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User:Jorend/Deterministic hash tables: Difference between revisions
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User:Jorend/Deterministic hash tables (view source)
Revision as of 19:39, 27 February 2012
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'''Speed.''' We consider the speed of lookups only. The <code>get</code>, <code>has</code>, <code>set</code>, and <code>delete</code> operations all start with a lookup. We will quantify the speed of lookups in terms of how many memory locations must be accessed. To simulate locality effects, we treat multiple reads from a single entry, or from the table object, as a single read. | '''Speed.''' We consider the speed of lookups only. The <code>get</code>, <code>has</code>, <code>set</code>, and <code>delete</code> operations all start with a lookup. We will quantify the speed of lookups in terms of how many memory locations must be accessed. To simulate locality effects, we treat multiple reads from a single entry, or from the table object, as a single read. | ||
In an open addressing hash table with fill factor ''f'', an unsuccessful lookup performs about ''1/(1-f)'' probes.[https://en.wikipedia.org/wiki/Hash_table#Performance_analysis] In an <code>OpenTable</code>, at worst ''f=F=3/4'' and so an unsuccessful lookup performs 4 probes on average. In the usual case, ''f=3F/4=9/16'', so about 2.3 probes on average. | In an open addressing hash table with fill factor ''f'', an unsuccessful lookup performs about ''1/(1-f)'' probes.[https://en.wikipedia.org/wiki/Hash_table#Performance_analysis] In an <code>OpenTable</code>, at worst ''f=F=3/4'' and so an unsuccessful lookup performs 4 probes on average. In the usual case, ''f=3F/4=9/16'', so about 2.3 probes on average. A successful lookup performs half as many probes on average. Before probing the table, the lookup must access the <code>OpenTable</code> struct itself, so the final numbers are as shown in the table: 5 reads, 3.3 reads, etc. | ||
A Close table uses direct chaining. Each lookup must read first the <code>CloseTable</code> object itself, and next the <code>hashTable</code>. Then, for an unsuccessful lookup, the whole chain is walked; the expected length of the chain is simply the number of keys in the table divided by the number of hash buckets. For a <code>CloseTable</code>, this quotient is at most 8/3 and typically 2. ... | |||
<table class="data"> | <table class="data"> | ||
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<td>5 | <td>5 | ||
<td>3.3 | <td>3.3 | ||
<td> | <td>3 | ||
<td> | <td>2.2 | ||
</tr> | </tr> | ||
<tr> | <tr> | ||
<td><code>CloseTable</code> | <td><code>CloseTable</code> | ||
<td> | <td>4.7 | ||
<td> | <td>3 | ||
<td>- | <td>- | ||
<td>- | <td>- | ||