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Hash: A few edits to the new rationale.

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[SVN r74963]
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Daniel James
2011-10-16 10:32:12 +00:00
parent 335930c652
commit 56293f4313
1 changed files with 11 additions and 16 deletions
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doc/rationale.qbk
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@@ -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