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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Linear congruential generator generates invalid values"
href="https://llvm.org/bugs/show_bug.cgi?id=27839">27839</a>
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<th>Summary</th>
<td>Linear congruential generator generates invalid values
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>3.7
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>david.g.hammen@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The linear congruential generator generates numbers outside the range of
`min()` and `max()` for some values of `a`, `c`, and `m`. For example,
`std::linear_congruential_engine <unsigned long long, 25214903917ull, 11ull,
1ull<<48 >` fails badly. These are the values used by `drand48`.
#include <random>
#include <iostream>
#include <cstdint>
using drand48_engine = std::linear_congruential_engine <
std::uint64_t, 25214903917ull, 11ull, (1ull<<48) >;
int main ()
{
drand48_engine rng;
int pass_fail[2] = {0,0};
for (int ii = 0; ii < 1000000; ++ii)
{
auto num = rng();
++pass_fail[num < (1ull<<48)];
}
std::cout << "#pass=" << pass_fail[1] << " #fail=" << pass_fail[0] <<
'\n';
}</pre>
</div>
</p>
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