freebsd-dev/contrib/libgmp/mpq/get_d.c
Mark Murray aa9bc17601 Clean import of libgmp 2.0.2, with only the non-x86 bits removed.
BMakefiles and other bits will follow.

Requested by:	Andrey Chernov
Made world by:	Chuck Robey
1996-10-20 08:49:26 +00:00

152 lines
4.5 KiB
C

/* double mpq_get_d (mpq_t src) -- Return the double approximation to SRC.
Copyright (C) 1995, 1996 Free Software Foundation, Inc.
This file is part of the GNU MP Library.
The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Library General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at your
option) any later version.
The GNU MP Library 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 Library General Public
License for more details.
You should have received a copy of the GNU Library General Public License
along with the GNU MP Library; see the file COPYING.LIB. If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */
#include "gmp.h"
#include "gmp-impl.h"
#include "longlong.h"
/* Algorithm:
1. Develop >= n bits of src.num / src.den, where n is the number of bits
in a double. This (partial) division will use all bits from the
denominator.
2. Use the remainder to determine how to round the result.
3. Assign the integral result to a temporary double.
4. Scale the temporary double, and return the result.
An alternative algorithm, that would be faster:
0. Let n be somewhat larger than the number of significant bits in a double.
1. Extract the most significant n bits of the denominator, and an equal
number of bits from the numerator.
2. Interpret the extracted numbers as integers, call them a and b
respectively, and develop n bits of the fractions ((a + 1) / b) and
(a / (b + 1)) using mpn_divrem.
3. If the computed values are identical UP TO THE POSITION WE CARE ABOUT,
we are done. If they are different, repeat the algorithm from step 1,
but first let n = n * 2.
4. If we end up using all bits from the numerator and denominator, fall
back to the first algorithm above.
5. Just to make life harder, The computation of a + 1 and b + 1 above
might give carry-out... Needs special handling. It might work to
subtract 1 in both cases instead.
*/
double
#if __STDC__
mpq_get_d (const MP_RAT *src)
#else
mpq_get_d (src)
const MP_RAT *src;
#endif
{
mp_ptr np, dp;
mp_ptr rp;
mp_size_t nsize = src->_mp_num._mp_size;
mp_size_t dsize = src->_mp_den._mp_size;
mp_size_t qsize, rsize;
mp_size_t sign_quotient = nsize ^ dsize;
unsigned normalization_steps;
mp_limb_t qlimb;
#define N_QLIMBS (1 + (sizeof (double) + BYTES_PER_MP_LIMB-1) / BYTES_PER_MP_LIMB)
mp_limb_t qp[N_QLIMBS + 1];
TMP_DECL (marker);
if (nsize == 0)
return 0.0;
TMP_MARK (marker);
nsize = ABS (nsize);
dsize = ABS (dsize);
np = src->_mp_num._mp_d;
dp = src->_mp_den._mp_d;
rsize = dsize + N_QLIMBS;
rp = (mp_ptr) TMP_ALLOC ((rsize + 1) * BYTES_PER_MP_LIMB);
count_leading_zeros (normalization_steps, dp[dsize - 1]);
/* Normalize the denominator, i.e. make its most significant bit set by
shifting it NORMALIZATION_STEPS bits to the left. Also shift the
numerator the same number of steps (to keep the quotient the same!). */
if (normalization_steps != 0)
{
mp_ptr tp;
mp_limb_t nlimb;
/* Shift up the denominator setting the most significant bit of
the most significant limb. Use temporary storage not to clobber
the original contents of the denominator. */
tp = (mp_ptr) TMP_ALLOC (dsize * BYTES_PER_MP_LIMB);
mpn_lshift (tp, dp, dsize, normalization_steps);
dp = tp;
if (rsize > nsize)
{
MPN_ZERO (rp, rsize - nsize);
nlimb = mpn_lshift (rp + (rsize - nsize),
np, nsize, normalization_steps);
}
else
{
nlimb = mpn_lshift (rp, np + (nsize - rsize),
rsize, normalization_steps);
}
if (nlimb != 0)
{
rp[rsize] = nlimb;
rsize++;
}
}
else
{
if (rsize > nsize)
{
MPN_ZERO (rp, rsize - nsize);
MPN_COPY (rp + (rsize - nsize), np, nsize);
}
else
{
MPN_COPY (rp, np + (nsize - rsize), rsize);
}
}
qlimb = mpn_divmod (qp, rp, rsize, dp, dsize);
qsize = rsize - dsize;
if (qlimb)
{
qp[qsize] = qlimb;
qsize++;
}
{
double res;
mp_size_t i;
res = qp[qsize - 1];
for (i = qsize - 2; i >= 0; i--)
res = res * MP_BASE_AS_DOUBLE + qp[i];
res = __gmp_scale2 (res, BITS_PER_MP_LIMB * (nsize - dsize - N_QLIMBS));
TMP_FREE (marker);
return sign_quotient >= 0 ? res : -res;
}
}