hash-collision

This is more of a cryptography theory question, but is it possible that the result of a hash algorithm will ever be the same value as the source? For example, say I have a string: baf34551fecb48acc3da868eb85e1b6dac9de356 If I get the SHA1 hash on it, the result is: 4d2f72adbafddfe49a726990a1bcb8d34d3da162 In theory, is there ever a case where these two values would match? I'm not asking about SHA1 specifically here - it's just my example. I'm just wondering if hashing algorithms are built in such a way as to prevent this. Well, it would depend on the hashing algorithm - but I'd be surprised to

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, regardless of original file size? Depends on your hash function, but in general, files that are of the same size

This is basically a math problem, but very programing related: if I have 1 billion strings containing URLs, and I take the first 64 bits of the MD5 hash of each of them, what kind of collision frequency should I expect? How does the answer change if I only have 100 million URLs? It seems to me that collisions will be extremely rare, but these things tend to be confusing. Would I be better off using something other than MD5? Mind you, I'm not looking for security, just a good fast hash function. Also, native support in MySQL is nice. EDIT : not quite a duplicate If the first 64 bits of the MD5

I need a 4-character hash. At the moment I am taking the first 4 characters of a md5() hash. I am hashing a string which is 80 characters long or less. Will this lead to collision? or, what is the chance of collision, assuming I'll hash less than 65,536 (16 4 ) different elements? Surprisingly high indeed. As you can see from this graph of an approximate collision probability (formula from the wikipedia page ), with just a few hundred elements your probability of having a collision is over 50%. Note, of course, if you're facing the possibility of an attacker providing the string, you can

When there is a collision during a put in a HashMap is the map resized or is the entry added to a list in that particular bucket? When you say 'collision', do you mean the same hashcode? The hashcode is used to determine what bucket in a HashMap is to be used, and the bucket is made up of a linked list of all the entries with the same hashcode. The entries are then compared for equality (using .equals()) before being returned or booted (get/put). Note that this is the HashMap specifically (since that's the one you asked about), and with other implementations, YMMV. Either could happen - it