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- <h3 class="subsection">D.12.3 Additional <code>mtwist</code> notes</h3>
- <p>Although the <code>mtwist</code> library is “external”, it requires no
- special configuration on the host because the uniform variate
- generator in the form developed by its original authors is short and
- elegant enough to be packaged easily within the <code>avram</code>
- distribution. All further embellishments are home grown despite the
- advice at the end of <a href="Implementing-new-library-functions.html#Implementing-new-library-functions">Implementing new library functions</a>.
- </p>
- <p>The <code>u_path</code> function is intended to allow sampling from a large
- population in logarithmic time when it is stored in a balanced tree. A
- left-heavy tree should be constructed initially with the data items
- all at the same level. Thereafter, a result returned by <code>u_path</code>
- with the appropriate dimensions can be used as an index into the tree
- for fast retrieval by the virtual machine’s <code>field</code> combinator
- (<a href="Field.html#Field">Field</a>).
- </p>
- <p>The last three functions, <code>u_enum</code>, <code>w_disc</code>, and
- <code>w_enum</code> use an inversion method with a binary search. The first
- draw from a given list will take a time asymptotically proportional to
- the length of the list, but subsequent draws from the same list are
- considerably faster due to a persistent cache maintained transparently
- by <code>avram</code>. For lists whose length is up to 2^16, the time
- required for a subsequent draw consists mainly of constant overhead
- with a small logarithmic component in the length of the list. For
- longer lists, the time ramps up linearly by a small factor.
- </p>
- <p>Information allowing fast draws from up to sixteen lists can be cached
- simultaneously. If an application uses more than sixteen, the cached
- data are replaced in first-in first-out order. The size of the cache
- and the maximum list length for logarithmic time access can be
- adjusted easily by redefining constants in ‘<tt>mtwist.c</tt>’ under the
- <code>avram</code> source tree, but will require recompilation.
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