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6861122
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2011-07-28T14:58:58.390
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2011-07-28T14:58:58.390
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Let's design the data structures for a caching proxy. We need a map from URLs to content; let's use a hash table. We also need a way to find pages to evict; let's use a FIFO queue to track the order in which URLs were last accessed, so that we can implement LRU eviction. In C, the data structure could look something like <pre><code>struct node { struct node *queueprev, *queuenext; struct node **hashbucketprev, *hashbucketnext; const char *url; const void *content; size_t contentlength; }; struct node *queuehead; /* circular doubly-linked list */ struct node **hashbucket; </code></pre> One subtlety: to avoid a special case and wasting space in the hash buckets, <code>x->hashbucketprev</code> points to the pointer that points to <code>x</code>. If <code>x</code> is first in the bucket, it points into <code>hashbucket</code>; otherwise, it points into another node. We can remove <code>x</code> from its bucket with <pre><code>x->hashbucketnext->hashbucketprev = x->hashbucketprev; *(x->hashbucketprev) = x->hashbucketnext; </code></pre> When evicting, we iterate over the least recently accessed nodes via the <code>queuehead</code> pointer. Without <code>hashbucketprev</code>, we would need to hash each node and find its predecessor with a linear search, since we did not reach it via <code>hashbucketnext</code>. (Whether that's really bad is debatable, given that the hash should be cheap and the chain should be short. I suspect that the comment you're asking about was basically a throwaway.)
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1. POHashtable with doubly linked lists?
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1. UScutting plane
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