usually this is implemented by tokenising (hashing pieces of) all the string input then only ever using the tokens (hashes, indices) outside of that context. In fact this sort of switch statement usage is such a good optimisation here that it is used by this perfect hash mechanism itself to solve its own problem, effectively offloading some of the cleverness requirement to the compiler.Īlso, in the context of programming languages this problem falls into the category i described of being beatable at compile-time. since these sorts of things are 4 or 8 bytes long the 'memcmp is slow' argument falls apart even if it generates a gigantic conditional statement. i'm still not sure i'd call it common, or even desirable.įor lots of chunk based formats using a switch statement on the chunk-ids is clean and gives the optimisation task to the compiler, which tends to do a pretty good job of these things. although this maybe different with XML and HTML parsers. the slow part of processing a png file is not identifying its contents. Right, that's a good use case, although i'm not sure its terribly important. but a perfect hash function isn't even applicable.Īll that being said, it is the right solution in those exceptionally rare cases where you know your data ahead of time, but need to look it up by value, rather than hash, at run-time. the usual, array of linked-list approach can be better for small data sets too. In most run-time cases a binary tree is good enough, its very rare that you need better, and even when you do the optimisation can be found by optimising that structure to pool allocate nodes or similar. which in my experience goes hand in hand with not knowing your data until run-time, which rules out a perfect hashing function. This is enormously faster than the necessarily more complicated perfect hash functions, and only falls down when you have a run-time requirement to do the lookup from the value instead of the key. I suspect this is why perfect hashes do not see the enormous widespread use, because there is a solution for the most common use cases with the exceptional run-time complexity of O(0). anything more is a failure to use the language to sufficiently inform the compiler of what is happening. In heavily compiled languages like C and C++, when you know everything at compile-time then the run-time should become zero. The special cases where you can construct perfect hashes are normally amenable to setting some fixed index 'hashes' for each item and storing them in an array, then never referring to their values for lookups in code, but always using the indices.
#Perfect hash calculator generator
With this online hash generator you can hash with many algorithms and that even several times, thus with iterations/hashing repeats.Ĥ.I still fail to see the enormous fuss about hash tables as if they are some revelatory data structure. The very popular algorithms md5 & sha1 are now being replaced by algorithms such as sha256. Therefore, it is particularly important to use a hash algorithm with greater collision security. A collision can lead to unexpected or undesirable results in programs. The stronger the algorithm, the better the so-called collisions - completely different inputs come to the same hash - can be avoided. They can compare dangerous files just by their hash/checksum without the need to send the whole file to the server.
#Perfect hash calculator password
In case of a hacking attack, the hacker can just use the password to hack also other accounts of this users, but he got only the hash. If someone gets access to the data, he gets only the hash. are often used to store passwords hashed, as a kind of 1-way-encryption.
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