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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. They serve like generic
    containers.
    </para>
    
    <section>
      <title>Lists</title>
      <para>
      HelenOS uses circular doubly linked lists to bind related data
      together. Lists are composed of an independent 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>
    </section>

    <section>
      <title>Hash tables</title>
      <para>
      The kernel, as well as userspace, makes use of the hash table implementation with
      separate chaining. The implementation 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>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>
    </section>

</chapter>