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35 lines
1.6 KiB
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35 lines
1.6 KiB
Plaintext
[def __tr1__ [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1745.pdf
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C++ Standard Library Technical Report]]
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[def __hash-function__ [@http://en.wikipedia.org/wiki/Hash_function
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hash function]]
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[section:intro Introduction]
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For accessing data based on keys, the C++ standard library offers `std::set`,
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`std::map`, `std::multiset` and `std::multimap`. These are generally
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implemented using balanced binary trees so accessing data by key is
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consistently of logarithmic complexity. Which is generally okay, but not great.
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Also, they require their elements to be ordered, and to supply a 'less than'
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comparison object. For some data types this is impractacle, It might be slow
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to calculate, or even impossible.
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So the __tr1__ provides unordered associative containers. These will store data
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with no ordering, and typically allow for fast constant time access. Their
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worst case complexity is linear, but this is generally rare, and with
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care can be avoided. There are four containers to match the existing
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associate containers:
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[classref boost::unordered_set unordered_set],
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[classref boost::unordered_map unordered_map],
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[classref boost::unordered_multiset unordered_multiset] and
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[classref boost::unordered_multimap unordered_multimap].
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The fast lookup speeds are acheived using a __hash-function__. The basic idea
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is that a function is called for the key value, which is used to index the
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data. If this hash function is carefully chosen the different keys will usually
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get different indicies, so there will only be a very small number of keys for
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each index, so a key lookup won't have to look at many keys to find the correct
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one.
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[endsect]
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