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<title>Issue 1437: Mersenne twister meaningless for word sizes less than two</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++11">C++11</a> status.</em></p>
<h3 id="1437"><a href="lwg-defects.html#1437">1437</a>. Mersenne twister meaningless for word sizes less than two</h3>
<p><b>Section:</b> 29.5.4.3 <a href="https://wg21.link/rand.eng.mers">[rand.eng.mers]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> INCITS <b>Opened:</b> 2010-08-25 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#rand.eng.mers">issues</a> in [rand.eng.mers].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses US-124</b></p>

<p>
The Mersenne twister algorithm is meaningless for word
sizes less than two, as there are then insufficient bits
available to be &#8220;twisted&#8221;.
</p>

<p><i>[
Resolution proposed by ballot comment:
]</i></p>

<blockquote><p>
Insert the following among the relations that are required to hold: <code>2u &lt; w</code>.
</p></blockquote>

<p><i>[
2010 Batavia: The working group concurred with the issue's Proposed Resolution
]</i></p>


<p><i>[
Adopted at 2010-11 Batavia
]</i></p>




<p id="res-1437"><b>Proposed resolution:</b></p>
<p>
Change 29.5.4.3 <a href="https://wg21.link/rand.eng.mers">[rand.eng.mers]</a> p. 4 as indicated:
</p>
<blockquote><p>
4 The following relations shall hold: <code>0 &lt; m</code>, <code>m &lt;= n</code>, <ins><code>2u &lt; w</code>,</ins>
<code>r &lt;= w</code>, <code>u &lt;= w</code>, <code>s &lt;= w</code>, <code>t &lt;= w</code>, 
<code>l &lt;= w</code>, <code>w &lt;= numeric_limits&lt;UIntType&gt;::digits</code>, 
<code>a &lt;= (1u&lt;&lt;w) - 1u</code>, <code>b &lt;= (1u&lt;&lt;w) - 1u</code>, 
<code>c &lt;= (1u&lt;&lt;w) - 1u</code>, <code>d &lt;= (1u&lt;&lt;w) - 1u</code>, 
and <code>f &lt;= (1u&lt;&lt;w) - 1u</code>.
</p></blockquote>





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