hash-collision

问题 My current implementation of an Hash Table is using Linear Probing and now I want to move to Quadratic Probing (and later to chaining and maybe double hashing too). I've read a few articles, tutorials, wikipedia, etc... But I still don't know exactly what I should do. Linear Probing, basically, has a step of 1 and that's easy to do. When searching, inserting or removing an element from the Hash Table, I need to calculate an hash and for that I do this: index = hash_function(key) % table_size;

问题 I need to compare large chunks of data for equality, and I need to compare many per second, fast . Every object is guaranteed to be the same size, and it is possible/likely they may only be slightly different (in unknown positions). I have seen, from the interactive session below, using == operator for byte strings can be slower if the differences are towards the end of the string, and it can be very fast if there is a difference near the start. I thought there might be some way to speed

问题 I need to compare large chunks of data for equality, and I need to compare many per second, fast . Every object is guaranteed to be the same size, and it is possible/likely they may only be slightly different (in unknown positions). I have seen, from the interactive session below, using == operator for byte strings can be slower if the differences are towards the end of the string, and it can be very fast if there is a difference near the start. I thought there might be some way to speed

问题 This probably never happened in the real-world yet, and may never happen, but let's consider this: say you have a git repository, make a commit, and get very very unlucky: one of the blobs ends up having the same SHA-1 as another that is already in your repository. Question is, how would Git handle this? Simply fail? Find a way to link the two blobs and check which one is needed according to the context? More a brain-teaser than an actual problem, but I found the issue interesting. 回答1: I did

问题 I'm hashing a large number of files, and to avoid hash collisions, I'm also storing a file's original size - that way, even if there's a hash collision, it's extremely unlikely that the file sizes will also be identical. Is this sound (a hash collision is equally likely to be of any size), or do I need another piece of information (if a collision is more likely to also be the same length as the original). Or, more generally: Is every file just as likely to produce a particular hash,

问题 Can any one give an example of 2 strings, consisting of alphabetical characters only, that will produce the same hash value with ELFHash? I need these to test my codes. But it doesn't seem like easy to produce. And to my surprise there there are a lot of example codes of various hash function on the internet but none of them provides examples of collided strings. Below is the ELF Hash, in case you need it. unsigned int ELFHash(const std::string& str) { unsigned int hash = 0; unsigned int x =

问题 I have the following hash algorithm: unsigned long specialNum=0x4E67C6A7; unsigned int ch; char inputVal[]=" AAPB2GXG"; for(int i=0;i<strlen(inputVal);i++) { ch=inputVal[i]; ch=ch+(specialNum*32); ch=ch+(specialNum/4); specialNum=bitXor(specialNum,ch); } unsigned int outputVal=specialNum; The bitXor simply does the Xor operation: int bitXor(int a,int b) { return (a & ~b) | (~a & b); } Now I want to find an Algorithm that can generate an "inputVal" when the outputVal is given.(The generated

问题 Is there more chance having collisions when betweens GUID's (128 bits) or SHA1 hashes of GUID's (160 bits) ? My opinion is there is less chance with a GUID (even if there is 32 bit less), because it has some special mechanisms to make sure it is (almost, because no guarantee) unique (ex : timestamp) Note : i already know that a GUID is very unlikely to have a collision with another GUID , no more debate about this please. 回答1: That's trivial: if two GUIDs are the same (that is, for each GUID

问题 I have two elements (6 and 747) that share their key ("eggs"). I want to find all the elements that share a key (let's say "eggs", but I would in real life do that for every key). How to do that? There must be a way to get a container or something back from the data structure . . . 回答1: You're still mistaking key's value with key's hash . But to answer question as asked: you can use unordered_map 's bucket() member function with bucket iterators: std::unordered_map<int,int,dumbest_hash> m; m

问题 I was reading the JavaDoc for Object.hashCode method, it says that As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer [...]) But whatever its implementation is, hashCode method always returns a (let's assume positive) integer, so given Integer.MAX+1 different objects, two of them are going to have the same hashcode.