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<!-- %A  dict.xml                  GAP documentation               Gene Cooperman -->
<!-- %A							         Scott Murray -->
<!-- %A	        					     Alexander Hulpke -->
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<!-- %Y  (C) 2000 School Math and Comp. Sci., University of St Andrews, Scotland -->
<!-- %Y  Copyright (C) 2002 The GAP Group -->
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<Chapter Label="Dictionaries and General Hash Tables">
<Heading>Dictionaries and General Hash Tables</Heading>

People and computers spend a large amount of time with searching.
Dictionaries are an abstract data structure which facilitates searching for
objects. Depending on the kind of objects the implementation will use a
variety of possible internal storage methods that will aim to provide the
fastest possible access to objects. These internal methods include
<P/>
<List>
<Mark>Hash Tables</Mark>
<Item>
  for objects for which a hash function has been defined.
</Item>
<Mark>Direct Indexing</Mark>
<Item>
  if the domain is small and cheaply enumerable
</Item>
<Mark>Sorted Lists</Mark>
<Item>
  if a total order can be computed easily
</Item>
<Mark>Plain lists</Mark>
<Item>
  for objects for which nothing but an equality test is available.
</Item>
</List>


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<Section Label="Using Dictionaries">
<Heading>Using Dictionaries</Heading>

The standard way to use dictionaries is to first create a dictionary (using
<Ref Func="NewDictionary"/>, and then to store objects (and associated information) in
it and look them up.
<P/>
For the creation of objects the user has to make a few choices: Is the
dictionary only to be used to check whether objects are known already, or
whether associated information is to be stored with the objects. This second
case is called a <E>lookup dictionary</E> and is selected by the second parameter
of <Ref Func="NewDictionary"/>.
<P/>
The second choice is to indicate which kind of objects are to be stored. This
choice will decide the internal storage used. This kind of objects is
specified by the first parameter to <Ref Func="NewDictionary"/>, which is a <Q>sample</Q>
object.
<P/>
In some cases however such a sample object is not specific enough. For
example when storing vectors over a finite field, it would not be clear
whether all vectors will be over a prime field or over a field extension.
Such an issue can be resolved by indicating in an (optional)  third
parameter to
<Ref Func="NewDictionary"/> a <E>domain</E> which has to be a collection that will
contain all objects to be used with this dictionary. (Such a domain may also
be used internally to decide that direct indexing can be used).
<P/>
The reason for this choice of giving two parameters is that in some cases no
suitable collection of objects has been defined in &GAP; - for example for
permutations there is no object representing the symmetric group on
infinitely many points.
<P/>
Once a dictionary has been created, it is possible to use
<Ref Oper="RepresentationsOfObject"/> to check which representation is used by &GAP;.
<P/>
In the following example, we create a dictionary to store permutations with
associated information.
<Example><![CDATA[
gap> d:=NewDictionary((1,2,3),true);;
gap> AddDictionary(d,(1,2),1);
gap> AddDictionary(d,(5,6),9);
gap> AddDictionary(d,(4,7),2);
gap> LookupDictionary(d,(5,6));
9
gap> LookupDictionary(d,(5,7));
fail
]]></Example>
A typical example of this use would be in an orbit algorithm. The dictionary
would be used to store the elements known in the orbit together with their
respective orbit positions.
<P/>
We observe that this dictionary is stored internally by a sorted list. On
the other hand, if we have an explicit, sorted element list, direct indexing
is to be used.
<Example><![CDATA[
gap> RepresentationsOfObject(d);
[ "IsComponentObjectRep", "IsNonAtomicComponentObjectRep",
  "IsDictionaryDefaultRep", "IsListDictionary",
  "IsListLookupDictionary", "IsSortDictionary",
  "IsSortLookupDictionary" ]
gap> d:=NewDictionary((1,2,3),true,Elements(SymmetricGroup(5)));;
gap> RepresentationsOfObject(d);
[ "IsComponentObjectRep", "IsNonAtomicComponentObjectRep",
  "IsDictionaryDefaultRep", "IsPositionDictionary",
  "IsPositionLookupDictionary" ]
]]></Example>
(Just indicating <C>SymmetricGroup(5)</C> as a third parameter would still keep
the first storage method, as indexing would be too expensive if no explicit
element list is known.)
<P/>
The same effect happens in the following example, in which we work with
vectors: Indicating only a vector only enables sorted index, as it cannot be
known whether all vectors will be defined over the prime field. On the other
hand, providing the vector space (and thus limiting the domain) enables
the use of hashing (which will be faster).
<Example><![CDATA[
gap> v:=GF(2)^7;;
gap> d:=NewDictionary(Zero(v),true);;
gap> RepresentationsOfObject(d);
[ "IsComponentObjectRep", "IsNonAtomicComponentObjectRep",
  "IsDictionaryDefaultRep", "IsListDictionary",
  "IsListLookupDictionary", "IsSortDictionary",
  "IsSortLookupDictionary" ]
gap> d:=NewDictionary(Zero(v),true,v);;
gap> RepresentationsOfObject(d);
[ "IsComponentObjectRep", "IsNonAtomicComponentObjectRep",
  "IsDictionaryDefaultRep", "IsPositionDictionary",
  "IsPositionLookupDictionary" ]
]]></Example>

</Section>


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<Section Label="Dictionaries">
<Heading>Dictionaries</Heading>

This section contains the formal declarations for dictionaries. For
information on how to use them, please refer to the previous
section&nbsp;<Ref Sect="Using Dictionaries"/>.

<#Include Label="[1]{dict}">
<#Include Label="[2]{dict}">
<#Include Label="NewDictionary">

</Section>


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<Section Label="Dictionaries via Binary Lists">
<Heading>Dictionaries via Binary Lists</Heading>

As there are situations where the approach via binary lists is explicitly
desired, such dictionaries can be created deliberately.

<#Include Label="DictionaryByPosition">
<#Include Label="IsDictionary">
<#Include Label="IsLookupDictionary">
<#Include Label="AddDictionary">
<#Include Label="KnowsDictionary">
<#Include Label="LookupDictionary">

</Section>


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<Section Label="General Hash Tables">
<Heading>General Hash Tables</Heading>

These sections describe some particularities for hash tables. These are
intended mainly for extending the implementation - programs requiring hash
functionality ought to use the dictionary interface described above.
<P/>
We hash by keys and also store a value.  Keys
cannot be removed from the table, but the corresponding value can be
changed.  Fast access to last hash index allows you to efficiently store
more than one array of values &ndash;this facility should be used with care.
<P/>
This code works for any kind of object, provided you have a
<Ref Oper="DenseIntKey"/> method to convert the key into a positive integer.
This method should ideally be implemented efficiently in the core.
<P/>Note that, for efficiency, it is currently impossible to create a
hash table with non-positive integers.

</Section>


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<Section Label="Hash keys">
<Heading>Hash keys</Heading>

The crucial step of hashing is to transform key objects into integers such
that equal objects produce the same integer.
<P/>
The actual function used will vary very much on the type of objects. However
&GAP; provides already key functions for some commonly encountered objects.

<#Include Label="DenseIntKey">
<#Include Label="SparseIntKey">

</Section>


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<Section Label="Dense hash tables">
<Heading>Dense hash tables</Heading>

Dense hash tables are used for hashing dense sets without collisions,
in particular integers.
Keys are stored as an unordered list and values as an
array with holes. The position of a value is given by
the function returned by <Ref Oper="DenseIntKey"/>,
and so <C>KeyIntDense</C> must be one-to-one.

<#Include Label="DenseHashTable">

</Section>


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<Section Label="Sparse hash tables">
<Heading>Sparse hash tables</Heading>

Sparse hash tables are used for hashing sparse sets.
Stores keys as an array with fail
denoting an empty position, stores values as an array with holes.
Uses
the result of calling <Ref Oper="SparseIntKey"/>) of the key.
<C>DefaultHashLength</C>
is the default starting hash table length; the table is doubled
when it becomes half full.
<P/>
In sparse hash tables, the integer obtained from the hash key is then
transformed to an index position by taking it modulo the length of the hash
array.

<#Include Label="SparseHashTable">
<#Include Label="DoubleHashArraySize">

</Section>
</Chapter>

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