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<title>Issue 800: Issues in 26.4.7.1 [rand.util.seedseq](6)</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#Resolved">Resolved</a> status.</em></p>
<h3 id="800"><a href="lwg-defects.html#800">800</a>. Issues in 26.4.7.1 [rand.util.seedseq](6)</h3>
<p><b>Section:</b> 29.5.8.1 <a href="https://wg21.link/rand.util.seedseq">[rand.util.seedseq]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Stephan Tolksdorf <b>Opened:</b> 2008-02-18 <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.util.seedseq">issues</a> in [rand.util.seedseq].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The for-loop in the algorithm specification has <code>n</code> iterations, where <code>n</code> is
defined to be <code>end - begin</code>, i.e. the number of supplied w-bit quantities.
Previous versions of this algorithm and the general logic behind it
suggest that this is an oversight and that in the context of the
for-loop <code>n</code> should be the number of full 32-bit quantities in <code>b</code> (rounded
upwards). If <code>w</code> is 64, the current algorithm throws away half of all bits
in <code>b</code>. If <code>w</code> is 16, the current algorithm sets half of all elements in <code>v</code>
to 0.
</p>

<p>
There are two more minor issues:
</p>

<ul>
<li>
Strictly speaking <code>end - begin</code> is not defined since
<code>InputIterator</code> is not required to be a random access iterator.
</li>
<li>
Currently all integral types are allowed as input to the <code>seed_seq</code>
constructor, including <code>bool</code>. IMHO allowing <code>bool</code>s unnecessarily
complicates the implementation without any real benefit to the user.
I'd suggest to exclude <code>bool</code>s as input.
</li>
</ul>

<p><i>[
Bellevue:
]</i></p>


<blockquote><p>
Move to Open: Bill will try to propose a resolution by the next meeting.
</p></blockquote>

<p><i>[
post Bellevue:  Bill provided wording.
]</i></p>


<p>
This issue is made moot if <a href="lwg-defects.html#803" title="Simplification of seed_seq::seq_seq (Status: Resolved)">803</a><sup><a href="https://cplusplus.github.io/LWG/issue803" title="Latest snapshot">(i)</a></sup> is accepted.
</p>



<p id="res-800"><b>Proposed resolution:</b></p>
<p>
Replace 29.5.8.1 <a href="https://wg21.link/rand.util.seedseq">[rand.util.seedseq]</a> paragraph 6 with:
</p>

<blockquote>
<p>
<i>Effects:</i> Constructs a <code>seed_seq</code> object by effectively concatenating the
low-order <code>u</code> bits of each of the elements of the supplied sequence <code>[begin,
end)</code> in ascending order of significance to make a (possibly very large) unsigned
binary number <code>b</code> having a total of <code>n</code> bits, and then carrying out the
following algorithm:
</p>

<blockquote><p style="white-space: pre;">
for( v.clear(); n &gt; 0; n -= 32 )
   v.push_back(b mod 2<sup>32</sup>), b /= 2<sup>32</sup>;
</p></blockquote>
</blockquote>


<p><b>Rationale:</b></p><p>
Addressed by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2836.pdf">N2836</a> 
"Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
</p>




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