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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Is hash calculated before/after compression?

I had a question regarding compression and calculation of checksum/hash of data. I would like to know if checksum has to be calculated before or after the compression of data before transmission. ...
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212 views

Why a hashtable? Why not just a non-hashed associative array?

I've been learning about using a hashtable to efficiently check for items in a list without looping through the whole thing, but there's one thing that I don't get: Why hashed keys? It seems like: ...
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1answer
26 views

Hash function for progressive changes

I am looking for a hashing algorithm that works like this. I start with a text file, and compute its hash. Now I know that I will remove one character at, say, position 67 and this character is a "m", ...
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133 views

What's the shortest generating one-way hash algorithm?

I have a 24 character long id which is guaranteed to be unique. I would like to shorten that to 7-10 (or even shorter) characters long. If I generate an short id randomly then I would have to check ...
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0answers
67 views

Managing debug information of C program for Debugger

I am not sure if the title is appropriate. I have written a parser for CDB files in C# and ANTLR that creates runtime objects for me such that I can pass it to the TCF Agent which takes care of ...
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112 views

Is there a known standard data structure which is a hash table that resolves collisions using a binary tree?

Is there a known standard data structure which is a hash table that resolves collisions using a binary tree? If so what is the name of this data structure? I imagine such a structure would be useful ...
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1answer
312 views

Complexity of ArrayList of LinkedHashSet

I get input strings from the console like this: while ((currentLine = bufferedReader.readLine()) != null ) { StringTokenizer string = new StringTokenizer(currentLine, " "); while ...
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2answers
4k views

Why do so many hashed and encrypted strings end in an equals sign?

I work in C# and MSSQL and as you'd expect I store my passwords salted and hashed. When I look at the hash stored in an nvarchar column (for example the out the box aspnet membership provider). I've ...
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4answers
96 views

Can I save & store a user's submission in a way that proves that the data has not been altered, and that the timestamp is accurate?

There are many situations where the validity of the timestamp attached to a certain post (submission of information) might be invaluable for the post owner's legal usage. I'm not looking for a service ...
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2answers
45 views

Hashing + security as it pertains to a theoretical file sharing site

These days it's possible to hash a file client-side, send the hash to the server, and have the server check whether or not that file is already uploaded. If it is, we can skip the file upload and make ...
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2answers
283 views

Overriding GetHashCode in a mutable struct - What NOT to do?

I am using the XNA Framework to make a learning project. It has a Point struct which exposes an X and Y value; for the purpose of optimization, it breaks the rules for proper struct design, since its ...
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2answers
156 views

Randomized Hash function with no collisions

Related to the question Which hashing algorithm is best for uniqueness and speed? Is there a way to create a hash function, or find one, whose hash length depends completely on the input length, has ...
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3answers
251 views

what is the javascript internal data structure?

Consider a basic js object: var obj={x:1,y:'2'}; Is this stored internally as a hashtable or does js use a different mechanism for key value pairs? If they are hash tables does anyone know how they ...
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2answers
608 views

Design Hash table with simple hash function

I want to learn to Design Hash table with simple hash function for better understanding. I understand that the hash table will work as long as the hash function maps each key to a non-negative integer ...
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1answer
121 views

Why Num&sizeMinusOne faster than num&(size-1)

I've been told that when I have a hash table of size m and m=2^k, I can use the & operator as num & (size-1) instead of num % size, to fit the hashCode to my table size. I've also been told ...
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1answer
155 views

Measuring “novelty” of data

I have a heuristic in mind that should allow me to "score" data based on "novelty" that I would like to work in real-ish time. In this case, I mean novelty in the sense that the data source is ...
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2answers
105 views

How do crackers determine number of iterations of a Hashing algorithm?

Does a cracker need to know the number of iterations a hashing algorithm uses to compute a hash? If they don't know it, how do they figure it out? How much longer does it take to figure it out, than ...
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3answers
672 views

Optimal way to implement this specific lookup table in C#?

I want to create a lookup table for this data: The "input variables" (what is used to "lookup") are 4 different doubles that can each take on 1 of 200 numbers (the numbers range from 1-1000 but there ...
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2answers
191 views

How deterministic are SessionIDs from SHA'd GUIDs?

Assume I'm using the following code to generate pseudo-random sessionID's: sessionID = SHA-512(GENERATE-GUID()) The GUIDs are pretty deterministic, i.e. I see lots of GUIDs with a lot of the same ...
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2answers
278 views

Would using rainbow tables to detect weak user passwords feasible?

As I understand in most security breach where the list of hashed password are compromised, attackers do use brute-force to try to find weak password and, invariably, they always find quite some (like ...
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4answers
312 views

Looking for monotonically increasing (integer) hash function

I'm looking for a HashFunction(X,Y: Integer): Integer that is monotonically increasing on X, then Y. So: HashFunction(x1,y1) > HashFunction(x2,y2) if x1>x2 HashFunction(x,y1) > HashFunction(x,y2) if ...
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4answers
207 views

Is there a name for the concept of a “cumulative checksum”?

Insofar as I understand it, tools like Git and Mercurial derive checksums from their data, and those checksums are used to derive other checksums used in aggregate, leading to a kind of accumulative ...
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1answer
460 views

Hashing Algorithm: Deleting an element in linear probing

While using Linear probing method to implement hashing, when we delete and element, the position of the deleted element is declared as a tombstone/ mark it as deleted. Why can't we just shift all the ...
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6answers
937 views

How to assure users that website and passwords are secure

On reliable websites I always see claims such as "All data is encrypted" or "All passwords are encrypted using 128bit encryption" and etc. However I have never come across a claim such as "All ...
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2answers
764 views

Data Aggregation of CSV files java

I have k csv files (5 csv files for example), each file has m fields which produce a key and n values. I need to produce a single csv file with aggregated data. I'm looking for the most efficient ...
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1answer
477 views

Sorting versus hashing

My problem is as follows. I have an array of n strings with m < n of them distinct. I want to create a one-to-one function which assigns each of the m distinct strings to the numbers 0 ... m-1. ...
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3answers
1k views

How can I reverse engineer a hash code?

I am building an application in C# that works with a Progress database. The passwords that are stored in this database are stored using a hash algorithm that Progress has not made public. However, I ...
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4answers
274 views

Preventing password hashing algorithm from overloading CPU

These days password hashing algorithms are designed to be slow. While it prevents black hats from guessing the password (at least partially), it also gives additional work for the server. I can ...
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1answer
403 views

ruby-idiomatic hashes vs arrays

So I am still fairly new to ruby, though I have noticed that it is very hard to create 2d-array and that hashes seem to be more of the go to data structure than arrays. I was wondering why the Ruby ...
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2answers
925 views

Finding duplicate files? [duplicate]

I am going to be developing a program that detects duplicate files and I was wondering what the best/fastest method would be to do this? I am more interested in what the best hash algorithm would be ...
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1answer
309 views

How can I benchmark concurrent key-value stores?

I have some concurrent key-value store implementations that are implemented with hash tables and search trees that I would like to compare. I would like to benchmark them with a real world application ...
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1answer
168 views

Is there any design pattern to remove elements from a hash map?

In my application I keep track of the running threads via a hash map, in that way I can retrieve the correct thread and do any actions. The app evolved in a way that new threads are created and old ...
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1answer
285 views

Linear probing hashing collision resolution

So I have a quick question about the linear probing method of collision resolution in hash tables. So by definition a linear probing method would look like: while (hashTable[hash] != null) hash = ...
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1answer
354 views

Max number of items mapped to same location in hashmap

I have a question about hashmaps. If you have this hashmap with m slots, and need to map n items to it, and n > m. There will be collisions for sure. But assuming there is simple uniform hashing ...
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6answers
984 views

Would md5 hashes allow detection of synced files?

We have to develop our own file management system in Java web application. We need to sync files between our main server and client severs and find out whether all the client server has all the latest ...
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2answers
805 views

How can “hash functions” be used to implement hash maps at all?

My understandment is that hash maps allow us to link, say, a string, to certain memory location. But if every string were to be linked to a unique place in memory it would need a huge block of empty ...
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2answers
845 views

How do scalable bloom filters work?

I was reading up on scalable bloom filters and could not understand how each time a constituent bloom filters fills up, a new bloom filter with larger size is added. The elements that contributed to ...
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1answer
730 views

Hash function classification interview question

In several places on the Internet there's the interview question Classify the Hashing Functions based on the various methods by which the key value is found. with answers like Direct method ...
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4answers
303 views

Concept behind SHA-1 Checksum

What's the basis behind SHA-1 or SHA-2 or other Checksum algorithms? I read about it here http://en.wikipedia.org/wiki/SHA-1#Data_Integrity But I am still wondering about an answer in a layman's ...
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1answer
436 views

How to test if a hashing algorithm is good? [duplicate]

Possible Duplicate: Which hashing algorithm is best for uniqueness and speed? I have created a hash algorithm and would like to test if it is usable. What do I need to test and how?
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3answers
1k views

Basic memcached question

I have been reading up on distributed hashing. I learnt that consistent hashing is used for distributing the keys among cache machines. I also learnt that, a key is duplicated on mutiple caches to ...
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1answer
461 views

Hash Algorithm Randomness Visualization

I'm curious if anyone here has any idea how the images were generated as shown in this response: Which hashing algorithm is best for uniqueness and speed? Ian posted a very well-received response but ...
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2answers
382 views

Looking for a non-cryptographic hash function that returns a single character

Suppose I have a dictionary of ASCII words stored in uppercase. I also want to save those words into separate files so that the total word count of each file is approximately the same. By simply ...
3
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2answers
461 views

History of Associative Array?

In quite a lot of modern scripting languages (e.g. Perl, Python, Ruby, PHP, Lua, JavaScript), associative arrays are supported as a primitive or first-class data type (with various names like map, ...
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1answer
625 views

Directory structure (file system design)

I was looking at how file systems are designed and noticed that most places say that the directory hierarchy can be implemented using a hash table. Could someone please explain me how using a hash ...
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0answers
60 views

Built-in Context-and-Input-to-Output-Hashing in Compilers

Why doesn't (open source) compilers contain builtin funtionality for (shared) caching and reuse-fetching (using SHA1-hash of compiler-version, build-flags, target-platform and inputs) of executable ...
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1answer
250 views

hash with file instead of array

Is it possible to use hash function but with File instead of Array, and it's gonna be saving the record in a file position and then search will fseek to that position, but I'm not sure how to open a ...
5
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1answer
267 views

Using account (login) password in generated PDF?

Ever since I heard about programming, I was told that any password (especially the one used on login) should be stored in database using any kind of one-way hashing algorithm, and never ever as plain ...
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1answer
593 views

Writing-Reading a hashtable to a text file

I'm implementing a hashtable structure for a dictionary. Dictionary is in a text file. There are 2 words on each line. I'm generating the hashtable by using the first word as a key. I'm holding the ...
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7answers
1k views

What makes a hashing algorithm “secure”?

After reading this interesting question, I felt like I had a good idea of which insecure hashing algorithm I'd use if I needed one, but no idea why I might use a secure algorithm instead. So what is ...