreSQL MD5 hash to calculate a 64 bit hash value for advisory lock functions?

I have a particular problem addressing the PostgreSQL advisory locking functions using the bigint variants. Basically I want to create a 64 bit bigint value from a text type obtained with the ...
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3answers
18k views

Where do “magic” hashing constants like 0x9e3779b9 and 0x9e3779b1 come from?

In code dealing with hash tables, I often find the constant 0x9e3779b9 or sometimes 0x9e3779b1. For example hash = n * 0x9e3779b1 >>> 24 Why is this particular value used?
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87 views

Hashing from within the database or outside?

I am in the process of evolving architecture for an 'equity analysis system'. I will be using SQLServer as the database, but I am not going the .NET route and won't be using the built-in 'membership' ...
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1answer
119 views

Hashset vs Treeset

I've always loved trees, that nice O(n*log(n)) and the tidiness of them. However, every software engineer I've ever known has asked me pointedly why I would use a TreeSet. From a CS background, I don'...
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Searching/Hashing Wikipedia - at each position for keys 1..31 long, the fastest way?

A C coder here. My wish is to find the best way to search words and phrases within the ever-growing English Wikipedia XML dump. As a first layer/step, hashing is the answer, yes? The problem is that ...
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1answer
91 views

Client Side Hashing + Server Side Hashing

Suppose one wishes to implement an account system for their service/website. There have been cases with Instagram for example where passwords were accidentally stored in plain text due to ...
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2answers
283 views

What are some disadvantages for a hashmap with timestamp as key? [closed]

"What are some problems that can arise from using a HashMap with timestamp as keys"? I, for one, do not see any major issues with this kind of "setup". Is this "setup" a no-go/an anti-pattern/...
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1answer
39 views

portable hashable string representation of data object

I have a class of (flat) objects that are going to be passed around between three different parties, each possibly running a different software stack. my_object: item: "string_a" money: ...
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6answers
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How to implement float hashing with approximate equality

Let's say we have the following Python class (the problem exists in Java just the same with equals and hashCode) class Temperature: def __init__(self, degrees): self.degrees = degrees ...
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1answer
84 views

Distribute jobs evenly among a set of servers

I have the need to distribute a set of long running jobs across a cluster of containers in ECS. These jobs essentially would need to open a socket connection with a remote server and begin streaming ...
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1answer
257 views

Explanation of how to resolve Hash conflicts in HAMT or hashtables in general

I am working on trying to understand HAMT and now am uncertain generally what to do when you run into a conflict in a hash. All I understand so far is you create a list to append the keys to, but I ...
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3answers
164 views

Is there a secure way to check previous passwords purely on the client-side?

We have a requirement for a security audit that our password policy must disallow the re-use of a previous password from the last 4 used passwords. We can accomplish this fairly easily by making a ...
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5answers
825 views

If passwords are stored hashed, how would a computer know that your password is similar to the last one if you try resetting your password?

If passwords are stored hashed, how would a computer know that your password is similar to the last one if you try resetting your password? Wouldn't the two passwords be totally different since one is ...
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0answers
265 views

LSH for a wordlist

I am trying to write a spellchecker, I have a huge wordlist (at least 500K, because of the nature of the language). The performance would suffer a lot if I got the lavenshtein distance of all of the ...
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1answer
385 views

Where and how to handle user password hashing in Clean Architecture?

I'm currently building a new application and trying to apply some of the principles of Clean Architecture. One of my first roadblocks is implementing my own Identity system (to avoid being tightly ...
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1answer
191 views

A collision-free hash-like function for use in hash tables and other data structures?

A short introduction to the problem: I'm working with a small database where I have a table of strings (web URLs, to be precise) as pairs: hash|string. Another table references these strings by hash ...
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1answer
55 views

Hashing tag list to perform faster search

I have a list of items. Each item have various properties. Each item can be tagged, like tagging a post here. Right now, tags are represented using a Tags property, that is a string and it looks like ...
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174 views

Hashing algorithm without the avalanche effect

I know that one of the most important features of hashing algorithms, is that minor changes in the input should reflect great change in the hash itself (the avalanche effect), but I'm currently ...
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1answer
356 views

Logic Behind a Basic Hash Table Hashing Algorithm

I am trying to write a Hash Table in Java based on some Princeton article about it. The most commonly used method for hashing integers is called modular hashing: we choose the array size M to be ...
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3answers
2k views

What is an example for a one-way hash function?

I am doing a bit of research on hash functions. I understand the concept that it is an equation that is easy to do one way (you take the number 00011010 for example and do reasonably simple math with ...
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1answer
238 views

What is the SLOWEST 32 hashing function without excessive collisions

I am currently using a CRC32 hash to generate a locality sensitive hash for some of my data. For much the same reasons as PKBDF2 makes a better choice than, say, SHA26, for hashing passwords I ...
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4answers
1k views

Why can't Dictionary(TKey, TValue) update it's hashcodes?

I've read that an object's hashcode should not change while it is part of a Dictionary. Because if it changed, there would be no way to find it in the Dictionary. Why doesn't the Dictionary class ...
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1answer
452 views

Reverse engineering a checksum or CRC

I am trying to reverse engineer a checksum or CRC wherein an 8 bit number* gets converted to a 5 bit number for error checking. I have an incomplete list of data values and checksums, and need to ...
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2answers
78 views

Hashable classes: Should I only keep the id's for results or entire class

Given 2 objects / classes: A class for a cargo ship. It has a small set of attributes, say current location, size, number of crew and some other things. Most important, it has an attribute for the id ...
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1answer
181 views

How does Steam resume downloads without computing a partial hash?

The Steam Client application allows one to download games purchased through the associated Steam account. A typical modern game uses quite a bit of disk space (in the order of 50-70 GB), so it is not ...
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3answers
248 views

What can I infer from Hilbert's map?

How do I understand Hilbert's map in this answer? The author of the answer has mapped the hash value (integer) of some strings to a 2D image. I understand that Hilbert's curve is a technique for 1D &...
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2answers
548 views

Map/store run time ID for objects

What I want to achieve: I need to obtain a run time (unique) id for "any object" (for classes either in java SE, classes I have created or classes provided by other third parties... any instance of a ...
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6answers
2k views

Is there an “anti-hash” or “similarity hash” / similarity measure algorithm?

The idea of hashes is they get drastically different results for even the smallest change of data. What I am asking for is the opposite of that. An algorithm that will produce proximity hash values ...
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0answers
189 views

Data structure for fast bi-directional SHA-1 to SHA-3 mapping

What data structure would be best choice for fast and performant bidirectional SHA-1 to SHA-3 mapping? Or in other words fixed-length string to fixed-length string mapping? To be more precise: ...
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3answers
488 views

algorithm for perfect substring temporarily replacement

I have an interesting question about string and delimiters. There is a random string str. There are some symbols (OR SEQUENCES OF SYMBOLS) that are predefined and should be temporarily replaced ...
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2answers
286 views

Why do we need datastructures other than HashMap

Map (or HashMap) takes Constant time for Insert, Remove and Retrieve. While all other data structures which I know so far, do not take constant time and their time for above operations depends on the ...
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1answer
271 views

When should a general purpose hash table assume hash equality implies logical equality?

For a general purpose hash table that aims for both high performance and correctness, when, if ever, does it make sense to assume that hash equality implies logical equality? To lay some ground rules ...
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1answer
2k views

If I am using just HashMap can I override only hashCode method?

If I need to use only HashMap, why do I need to override equals() method along with hashCode()? How will not implementing equals() affect the working of HashMap?
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2answers
264 views

Which is the best hash algorithm considering uniqueness and slowness

I couldn't find an answer, so let's try here... I thought a bit about security and came to the conclusion, that the used hash algorithm should be as slow as possible, to slow potential attackers. (...
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3answers
2k views

Is it possible to implement a well-distributed hash table without using the % operator?

I'm looking to implement a fast, well-distributed hash table in C#. I'm having trouble choosing my hash-constraining function that takes an arbitrary hash code and "constrains" it so it can be used to ...
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2answers
2k views

Best way to hash passwords in PHP?

I've switched to PHP 7.0 very recently and I was wondering that if password_hash was better than making your own salt and using the crypt function. I have three examples of code and I don't know which ...
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1answer
384 views

Secure communication between a mobile app and a microcontroller

I have a MCU-based design that essentially controls other hardware and this MCU is controlled through an app. I was hoping someone could help me figure out how to secure this communication between the ...
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1answer
209 views

Reproducible uniqueness

So, I have a bunch of calls that are all being generated with a UUID1 throughout the day. At the end of the call the call is processed and some metrics around that call are generated and stored in ...
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2answers
255 views

Does it make sense to check multiple hashes when detecting a file duplicate?

Please forgive me if this question belongs on different StackExchange site. A much more experienced colleague (like, 8 times more experienced than me) has vigorously defended an idea that you should ...
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2answers
119 views

How can I tell if a web sign up form actually encrypts my password?

Recently I signed up for new account for a service provider's web site. I entered the usual the name, email , password (strong) , repeat password and I was signed up. Great everything works well. A ...
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Consistent hash ring with subsets in different buckets

I have N buckets. I have M users. Each user can buy anywhere between 1 and N tickets to an upcoming lottery. I would like to distribute the tickets more or less evenly between the buckets, with the ...
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1answer
684 views

How does Pearson hashing compare with other non-cryptographic hashing algorithms?

FNV-1, Murmur2, and DJB2 are examples of non-cryptographic hashing functions used in actual applications (see Which hashing algorithm is best for uniqueness and speed?). These are all similar in that ...
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1answer
60 views

Hash system to generate same (specified) hash from multiple datasets?

I have a hashed value X and need to ensure that any of the data sets D1...Dn can be concatenated with a correctional value C1...Cn and passed into a hashing algorithm to result in the value of X. ...
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1answer
120 views

Efficient irreversible “serialization”?

Suppose we have a function f that gets a complex (non-flat) object as an input, serializes it, calculates hash of serialization results (e.g. md5) and returns calculated hash. It may use some exiting ...
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1answer
389 views

Hashing a tuple of maximum-valued integers

I'm considering a hash function for a tuple of and maximum-valued integers for example (3 ,2, 5 ) -- where each element of the tuple has some maximum value N. My plan is to treat the elements of the ...
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15 views

Partition stream of data between n servers, with m duplicates

I'm partitioning a stream of input data between n servers. The simple way of doing it is to use the hash of some property of each input data packet to assign a single server, using mod or similar, and ...
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1answer
1k views

What kind of members should be used in a GetHashCode() implementation

We're building some Roslyn analyzers concerning GetHashCode(), including an analyzer that implements it for you in a given class. While researching the subject we've found that there are many ...
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2answers
110 views

Does sql use hastables for indexes? [closed]

Are there other ways of indexing?, Which are the more used? Does sql have an standard for indexes, it uses hash tables?
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1answer
201 views

Does murmur3 hash have memory?

This implementation of Murmur hash yields different results when called twice in a row: Murmur3 m3 = new Murmur3(); var la = m3.ComputeHash(new byte[] { 1, 2 }); var lb = m3.ComputeHash(new byte[] { ...
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3answers
719 views

How do I generate multiple hashes that can be resolved to a single value?

I need multiple "random" URLs that just mask the original URL, i.e.: /ABC /CBA /CAB All resolve to /6 I came up with matching a letter to a number and then you can sum them to the single value. A =...