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# Good hash function for strings c

FNV-1 is rumoured to be a good hash function for strings. For long strings (longer than, say, about 200 characters), you can get good performance out of the MD4 hash function. As a cryptographic function, it was broken about 15 years ago, but for non cryptographic purposes, it is still very good, and surprisingly fast What is a good hash function for strings? The basic approach is to use the characters in the string to compute an integer, and then take the integer mod the size of the table; How to compute an integer from a string? You could just take the last two 16-bit chars of the string and form a 32-bit in Calculation of the hash of a string. The good and widely used way to define the hash of a string $s$ of length $n$ is \begin{align} \text{hash}(s) &= s + s \cdot p + s \cdot p^2 + + s[n-1] \cdot p^{n-1} \mod m \\ &= \sum_{i=0}^{n-1} s[i] \cdot p^i \mod m, \end{align} where $p$ and $m$ are some chosen, positive numbers Perhaps even some string hash functions are better suited for German, than for English or French words. Many software libraries give you good enough hash functions, e.g. Qt has qhash, and C++11 has std::hash in <functional>, Glib has several hash functions in C, and POCO has some hash function The function should expect a valid null-terminated string, it's responsibility of the caller to ensure correct argument. You don't need to know the string length. Check for null-terminator right in the hash loop. It's possible to write it shorter and cleaner

### Good Hash Function for Strings - ExceptionsHu

• A function that converts a given big phone number to a small practical integer value. The mapped integer value is used as an index in the hash table. In simple terms, a hash function maps a big number or string to a small integer that can be used as the index in the hash table. What is meant by Good Hash Function
• This one is quite popular and works nicely with ASCII strings. unsigned long hashstring(unsigned char *str) { unsigned long hash = 5381; int c; while (c = *str++) hash = ((hash << 5) + hash) + c; /* hash * 33 + c */ return hash; } More info here. If you need more alternatives and some perfomance measures, read here
• e some hash functions suitable for storing strings of characters. We start with a simple summation function. int h(String x, int M) { char ch[]; ch = x.toCharArray(); int xlength = x.length(); int i, sum; for (sum=0, i=0; i . x.length(); i++) sum += ch[i]; return sum % M;
• A hash function may not always lend itself to being of O(1) complexity, however in general the linear traversal through a string or byte array of data that is to be hashed is so quick and the fact that hash functions are generally used on primary keys which by definition are supposed to be much smaller associative identifiers of larger blocks of data implies that the whole operation should be.
• 5. This loads a dictionary text file into memory to be used as part of a spell checker. It's part of a larger program, but I wanted general comments so I can clean it up further. #define TABLESIZE 500 #define LENGTH 45 bool load (const char* dictionary) { //initiate hash table node* hashtable [TABLESIZE]; //open dictionary and check FILE* dict.
• Since C++11, C++ has provided a std::hash< string >( string ). That is likely to be an efficient hashing function that provides a good distribution of hash-codes for most strings. Furthermore, if you are thinking of implementing a hash-table, you should now be considering using a C++ std::unordered_map instead
• A hash function turns a key into a random-looking number, and it must always return the same number given the same key. For example, with the hash function we're going to use (64-bit FNV-1a), the hashes of the keys above are as follows

### Hash functions for strings - Computer Scienc

Answer: Hashtable is a widely used data structure to store values (i.e. keys) indexed with their hash code. Hash code is the result of the hash function and is used as the value of the index for storing a key. If two distinct keys hash to the same value the situation is called a collision and a good hash function minimizes collisions I've changed the original syntax of the hash function djib2 that OP used in the following ways: I added the function tolower to change every letter to be lowercase. This is important, because you want the words And and and (for example) in the original text to give the same hash result

general hashing function with good distribution. the actual function is hash(i) = hash(i - 1) * 65599 + str[i];what is included below is the faster version used in gawk. [there is even a faster, duff-device version] the magic constant 65599 was picked out of thin air while experimenting wit I'm trying to think of a good hash function for strings. And I think it might be a good idea, to sum up the unicode values for the first five characters in the string (assuming it has five, otherwise stop where it ends) Dr. Rob Edwards from San Diego State University demonstrates a common method of creating an integer for a string, and some of the problems you can get into Characteristics of good hashing function The hash function should generate different hash values for the similar string. The hash function is easy to understand and simple to compute. The hash function should produce the keys which will get distributed, uniformly over an array

Uniformity. A good hash function should map the expected inputs as evenly as possible over its output range. That is, every hash value in the output range should be generated with roughly the same probability.The reason for this last requirement is that the cost of hashing-based methods goes up sharply as the number of collisions—pairs of inputs that are mapped to the same hash value. A Hash Table in C/C++ (Associative array) is a data structure that maps keys to values. This uses a hash function to compute indexes for a key. Based on the Hash Table index, we can store the value at the appropriate location. If two different keys get the same index, we need to use other data structures (buckets) to account for these collisions A good hash function will try to minimize collisions as much as possible, which will imply that most of our buckets are either empty or store just a single entry. Assume we use a good hash function to index the n entries of our map in a bucket array of capacity N, we expect each bucket to be of size n/N String hashing is the way to convert a string into an integer known as a hash of that string. An ideal hashing is the one in which there are minimum chances of collision (i.e 2 different strings having the same hash). P: The value of P can be any prime number roughly equal to the number of different. A simple hash function for dictionary words is made up of addition of its ASCII values. However it is not good enough, as many words have same sum. This will result into lot of collisions. So the alternative method is to use polynomial coefficient

### String Hashing - Competitive Programming Algorithm

• No space limitation: trivial hash function with key as address.! No time limitation: trivial collision resolution = sequential search.! Limitations on both time and space: hashing (the real world) . 4 Choosing a Good Hash Function Goal: scramble the keys.! Efficiently computable.! Each table position equally likely for each key
• I Good internal state diffusion—but not too good, cf. Rogaway's Bucket Hashing. Portability For speed without total loss of portability, assume: I 64-bit registers I pipelined and superscalar I fairly cheap multiplication CityHash: Fast Hash Functions for Strings.
• Hash function (e.g., MD5 and SHA-1) are also useful for verifying the integrity of a file. Hash the file to a short string, transmit the string with the file, if the hash of the transmitted file differs from the hash value then the data was corrupted. Cuckoo hashing. Maximum load with uniform hashing is log n / log log n
• If you are a programmer, you must have heard the term hash function. In its most general form, a hash function projects a value from a set with many members to a value from a set with a fixed number of members. We basically convert the input into a different form by applying a transformation function
• ed by the data being hashed. 2) The hash function uses all the input data. 3) The hash function uniformly distributes the data across the entire set.
• Any strings set with the same prefix/suffix will trash your hash. hashval = hashval << 8; hashval += key[i]; Most popular way to calculate some simple hash for strings is something like hashval = key[i] + 31 * hashval, with possible modifications of some prime number instead of 31

### c++ - Hash function for a string - Stack Overflo

Choose a Hash Function. The first step is to choose a reasonably good hash function that has a low chance of collision. But, since this is for illustration, I will be doing the opposite! Reverse Psychology, eh? We will be working only with strings (or character arrays in C) in this article Disadvantage. Hash table has fixed size, assumes good hash function. Sybol Table: Implementations Cost Summary fix: use repeated doubling, and rehash all keys S orted ay Implementation Unsorted list lgN Get N Put N Get N / 2 /2 Put N Remove N / 2 Worst Case Average Case Remove N Separate chaining N N N 1* 1* 1* * assumes hash function is rando

Hash function. C / C++ Forums on Bytes. Hi there! I have to do a B-tree, which can use strings or ints as keys. Anyway, keys can be repeated, and I have to use a has function in order to converto from string to int.. Hash Function Efficiency. This is the measure of how efficiently the hash function produces hash values for elements within a set of data. When algorithms which contain hash functions are analyzed it is generally assumed that hash functions have a complexity of O (1), that is why look-ups for data in a hash-table are said to be on average of O. Thanks, but when I implemented your hash function it took nearly twice as long. OK, by optimize you mean speed and not collisions. Your algorithm is about as fast as it gets without having excessive collisions or doing micro optimizations Using primes for hash tables is a good idea because it minimizes clustering in the hashed table. Item (2) is nice because it is convenient for growing a hash table in the face of expanding data. Item (3) has, allegedly, been shown to yield especially good results in practice Approach: The idea is to use Map Data Structure for grouping together the strings with the same hash values. Follow the steps below to solve the problem: Initialize a Map, say mp, to map hash values with respective strings in a vector.; Traverse the given array of strings and perform the following steps: . Calculate the hash value of the current string according to the given function

character, C[n], is random-equally likely to take any value, and uncorrelated with any preceding charac- ter-then all final values of h are equally likely. If two input strings differ by a single bit, will their hash function values collide more often than by June 1990 Volume 33 Number 6 Communications of the ACM 67 Keys might be character strings, numbers, bit-arrays, or weirder things. Table sizes can be anything, including powers of 2. The hash must be faster than its predecessor. The hash must do a good job. I developed this hash function with an elaborate search mechanism that tested for a variety of weaknesses

And then it turned into making sure that the hash functions were sufficiently random. FNV-1a algorithm. The FNV1 hash comes in variants that return 32, 64, 128, 256, 512 and 1024 bit hashes. The FNV-1a algorithm is: hash = FNV_offset_basis for each octetOfData to be hashed hash = hash xor octetOfData hash = hash * FNV_prime return hash put it at the top of the list. If h is a good hash function, then our hope is that the lists will be small. One great property of hashing is that all the dictionary operations are incredibly easy to implement. To perform a lookup of a key x, simply compute the index i = h(x) and then walk down the list at A[i] until you ﬁnd it (or walk oﬀ. I offer you a new hash function for hash table lookup that is faster and more thorough than the one you are using now. I also give you a way to verify that it is more thorough. All the text in this color wasn't in the 1997 Dr Dobbs article. The code given here are all public domain. The Hash

### c - Simple string hashing algorithm implementation - Code

1. Strings. Modular hashing works for long keys such as strings, too: we simply treat them as huge integers. For example, the code below computes a modular hash function for a String s, where R is a small prime integer (Java uses 31)
2. Need for a good hash function. Let us understand the need for a good hash function. Assume that you have to store strings in the hash table by using the hashing technique {abcdef, bcdefa, cdefab , defabc }. To compute the index for storing the strings, use a hash function that states the following
3. Therefore, it's inevitable that there is a pair of non-equal strings for which a hash function produces equal values. This phenomenon is called collision. We are not going to dive into the engineering details behind hash functions, but it's clear that a good hash function should try to map uniformly the strings on which it's defined into numbers

A hash table is a randomized data structure that supports the INSERT, DELETE, and FIND operations in expected O(1) time. The core idea behind hash tables is to use a hash function that maps a large keyspace to a smaller domain of array indices, and then use constant-time array operations to store and retrieve the data.. 1. Dictionary data types. A hash table is typically used to implement a. Again, practically any good multiplier works. I think you're worrying about the fact that 31c + d doesn't cover any reasonable range of hash values if c and d are between 0 and 255. That's why, when I discovered the 33 hash function and started using it in my compressors, I started with a hash value of 5381. I think you'll find that this does. Rabin-Karp improves upon this concept by utilising the fact that comparing the hashes of two strings can be done in linear time and is far the algorithm is only as good as its hash function

### What are Hash Functions and How to choose a good Hash

Hash-then-XOR first hashes each input value, then combines all the hashes with XOR. Hash-then-XOR seems plausible, but is it a good hash function? Think about it for a moment. No, hash-then-XOR is not a good hash function! A good hash function makes it hard to find collisions, distinct inputs which produc This function returns a positive integer between 0 and NBUCKETS, which can be used as an index into the key and the value arrays. For example, hash_function(Tudor) returns 31687 and hash_function(Dumitras) returns 48160. Implement the hash function in enee140_hashtable.c. In the same file, declare the storage of the hash table as follows Hash Table Program in C. Hash Table is a data structure which stores data in an associative manner. In hash table, the data is stored in an array format where each data value has its own unique index value. Access of data becomes very fast, if we know the index of the desired data Strings are among the most common kinds of keys, so let's look at finding a hash function for strings. One idea is to get the integer values of the characters in the string and to add them up. For example, 'c' = 99, 'a' = 97 and 't' = 116, so this hash function would yield 99 + 97 + 116 = 312 for cat

### c++ - What's a good hash function for English words

1. Fowler-Noll-Vo is a non-cryptographic hash function created by Glenn Fowler, Landon Curt Noll, and Kiem-Phong Vo.. The basis of the FNV hash algorithm was taken from an idea sent as reviewer comments to the IEEE POSIX P1003.2 committee by Glenn Fowler and Phong Vo in 1991. In a subsequent ballot round, Landon Curt Noll improved on their algorithm
2. I'm trying to hash two unique strings together to create a hash. The most obvious way would be simply to concatenate the two and run a hash function on it: hash = sha256(strA + strB) But I was wondering if this is how most people do it, and if there is a standard way of doing something like this in a most secure way
3. A hash function can result in a many-to-one mapping (causing collision)(causing collision) Collision occurs when hash function maps two or more keys to same array index C lli i t b id d b t it h bCollisions cannot be avoided but its chances can be reduced using a good hash function Cpt S 223. School of EECS, WSU
4. For a hash function to be collision-free, no two strings can map to the same output hash. In other words, every input string must generate a unique output string. This type of hash function is also referred to as a cryptographic hash function
5. In general, a hash function is a function from E to 0..size-1, where E is the set of all possible keys, and size is the number of entry points in the hash table. We want this function to be uniform: it should map the expected inputs as evenly as possible over its output range. Java's implementation of hash functions for strings is a good example
6. Seems to be from ga.js (Google Analytics), and was used to hash domain name strings. It's not that good honestly. Distribution and collision resistance is very poor. Only useful for its original purpose: hashing domain name strings. For example, hash for a is 00184061 and b is 00188062. revenge is 097599b9 and revenue is 0575be19 ### Hashing Tutorial: Section 2

So we can choose a random hash function from this two staged family. And store our names, and phone numbers in the hash table, using this hash function. In conclusion, you learned how to hash integers, and strings, really good, so that probability of collision is small Hash tables can add new key-values quickly. Hash tables store data in a large array, and work by hashing the keys. A good hash should be fast, distribute keys uniformly, and be deterministic. Separate chaining and linear probing are two strategies used to deal with two keys that hash to the same index. When in doubt, use a hash table String hash function #1. This hash function adds up the integer values of the chars in the string (then need to take the result mod the size of the table): This function is simple to compute, but it often doesn't work very well in practice: Suppose the keys are strings of 8 ASCII capital letters and spaces Writing A Good Hash Function. If you want to create a good hash function or mechanism, you need to understand the basic requirements of creating one. Let's list them below: The hash function needs to be easy to compute. That means that it shouldn't take many resources to execute. The hash function needs to be uniformly distributed C Hash Table Source code for a hash table data structure in C. This code is made available under the terms of the new BSD license. If you use this code, drop me an email. It's nice to feel useful occasionally. I promise not to sell your email address to Nigerian spam bandits. Thanks. Christopher Clark (firstname.lastname @ cl.cam.ac.uk.

### General Purpose Hash Function Algorithms - By Arash Parto

In practice, we would first compare the hash codes of the two strings. That quickly detects almost all different strings — it wouldn't be a very good hash function if it didn't. But when the two hashes are the same, we still have to compare characters to make sure we didn't have a hash collision on different strings This also adds a new API json_global_set_string_hash() which permits to select the hash function. The default one is the only one that was previously present. So there are no changes to existing ap..

### c - Dictionary load function using hash table - Code

Hash functions for hash table lookup. A hash function for hash table lookup should be fast, and it should cause as few collisions as possible. If you know the keys you will be hashing before you choose the hash function, it is possible to get zero collisions -- this is called perfect hashing.Otherwise, the best you can do is to map an equal number of keys to each possible hash value and make. For completeness the function Ada.Strings.Fixed.Hash is a renaming of Ada.Strings.Hash. These are provided because it is often the case that the key is a string and they save the user from devising good hash functions for strings which might cause a nasty headache Abstract. A good cryptographic hash function should behave like a random oracle: it should Note that as opposed to most hash functions, a sponge function generates inﬁnite output strings like a random oracle. This makes it suited to also serve as reference for stream ciphers and so-called mask generation functions [13,8] 1. Pretend hash function is really this good 2. Design a secure cryptosystem using it Prove security relative to a random oracle 3. Replace oracle with a hash function Hope that it remains secure Very successful paradigm, many schemes - E.g., OAEP encryption, FDH,PSS signatures Also all the examples from before

Hash tables! They're everywhere. They're also pretty boring, but I've had GLib issue #1198 sitting around for a while, and the GNOME move to GitLab resulted in a helpful reminder (or two) being sent out that convinced me to look into it again with an eye towards improving GHashTable and maybe answering some domain-typical questions, like You're using approach X, but I've heard. Hash faster to find Hash can insert easily Comparison with Tree Binary Search Hash generally faster when not too full (except for small tables) Hash disadvantages: Tree easily finds next larger and next smaller Tree easily traversed in order (hash unordered Converts a hash table to a list of key/value pairs. Hash functions This implementation of hash tables uses the low-order n bits of the hash value for a key, where n varies as the hash table grows. A good hash function therefore will give an even distribution regardless of n.. If your keyspace is integrals such that the low-order bits between keys are highly variable, then you could get away.  Keep in mind that hash tables can be used to store data of all types, but for now, let's consider a very simple hash function for strings. This hash function uses the first letter of a string to determine a hash table index for that string, so words that start with the letter 'a' are assigned to index 0, 'b' to index 1, and so on TIL the current hash function for Java strings is of unknown author. In 2004 Joshua Bloch went so far as to call up Dennis Ritchie, who said that he did not know where the hash function came from. He walked across the hall and asked Brian Kernighan, who also had no recollection. [x-post /r/java Our hash method needs to take our key, which will be a string of any length, and produce an index for our internal buckets array. We will be creating a hash function to convert the string to an index. There are many properties of a good hash function, but for our purposes the most important characteristic for our function to have is uniformity Hash Function. Hash Function is used to index the original value or key and then used later each time the data associated with the value or key is to be retrieved. Thus, hashing is always a one-way operation. There is no need to reverse engineer the hash function by analyzing the hashed values. Characteristics of Good Hash Function 0 1 2 abcde f bcdefa cdefab defabc 3 4----Here, it will take O(n) time (where n is the number of strings) to access a specific string. This shows that the hash function is not a good hash function. Let's try a different hash function. The index for a specific string will be equal to sum of ASCII values of characters multiplied by their respective order in the string after which it is modulo. ### Have a good hash function for a C++ hash table

Finding a good hash Function It is difficult to find a perfect hash function, that is a function that has no collisions. But we can do better by using hash functions as follows. Suppose we need to store a dictionary in a hash table. A dictionary is a set of Strings and we can define a hash function as follows What is a good hash function Ideally the hash function satisfies the assumption from SC 3050 at New York Universit By definition, if a function has good avalanche behavior, a minute change almost always gives a grossly different result. If one needs K different hash functions, and starts with a good algorithm, say, Spooky, is it sufficient, in your view, to implement this by simply concatenating a different byte onto each of K instances of the input Update December 6, 2011: To speed up Debug mode, the downloadable fnv.h is slightly different (fnv1a is explicitly inlined for C-style strings). Unrolling The Inner Loop Often it's a good idea to (partially) unroll the most inner loop. I couldn't observe a significant speed-up when I wrote an unrolled version of the FNV1a hash A good hash function requires avalanching from all input bits to all the output bits. (Incidentally, Bob Jenkins overly chastizes CRCs for their lack of avalanching -- CRCs are not supposed to be truncated to fewer bits as other more general hash functions are; you are supposed to construct a custom CRC for each number of bits you require.

A hash function takes an item of a given type and generates an integer hash value within a given range. The input items can be anything: strings, compiled shader programs, files, even directories. The same input always generates the same hash value, and a good hash function tends to generate different hash values when given different inputs hash function for string . I'm working on hash table in C language and I'm testing hash function for string. The first function I've tried is to add ascii code and use modulo(%100) but i've got poor results with the first tes Now the need of a good hash function is surely a concern because collision can be bad issue in case of a hash function that is not that diverse and robust. So sometimes it is also required we build some user defined hash function so that in collision cases we can handle it properly. That was almost everything about std::hash. Hope you liked it Passing Strings in C Jul 20, 2019 C, Strings David Egan. If you have allocated storage for the string outside the function, which you cannot exceed within the function, it's probably a good idea to pass in the size. In C, function arguments are passed by value Hey all, I need a good hash function (for strings) that will produce the same hash value for a specific string on different platforms, My app invovles a server farm, and a bank server, Each server produces hash keys for strings and the bank stores the keys

### How to implement a hash table (in C) - Ben Hoy

Characteristics of good hash function and collision resolution technique are also prescribed in this article. Submitted by Abhishek Kataria, on June 21, 2018 Hashing. There are many possibilities for representing the dictionary and one of the best methods for representing is hashing Using a secondary hash function is like a safe guard. When users choose good hash functions, this secondary function only wastes time, a little bit. Caching hash values. When we use long strings as keys, comparing two keys may take significant time. This comparison is often unnecessary. Note that the hash of a string is a good summary of the.

### Hash Function for String data in C# - GeeksforGeek

Note that the converse of this statement is not always true, but a good hash function tries to reduce the number of such hash collisions. Rabin-Karp computes hash value of the pattern, and then goes through the string computing hash values of all of its substrings and checking if the pattern's hash value is equal to the substring hash value, and advancing by 1 character every time Important considerations in designing a hash function for use with a hash table • It is fast to compute (must be O(1)) • It distributes keys evenly • It is consistent with the equality testing function (i.e. two keys that are equal will have the same hash value) Designing a good hash function is not easy However, if we are careful, we can design the functions in such a way that is a good hash function. The hashing methods discussed in the preceding section deal with integer-valued keys. But this is precisely the domain of the function g. Consequently, we have already examined several different alternatives for the function g

Running example: design a hash function that maps strings to 32-bit integers [ -2147483648, 2147483647] A good hash function exhibits the following properties: Deterministic: same input should generate the same output Efficient: should take a reasonable amount of time Uniform: should spread inputs evenly over its output range A good hash function minimizes the possibility of collisions; a Computers 8 Security Vol. 17, No.2, pp. 171- 174, 1998 hash function is said to be collision resistant if it is hard to find two input strings that map to the same hash value.The problem of constructing fast hash functions that also have low collision rates is studied in [S] Take 'hash_function_b' that will give '1' and '2' the hash '3'. If I were to use it as a 'secondary hash' after 'hash_function_a' then even if the password is 'a' I could use 'b', 'c' or 'd'. On top of all of that, I get that salts should be used, but they don't really change the fact that each time we are mapping 'x' inputs to 'less than x.

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