64 lines
2.5 KiB
HTML
64 lines
2.5 KiB
HTML
<div class="post-2-1">
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<div>
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<p>Skiplists are possibly one of the most useful data structures that I don't
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see talked about all that often. They are a special type of list that is
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probabilistic in nature, and allows for O(log n) indexing and searching, and
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O(1) insertion when provided an iterator. They are always sorted, and therefore
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have great ordered traversal. Its insane just how fast these things are.</p>
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<p>The general concept starts with a normal linked list, usually singly linked
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lists. From there we make it so that each node in the list has pointers to more
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than just the next element. With this we can skip forward through the last much
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faster. By randomly placing pointers forward when traversing normally, we can
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build up a well distributed set of pointers, allowing us to skip forward large
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amounts at first, then slowing getting more precise as we find the element we
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want.</p>
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</div>
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<div class="col-sm-4">
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<figure>
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<img src="../img/Skip_list.svg" class="fig-img"></img>
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<figcaption>Diagram of a skiplist, from <a
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href="https://en.wikipedia.org/wiki/Skip_list">Wikipedia</a>
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</figcaption>
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</figure>
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</div>
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</div>
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<div class="row no-gutters">
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<div class="col-sm">
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<h4>Complexities</h4>
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<h5>Average case</h5>
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<ul>
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<li>Search - log n</li>
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<li>Insert - log n</li>
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<li>Delete - log n</li>
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<li>Index - log n (indexable skiplist implementation)</li>
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<li>Space usage - n log n</li>
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</ul>
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<h5>Use cases</h5>
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<p>The use case that lead me to find this was implementing a cache for often
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repeated malloc() operations. I have a use case where textures need to be
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allocated with strings on those textures. The set of string will repeat itself
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often, however the strings to be shown are somewhat unpredictable. Therefore, I
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am keeping a hash table of pointers to the allocated textures, and a skip list
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associating the age of the entry with the string on the texture. Using a
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skiplist allows for log n lookup and removal of the entries so we can keep
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accurate track of what entries are how old. Due to its fast access of its
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elements, and having O(1) access time to the head element, there is a slight
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performance advantage to using this versus a Heap.</p>
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<h5>Implementations</h5>
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<p>I am using <a href="https://github.com/Samyak2/skip-list">this</a>
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implementation right now to benchmark this cache system in context, then I
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plan to build my own skip list. I hope this helps in your own projects!</p>
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</div>
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</div>
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