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/*
* IBM Accurate Mathematical Library
* written by International Business Machines Corp.
* Copyright (C) 2001-2013 Free Software Foundation, Inc.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2.1 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this program; if not, see <http://www.gnu.org/licenses/>.
*/
/*************************************************************************/
/* MODULE_NAME:mpexp.c */
/* */
/* FUNCTIONS: mpexp */
/* */
/* FILES NEEDED: mpa.h endian.h mpexp.h */
/* mpa.c */
/* */
/* Multi-Precision exponential function subroutine */
/* ( for p >= 4, 2**(-55) <= abs(x) <= 1024 ). */
/*************************************************************************/
#include "endian.h"
#include "mpa.h"
#include <assert.h>
#ifndef SECTION
# define SECTION
#endif
/* Multi-Precision exponential function subroutine (for p >= 4,
2**(-55) <= abs(x) <= 1024). */
void
SECTION
__mpexp (mp_no *x, mp_no *y, int p)
{
int i, j, k, m, m1, m2, n;
double b;
static const int np[33] =
{
0, 0, 0, 0, 3, 3, 4, 4, 5, 4, 4, 5, 5, 5, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 8
};
/* Factorials for the values of np above. */
static const double nfa[33] =
{
1.0, 1.0, 1.0, 1.0, 6.0, 6.0, 24.0, 24.0, 120.0, 24.0, 24.0,
120.0, 120.0, 120.0, 720.0, 720.0, 720.0, 720.0, 720.0, 720.0,
720.0, 720.0, 720.0, 720.0, 5040.0, 5040.0, 5040.0, 5040.0,
40320.0, 40320.0, 40320.0, 40320.0, 40320.0
};
static const int m1p[33] =
{
0, 0, 0, 0,
17, 23, 23, 28,
27, 38, 42, 39,
43, 47, 43, 47,
50, 54, 57, 60,
64, 67, 71, 74,
68, 71, 74, 77,
70, 73, 76, 78,
81
};
static const int m1np[7][18] =
{
{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{0, 0, 0, 0, 36, 48, 60, 72, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{0, 0, 0, 0, 24, 32, 40, 48, 56, 64, 72, 0, 0, 0, 0, 0, 0, 0},
{0, 0, 0, 0, 17, 23, 29, 35, 41, 47, 53, 59, 65, 0, 0, 0, 0, 0},
{0, 0, 0, 0, 0, 0, 23, 28, 33, 38, 42, 47, 52, 57, 62, 66, 0, 0},
{0, 0, 0, 0, 0, 0, 0, 0, 27, 0, 0, 39, 43, 47, 51, 55, 59, 63},
{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 43, 47, 50, 54}
};
mp_no mps, mpk, mpt1, mpt2;
/* Choose m,n and compute a=2**(-m). */
n = np[p];
m1 = m1p[p];
b = X[1];
m2 = 24 * EX;
for (; b < HALFRAD; m2--)
b *= TWO;
if (b == HALFRAD)
{
for (i = 2; i <= p; i++)
{
if (X[i] != ZERO)
break;
}
if (i == p + 1)
m2--;
}
m = m1 + m2;
if (__glibc_unlikely (m <= 0))
{
/* The m1np array which is used to determine if we can reduce the
polynomial expansion iterations, has only 18 elements. Besides,
numbers smaller than those required by p >= 18 should not come here
at all since the fast phase of exp returns 1.0 for anything less
than 2^-55. */
assert (p < 18);
m = 0;
for (i = n - 1; i > 0; i--, n--)
if (m1np[i][p] + m2 > 0)
break;
}
/* Compute s=x*2**(-m). Put result in mps. This is the range-reduced input
that we will use to compute e^s. For the final result, simply raise it
to 2^m. */
__pow_mp (-m, &mpt1, p);
__mul (x, &mpt1, &mps, p);
/* Compute the Taylor series for e^s:
1 + x/1! + x^2/2! + x^3/3! ...
for N iterations. We compute this as:
e^x = 1 + (x * n!/1! + x^2 * n!/2! + x^3 * n!/3!) / n!
= 1 + (x * (n!/1! + x * (n!/2! + x * (n!/3! + x ...)))) / n!
n! is pre-computed and saved while k! is computed on the fly. */
__cpy (&mps, &mpt2, p);
double kf = 1.0;
/* Evaluate the rest. The result will be in mpt2. */
for (k = n - 1; k > 0; k--)
{
/* n! / k! = n * (n - 1) ... * (n - k + 1) */
kf *= k + 1;
__dbl_mp (kf, &mpk, p);
__add (&mpt2, &mpk, &mpt1, p);
__mul (&mps, &mpt1, &mpt2, p);
}
__dbl_mp (nfa[p], &mpk, p);
__dvd (&mpt2, &mpk, &mpt1, p);
__add (&mpone, &mpt1, &mpt2, p);
/* Raise polynomial value to the power of 2**m. Put result in y. */
for (k = 0, j = 0; k < m;)
{
__mul (&mpt2, &mpt2, &mpt1, p);
k++;
if (k == m)
{
j = 1;
break;
}
__mul (&mpt1, &mpt1, &mpt2, p);
k++;
}
if (j)
__cpy (&mpt1, y, p);
else
__cpy (&mpt2, y, p);
return;
}
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