set_q.c
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上传日期:2022-08-06
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- /* mpf_set_q (mpf_t rop, mpq_t op) -- Convert the rational op to the float rop.
- Copyright 1996, 1999, 2001, 2002, 2004, 2005 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 Lesser General Public License as published by
- the Free Software Foundation; either version 3 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 Lesser General Public
- License for more details.
- You should have received a copy of the GNU Lesser General Public License
- along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
- #include <stdio.h> /* for NULL */
- #include "gmp.h"
- #include "gmp-impl.h"
- #include "longlong.h"
- /* As usual the aim is to produce PREC(r) limbs, with the high non-zero.
- The basic mpn_tdiv_qr produces a quotient of nsize-dsize+1 limbs, with
- either the high or second highest limb non-zero. We arrange for
- nsize-dsize+1 to equal prec+1, hence giving either prec or prec+1 result
- limbs at PTR(r).
- nsize-dsize+1 == prec+1 is achieved by adjusting num(q), either dropping
- low limbs if it's too big, or padding with low zeros if it's too small.
- The full given den(q) is always used.
- We cannot truncate den(q), because even when it's much bigger than prec
- the last limbs can still influence the final quotient. Often they don't,
- but we leave optimization of that to a prospective quotient-only mpn
- division.
- Not done:
- If den(q) is a power of 2 then we may end up with low zero limbs on the
- result. But nothing is done about this, since it should be unlikely on
- random data, and can be left to an application to call mpf_div_2exp if it
- might occur with any frequency.
- Enhancements:
- The high quotient limb is non-zero when high{np,dsize} >= {dp,dsize}. We
- could make that comparison and use qsize==prec instead of qsize==prec+1,
- to save one limb in the division.
- Future:
- If/when mpn_tdiv_qr supports its qxn parameter we can use that instead of
- padding n with zeros in temporary space.
- If/when a quotient-only division exists it can be used here immediately.
- remp is only to satisfy mpn_tdiv_qr, the remainder is not used. */
- void
- mpf_set_q (mpf_t r, mpq_srcptr q)
- {
- mp_srcptr np, dp;
- mp_size_t prec, nsize, dsize, qsize, prospective_qsize, tsize, zeros;
- mp_size_t sign_quotient, high_zero;
- mp_ptr qp, tp, remp;
- mp_exp_t exp;
- TMP_DECL;
- ASSERT (SIZ(&q->_mp_den) > 0); /* canonical q */
- nsize = SIZ (&q->_mp_num);
- dsize = SIZ (&q->_mp_den);
- if (UNLIKELY (nsize == 0))
- {
- SIZ (r) = 0;
- EXP (r) = 0;
- return;
- }
- TMP_MARK;
- prec = PREC (r);
- qp = PTR (r);
- sign_quotient = nsize;
- nsize = ABS (nsize);
- np = PTR (&q->_mp_num);
- dp = PTR (&q->_mp_den);
- prospective_qsize = nsize - dsize + 1; /* q from using given n,d sizes */
- exp = prospective_qsize; /* ie. number of integer limbs */
- qsize = prec + 1; /* desired q */
- zeros = qsize - prospective_qsize; /* n zeros to get desired qsize */
- tsize = nsize + zeros; /* possible copy of n */
- if (WANT_TMP_DEBUG)
- {
- /* separate alloc blocks, for malloc debugging */
- remp = TMP_ALLOC_LIMBS (dsize);
- tp = NULL;
- if (zeros > 0)
- tp = TMP_ALLOC_LIMBS (tsize);
- }
- else
- {
- /* one alloc with a conditionalized size, for efficiency */
- mp_size_t size = dsize + (zeros > 0 ? tsize : 0);
- remp = TMP_ALLOC_LIMBS (size);
- tp = remp + dsize;
- }
- if (zeros > 0)
- {
- /* pad n with zeros into temporary space */
- MPN_ZERO (tp, zeros);
- MPN_COPY (tp+zeros, np, nsize);
- np = tp;
- nsize = tsize;
- }
- else
- {
- /* shorten n to get desired qsize */
- nsize += zeros;
- np -= zeros;
- }
- ASSERT (nsize-dsize+1 == qsize);
- mpn_tdiv_qr (qp, remp, (mp_size_t) 0, np, nsize, dp, dsize);
- /* strip possible zero high limb */
- high_zero = (qp[qsize-1] == 0);
- qsize -= high_zero;
- exp -= high_zero;
- EXP (r) = exp;
- SIZ (r) = sign_quotient >= 0 ? qsize : -qsize;
- TMP_FREE;
- }