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<span class="hidden">CLiki - MT19937</span>
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<div id="content"><div id="content-area"><div id="article-title">MT19937</div><div id="article">MT19937 is a portable <a href="Mersenne Twister.html" class="category">Mersenne Twister</a> random number generator. It
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is mainly a modification of <a href="CMUCL.html" class="internal">CMUCL</a>'s random number generator with all
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the CMUCL-specific parts taken out.<p>It is faster than the <a href="https://www.cliki.net/JMT" class="new">JMT</a> Mersenne Twister
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implementation, but significantly slower than native random number
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generators provided by major Common Lisp implementations. For light
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use this shouldn't be a problem, since it is still very fast.<p>It should be very stable, since it's based on stable CMUCL code. It
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has been tested on <a href="CMUCL.html" class="internal">CMUCL</a>, <a href="SBCL.html" class="internal">SBCL</a>, (LispWorks), <a href="Allegro.html" class="internal">Allegro</a> CL, <a href="GCL.html" class="internal">GCL</a>, <a href="CLISP.html" class="internal">CLISP</a>, and <a href="https://www.cliki.net/Corman%20Lisp" class="new">Corman Lisp</a>. MT19937 is in the public domain.<p><h2>What's the point?</h2>
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Why, you might ask, would you want to use a portable, slower random
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number generator? The answer is consistency. The portable version of
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this code was originally created for <a href="Maxima.html" class="internal">Maxima</a>, a computer algebra
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system. They wanted the results of the random number generator to be
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portable across several implementations and platforms, so that if you
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used a certain seed on CMUCL the numbers generated would be the same
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as you would get with the same seed on GCL or CLISP. This was more
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important than achieving the maximum possible speed. You may have
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similar problems.<p><h2>Usage</h2>
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MT19937 is a plug-in replacement for the Common Lisp random-number
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generator. The MT19937 package exports all the Common Lisp symbols
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related to random number generation, so you just need to load MT19937
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and call its functions instead of the built-in ones. For example:<p><pre>
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;; Load MT19937
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(asdf:oos 'asdf:load-op :mt19937)
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;; Make random numbers with your implementation's random number generator.
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(random 1234567)
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(random 42.56)
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(random 3.1415d0)
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;; Make random numbers using MT19937
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(mt19937:random 1234567)
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(mt19937:random 42.56)
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(mt19937:random 3.1415d0)
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;; MT19937 has its own random state
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(eq *random-state*
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mt19937:*random-state*) => nil
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</pre><p><h2>Download</h2><p>MT19937 can be downloaded from the <a href="http://common-lisp.net/project/asdf-packaging/">asdf-packaging home page</a>.<p><h2>Bug</h2>
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This code has a bug in the definition of the "random" compiler macro at the bottom of the file. Because it is a macro the default value for the state argument needs a quote in front of it, so that the macro argument gets bound to a symbol, not the value of a symbol. <p>Without this fix, code calling mt19937:random with a constant argument probably won't compile at all (it would not on ACL 8.0) for lack of a make-load-form method for random-state's. If one is added so that it compiles, it will behave differently than expected: calls using constant arguments with no state argument will fail to use the dynamic value of *random-state* at the time of the call. <p>The macro definition should read
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<pre>
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(define-compiler-macro random (&whole form arg &optional (state '*random-state*)
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</pre>
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with single quote added in front of *random-state*.<p>Or it might be safest to delete this macro entirely.<p>Another weakness is that it is very unclear how to contact a maintainer of the code to correct a bug like this, short of modifying this page!<p>Confirmed on SBCL 1.0.29. I believe the asdf-packaging mailing list is probably the best place to report the bug. I have asked on the list but have not yet received a response. -- Elliott Slaughter 2010-01-05<p>Update: As of 2011-01-30, this bug has finally been fixed. -- Elliott Slaughter</div></div>
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<div id="footer" class="buttonbar"><ul><li><a href="MT19937.html">Current version</a></li>
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<li><a href="https://www.cliki.net/site/history?article=MT19937">History</a></li>
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<li><a href="https://www.cliki.net/site/backlinks?article=MT19937">Backlinks</a></li><li><a href="https://www.cliki.net/site/edit-article?title=MT19937&from-revision=3686398038">Edit</a></li><li><a href="https://www.cliki.net/site/edit-article?create=t">Create</a></li></ul></div>
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