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IMP Reference Guide
2.18.0
The Integrative Modeling Platform
IMP Manual
Reference Guide
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include
IMP
em
version 2.18.0
exp.h
Go to the documentation of this file.
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/**
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* \file IMP/em/exp.h
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* \brief An approximation of the exponential function.
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*
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* Copyright 2007-2022 IMP Inventors. All rights reserved.
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*
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*/
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#ifndef IMPEM_EXP_H
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#define IMPEM_EXP_H
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#include <IMP/em/em_config.h>
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/* An approximation of the exponential function.
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Schraudolph, Nicol N. "A Fast, Compact Approximation of the exponential
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function" Technical Report IDSIA-07-98
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(Neural Computation 11(4))
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The function returns values within 1-2% deviation of the correct ones in
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the range (-700,700)
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*/
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#include <IMP/em/em_config.h>
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#include <
math.h
>
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#include <boost/version.hpp>
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#if BOOST_VERSION < 106900
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#include <boost/detail/endian.hpp>
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#else
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#include <boost/predef/other/endian.h>
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#endif
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IMPEM_BEGIN_NAMESPACE
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/* Unions are similar to structs, but they differ in one aspect:
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the fields of a union share the same position in memory.
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The size of the union is the size of its largest field
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(or larger if alignment so requires, for example on a SPARC machine
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a union contains a double and a char [17] so its size is likely to be
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24 because it needs 64-bit alignment).
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What is the point of this? Unions provide multiple ways of viewing
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the same memory location, allowing for \more efficient use of memory.
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Most of the uses of unions are covered by object-oriented features of C++,
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so it is more common in C. However, sometimes it is convenient to avoid
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the formalities of object-oriented programming when performance is
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important or when one knows that the item in question will not be extended.
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http://en.wikibooks.org/wiki/C++_Programming/Union
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*/
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typedef
union
{
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double
d;
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struct
{
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#if defined(BOOST_ENDIAN_LITTLE_BYTE) || defined(BOOST_LITTLE_ENDIAN)
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int
j, i;
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#else
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int
i, j;
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#endif
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} n;
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} _eco;
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/* Originally was (1048576/M_LN2); M_LN2 expanded out so that this header
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can be correctly #include'd in other code on Windows machines without having
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to #define the non-standard _USE_MATH_DEFINES macro */
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#define IMP_EXP_A (1048576 / 0.693147180559945309417232121458176568)
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#define IMP_EXP_C 60801
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inline
double
EXP(
float
y) {
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static
_eco eco;
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eco.n.i = (int)(IMP_EXP_A * (y) + (1072693248 - IMP_EXP_C));
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return
eco.d;
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}
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IMPEM_END_NAMESPACE
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#endif
/* IMPEM_EXP_H */
math.h
Helper functions to check for NaN or infinity.