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<li ><a href="#method-i-5B-5D">#[]</a>
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<h1 id="class-Prime::TrialDivision" class="class">
class Prime::TrialDivision
</h1>
<section class="description">
<p>Internal use. An implementation of prime table by trial division method.</p>
</section>
<section id="5Buntitled-5D" class="documentation-section">
<section id="public-instance-5Buntitled-5D-method-details" class="method-section">
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<h3>Public Instance Methods</h3>
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<div id="method-i-5B-5D" class="method-detail ">
<div class="method-heading">
<span class="method-name">[]</span><span
class="method-args">(index)</span>
<span class="method-click-advice">click to toggle source</span>
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<div class="method-description">
<p>Returns the +index+th prime number.</p>
<p><code>index</code> is a 0-based index.</p>
<div class="method-source-code" id="5B-5D-source">
<pre><span class="ruby-comment"># File lib/prime.rb, line 397</span>
<span class="ruby-keyword">def</span> <span class="ruby-identifier ruby-title">[]</span>(<span class="ruby-identifier">index</span>)
<span class="ruby-keyword">while</span> <span class="ruby-identifier">index</span> <span class="ruby-operator">>=</span> <span class="ruby-ivar">@primes</span>.<span class="ruby-identifier">length</span>
<span class="ruby-comment"># Only check for prime factors up to the square root of the potential primes,</span>
<span class="ruby-comment"># but without the performance hit of an actual square root calculation.</span>
<span class="ruby-keyword">if</span> <span class="ruby-ivar">@next_to_check</span> <span class="ruby-operator">+</span> <span class="ruby-value">4</span> <span class="ruby-operator">></span> <span class="ruby-ivar">@ulticheck_next_squared</span>
<span class="ruby-ivar">@ulticheck_index</span> <span class="ruby-operator">+=</span> <span class="ruby-value">1</span>
<span class="ruby-ivar">@ulticheck_next_squared</span> = <span class="ruby-ivar">@primes</span>.<span class="ruby-identifier">at</span>(<span class="ruby-ivar">@ulticheck_index</span> <span class="ruby-operator">+</span> <span class="ruby-value">1</span>) <span class="ruby-operator">**</span> <span class="ruby-value">2</span>
<span class="ruby-keyword">end</span>
<span class="ruby-comment"># Only check numbers congruent to one and five, modulo six. All others</span>
<span class="ruby-comment"># are divisible by two or three. This also allows us to skip checking against</span>
<span class="ruby-comment"># two and three.</span>
<span class="ruby-ivar">@primes</span>.<span class="ruby-identifier">push</span> <span class="ruby-ivar">@next_to_check</span> <span class="ruby-keyword">if</span> <span class="ruby-ivar">@primes</span>[<span class="ruby-value">2</span><span class="ruby-operator">..</span><span class="ruby-ivar">@ulticheck_index</span>].<span class="ruby-identifier">find</span> {<span class="ruby-operator">|</span><span class="ruby-identifier">prime</span><span class="ruby-operator">|</span> <span class="ruby-ivar">@next_to_check</span> <span class="ruby-operator">%</span> <span class="ruby-identifier">prime</span> <span class="ruby-operator">==</span> <span class="ruby-value">0</span> }.<span class="ruby-identifier">nil?</span>
<span class="ruby-ivar">@next_to_check</span> <span class="ruby-operator">+=</span> <span class="ruby-value">4</span>
<span class="ruby-ivar">@primes</span>.<span class="ruby-identifier">push</span> <span class="ruby-ivar">@next_to_check</span> <span class="ruby-keyword">if</span> <span class="ruby-ivar">@primes</span>[<span class="ruby-value">2</span><span class="ruby-operator">..</span><span class="ruby-ivar">@ulticheck_index</span>].<span class="ruby-identifier">find</span> {<span class="ruby-operator">|</span><span class="ruby-identifier">prime</span><span class="ruby-operator">|</span> <span class="ruby-ivar">@next_to_check</span> <span class="ruby-operator">%</span> <span class="ruby-identifier">prime</span> <span class="ruby-operator">==</span> <span class="ruby-value">0</span> }.<span class="ruby-identifier">nil?</span>
<span class="ruby-ivar">@next_to_check</span> <span class="ruby-operator">+=</span> <span class="ruby-value">2</span>
<span class="ruby-keyword">end</span>
<span class="ruby-ivar">@primes</span>[<span class="ruby-identifier">index</span>]
<span class="ruby-keyword">end</span></pre>
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