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POWhat is a data structure kind of like a hash table, but infrequently-used keys are deleted?
 

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  1. POWhat is a data structure kind of like a hash table, but infrequently-used keys are deleted?
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    <p>I am looking for a data structure that operates similar to a hash table, but where the table has a size limit. When the number of items in the hash reaches the size limit, a culling function should be called to get rid of the least-retrieved key/value pairs in the table.</p> <p>Here's some pseudocode of what I'm working on:</p> <pre><code>class MyClass { private Map&lt;Integer, Integer&gt; cache = new HashMap&lt;Integer, Integer&gt;(); public int myFunc(int n) { if(cache.containsKey(n)) return cache.get(n); int next = . . . ; //some complicated math. guaranteed next != n. int ret = 1 + myFunc(next); cache.put(n, ret); return ret; } } </code></pre> <p>What happens is that there are some values of <code>n</code> for which <code>myFunc()</code> will be called lots of times, but many other values of <code>n</code> which will only be computed once. So the cache could fill up with millions of values that are never needed again. I'd like to have a way for the cache to automatically remove elements that are not frequently retrieved.</p> <p>This feels like a problem that must be solved already, but I'm not sure what the data structure is that I would use to do it efficiently. Can anyone point me in the right direction?</p> <hr> <p><strong>Update</strong> I knew this had to be an already-solved problem. It's called an LRU Cache and is easy to make by extending the LinkedHashMap class. Here is the code that incorporates the solution:</p> <pre><code>class MyClass { private final static int SIZE_LIMIT = 1000; private Map&lt;Integer, Integer&gt; cache = new LinkedHashMap&lt;Integer, Integer&gt;(16, 0.75f, true) { protected boolean removeEldestEntry(Map.Entry&lt;Integer, Integer&gt; eldest) { return size() &gt; SIZE_LIMIT; } }; public int myFunc(int n) { if(cache.containsKey(n)) return cache.get(n); int next = . . . ; //some complicated math. guaranteed next != n. int ret = 1 + myFunc(next); cache.put(n, ret); return ret; } } </code></pre>
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    1. USRobert Gamble
    1. USKip
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    1. COSee also http://stackoverflow.com/questions/224868/easy-simple-to-use-lru-cache-in-java.
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      1. USMichael Myers
 

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