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  <title>Data structures</title>

  <para>There is lots of data that either flows through various HelenOS
  subsystems or is stored directly by them. Each subsystem uses its own data
  structures to represent the data. These data structures need to be kept
  somewhere. In order to work efficiently, HelenOS, and especially its kernel,
  deploys several house keeping data types that are designed to faciliate
  managing other data structures. Most of them serve like generic
  containers.</para>

  <section>
    <title>Lists</title>

    <para>HelenOS uses doubly-circularly-linked lists to bind related data
    together. Lists are composed of an independent sentinel node called head
    and links that are always part of the object that is to be put into the
    list. Adding items to a list thus doesn't require any further memory
    allocations. Head and each link then contains forward and backward
    pointer. An empty list is composed of a sole head whose both pointers
    reference the head itself. The expense of two times bigger memory
    consumption as compared to memory consumption of singly linked lists is
    justified by constant insertion and removal times at random positions
    within the list.</para>

    <para>Lists are frequently used to implement FIFO behaviour (e.g.
    scheduler run queues or synchronization wait queues). Contrary to the FIFO
    type, which is also supported by HelenOS, they don't take up any unused
    space and are more general. On the other hand, they are slower than
    in-array FIFOs and can be hardly used to implement buffers.</para>
  </section>

  <section>
    <title>FIFO queues</title>

    <para>FIFO queues are implemented as either statically or dynamically
    allocated arrays<footnote>
        <para>Depending on the array size.</para>
      </footnote> of some generic type with two indices. The first index
    points to the head of the FIFO queue and the other points to the tail
    thereof. There can be as many items in the FIFO as is the number of
    elements in the array and no more. The indices are taken modulo size of
    the queue because as a consequence of insertions and deletions, the tail
    can have numericaly lower index than the head.</para>

    <para>FIFO queues are used, for example, in ASID management code to store
    inactive ASIDs or in userspace keyboard driver to buffer read
    characters.</para>

    <figure>
    <mediaobject id="fifo" xreflabel="">
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        <imagedata fileref="images/fifo.png" format="PNG" />
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    <imageobject role="fop">
        <imagedata fileref="images.vector/fifo.svg" format="SVG" />
    </imageobject>
                    
    </mediaobject>
    <title>FIFO queue showing the wrap around the end of the array.</title>
    </figure>

  </section>

  <section>
    <title>Hash tables</title>

    <para>The kernel, as well as userspace, provides hash table data type
    which uses separate chaining. The hash table type is very generic in that
    it forces the user to supply methods for computing the hash index,
    comparing items against a set of keys and the item removal callback
    function. Besides these virtual operations, the hash table is composed of
    a dynamically allocated array of list heads that represent each chain,
    number of chains and the maximal number of keys.</para>
  </section>

  <section>
    <title>Bitmaps</title>

    <para>Several bitmap operations such as clearing or setting consecutive
    bit sequences as well as copying portions of one bitmap into another one
    are supported.</para>
  </section>

  <section>
    <title>B+trees</title>

    <para>HelenOS makes use of a variant of B-tree called B+tree. B+trees, in
    HelenOS implementation, are 3-4-5 balanced trees. They are characteristic
    by the fact that values are kept only in the leaf-level nodes and that
    these nodes are linked together in a list. This data structure has
    logaritmic search, insertion and deletion times and, thanks to the
    leaf-level list, provides fantastic means of walking the nodes containing
    data. Moreover, B+trees can be used for easy storing, resizing and merging
    of disjunctive intervals.</para>

    <figure>
    <mediaobject id="btree" xreflabel="">
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        <imagedata fileref="images/btree.png" format="PNG" />
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    <imageobject role="fop">
        <imagedata fileref="images.vector/btree.svg" format="SVG" />
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    </mediaobject>
    <title>B+tree containing keys ranging from 1 to 12.</title>
    </figure>

  </section>
</chapter>