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Peter Dimov
46 lines
2.2 KiB
Plaintext
46 lines
2.2 KiB
Plaintext
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Copyright 2005-2008 Daniel James
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Copyright 2022 Christian Mazakas
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Copyright 2022 Peter Dimov
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Distributed under the Boost Software License, Version 1.0.
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https://www.boost.org/LICENSE_1_0.txt
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////
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[#notes]
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= Design and Implementation Notes
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:idprefix: notes_
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== Quality of the Hash Function
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Many hash functions strive to have little correlation between the input and
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output values. They attempt to uniformally distribute the output values for
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very similar inputs. This hash function makes no such attempt. In fact, for
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integers, the result of the hash function is often just the input value. So
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similar but different input values will often result in similar but different
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output values. This means that it is not appropriate as a general hash
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function. For example, a hash table may discard bits from the hash function
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resulting in likely collisions, or might have poor collision resolution when
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hash values are clustered together. In such cases this hash function will
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perform poorly.
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But the standard has no such requirement for the hash function, it just
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requires that the hashes of two different values are unlikely to collide.
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Containers or algorithms designed to work with the standard hash function will
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have to be implemented to work well when the hash function's output is
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correlated to its input. Since they are paying that cost a higher quality hash
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function would be wasteful.
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For other use cases, if you do need a higher quality hash function, then
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neither the standard hash function or `boost::hash` are appropriate. There are
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several options available. One is to use a second hash on the output of this
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hash function, such as
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http://web.archive.org/web/20121102023700/http://www.concentric.net/~Ttwang/tech/inthash.htm[Thomas Wang's hash function].
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This this may not work as well as a hash algorithm tailored for the input.
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For strings there are several fast, high quality hash functions available
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(for example http://code.google.com/p/smhasher/[MurmurHash3] and
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http://code.google.com/p/cityhash/[Google's CityHash]), although they tend to
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be more machine specific. These may also be appropriate for hashing a binary
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representation of your data - providing that all equal values have an equal
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representation, which is not always the case (e.g. for floating point values).
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