diff --git a/hash/doc/rationale.qbk b/hash/doc/rationale.qbk index 8323dfa..f01605b 100644 --- a/hash/doc/rationale.qbk +++ b/hash/doc/rationale.qbk @@ -14,8 +14,8 @@ Many hash functions strive to have little correlation between the input and output values. They attempt to uniformally distribute the output values for very similar inputs. This hash function makes no such attempt. In fact, for integers, the result of the hash function is often -just the input value. So similar but different input values will result -in similar but different output values. +just the input value. So similar but different input values will often +result in similar but different output values. This means that it is not appropriate as a general hash function. For example, a hash table may discard bits from the hash function resulting @@ -25,28 +25,23 @@ preform poorly. So why not implement a higher quality hash function? Well, the standard makes no such guarantee, it just requires that the hashes of two -different values are unlikely to collide. So containers or algorithms +different values are unlikely to collide. Containers or algorithms designed to work with the standard hash function will have to be implemented to work well when the hash function's output is correlated -to its input. Since they are paying that cost it would be wasteful to -expand the effort to make a higher quality hash function. +to its input. Since they are paying that cost a higher quality hash function +would be wasteful. -If you do need a higher quality hash function, there are several options +For other use cases, if you do need a higher quality hash function, +there are several options available. One is to use a second hash on the output of this hash function, such as [@http://www.concentric.net/~ttwang/tech/inthash.htm -Thomas Wang's hash function]. But for many types this might not work as +Thomas Wang's hash function]. This this may not work as well as a hash algorithm tailored for the input. For strings that are several fast, high quality hash functions -available, such as: - -* [@http://burtleburtle.net/bob/hash/index.html Bob Jenkins' hash - functions] -* [@http://www.azillionmonkeys.com/qed/hash.html Paul Hsieh's hash - functions] -* [@http://code.google.com/p/cityhash/ Google's CityHash] -* [@http://code.google.com/p/smhasher/ MurmurHash3] - +available (for example [@http://code.google.com/p/smhasher/ MurmurHash3] +and [@http://code.google.com/p/cityhash/ Google's CityHash]), +although they tend to be more machine specific. These may also be appropriate for hashing a binary representation of your data - providing that all equal values have an equal representation, which is not always the case (e.g. for floating point