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<head>
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<title>List Updates</title>
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<meta name="GENERATOR" content="Microsoft Visual Studio .NET 7.1">
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<body bgcolor = "white">
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<h1>List-Update Containers</h1>
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<p>
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This section describes list-based containers. It is organized as follows.
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</p>
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<ol>
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<li> <a href = "#overview">Overview</a> is an overview.</li>
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<li> <a href = "#list_updates">List Updates</a> describes updating lists
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as elements are accessed.</li>
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</ol>
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<h2><a name = "overview">Overview</a></h2>
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<p>
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Associative containers typically use some attributes of the keys of which they store: tree-based
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containers use the ability to compare keys; hash-based containers use the ability to map
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keys into numbers.
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</p>
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<p>
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In some cases it is better to avoid this:
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</p>
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<ol>
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<li>
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Hash-based and tree-based containers typically require additional memory
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for time efficiency.
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</li>
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<li>
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Hash-based and tree-based containers require extra information
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about keys: hash-based containers need hash functors, tree-based containers need
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comparison functors. In some (rare) cases, a key might be encapsulated to the extent that it is not possible to supply these functors.
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</li>
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</ol>
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<p>
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In such cases, storing the entries in a unique-key list is a reasonable solution.
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This uses the minimal amount of memory, and requires only an equivalence functor.
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Clearly, the order of the elements within the list affects performance; ideally, frequently accessed elements
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should be at the front of the list.
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</p>
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<p>
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Many remarkable (online competitive
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[<a href = "references.html#motwani95random">motwani95random</a>])
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algorithms exist for reordering lists to reflect access prediction
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[<a href = "references.html#andrew04mtf">andrew04mtf</a>].
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</p>
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<p>
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Figure
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<a href = "#lu_cd">List-update containers</a>
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shows the container-hierarchy; the list-based container is circled.
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</p>
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<h6 align = "center">
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<a name = "lu_cd">
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<img src = "lu_cd.jpg" width = "70%" alt = "no image">
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</h6>
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<h6 align = "center">
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</a>
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List-update containers.
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</h6>
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<p>
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The list-based container has the following declaration:
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</p>
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<pre>
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<b>template</b><
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<b>typename</b> Key,
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<b>typename</b> Data,
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<b>class</b> Eq_Fn = std::equal_to<Key>,
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<b>class</b> Update_Policy =
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<a href = "move_to_front_lu_policy.html">move_to_front_lu_policy<></a>,
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<b>class</b> Allocator =
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std::allocator<<b>char</b>> >
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<b>class</b> <a href = "lu_assoc_cntnr.html">lu_assoc_cntnr</a>;
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</pre>
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<p>
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The parameters have the following meaning:
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</p>
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<ol>
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<li> <tt>Key</tt> is the key type.
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</li>
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<li> <tt>Data</tt> is the data-policy, and is explained in
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<a href = "ms_gen.html#ds_policy">Mapping-Semantics Genericity::Data Types as a Policy</a>.
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</li>
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<li> <tt>Eq_Fn</tt> is a key equivalence functor.</li>
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<li> <tt>Update_Policy</tt> is a policy updating
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positions in the list based on access patterns. It is described in
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the following subsection.
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</li>
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<li> <tt>Allocator</tt> is (surprisingly) an allocator type.
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</li>
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</ol>
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<h2><a name = "list_updates">List Updates</a></h2>
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<p>
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This subsection describes list-update policies. It is organized as follows.
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</p>
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<ol>
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<li> <a href = "#general">General Terms</a> describes general
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terms.
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</li>
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<li> <a href = "#imp_pb_assoc">Implementation in <tt>pb_assoc</tt></a>
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describes the implementation of these concepts in <tt>pb_assoc</tt>.
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</li>
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</ol>
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<h3><a name = "general">General Terms</a></h3>
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<p>
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For example, Figure
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<a href = "#lu">-A
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The counter algorithm
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</a>
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shows the counter algorithm. Each node contains both a key and a count metadata (shown in bold).
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When an element is accessed (<i>e.g.</i> 6)
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its count is incremented, as shown in
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Figure
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<a href = "#lu">
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The counter algorithm
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</a>-B.
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If the count reaches some predetermined value, say 10, as shown in
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Figure
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<a href = "#lu">
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The counter algorithm
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</a>-C,
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the count is set to 0
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and the node is moved to the front of the list, as in
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Figure
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<a href = "#lu">
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The counter algorithm
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</a>-D.
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</p>
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<h6 align = "center">
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<a name = "lu">
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<img src = "lu_ops.jpg" width = "65%">
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</a>
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</h6>
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<h6 align = "center">
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The counter algorithm.
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</h6>
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<h3><a name = "imp_pb_assoc">Implementation in <tt>pb_assoc</tt></a></h3>
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<p>
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The <tt>pb_assoc</tt> library allows instantiating lists with policies
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implementing any algorithm moving nodes to the front of the list (policies implementing
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algorithms interchanging nodes are currently unsupported).
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</p>
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<p>
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Associative containers based on lists are parameterized by a <tt>Update_Policy</tt> parameter.
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This parameter defines the type of metadata each node contains, how to create the metadata, and how to
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decide, using this metadata, whether to move a node to the front of the list.
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A list-based associative container object derives (publicly) from its update policy.
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</p>
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<p>
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An instantiation of <tt>Update_Policy</tt> must define internally <tt>update_metadata</tt> as the metadata
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it requires. Internally, each node of the list contains, besides the usual key and data, an instance
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of <tt><b>typename</b> Update_Policy::update_metadata</tt>.
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</p>
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<p>
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An instantiation of <tt>Update_Policy</tt> must define internally two operators:
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</p>
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<pre>
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update_metadata
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<b>operator</b>()
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();
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<b>bool</b>
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<b>operator</b>()
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(update_metadata &);
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</pre>
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<p>
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The first is called by the container object, when creating a new node, to create the node's metadata. The
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second is called by the container object, when a node is accessed (<i>e.g.</i>, when a find operation's key
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is equivalent to the key of the node), to determine whether to move the node to the front of the list.
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</p>
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<p>
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Additionally, the library contains implementations of the move-to-front and counter policies. These
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are described in
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<a href="interface.html#policy_classes">Policy Classes</a>.
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</p>
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</body>
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</html>
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