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/* double mpq_get_d (mpq_t src) -- Return the double approximation to SRC.
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Copyright 1995, 1996, 2001, 2002 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
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MA 02111-1307, USA. */
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1. Develop >= n bits of src.num / src.den, where n is the number of bits
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in a double. This (partial) division will use all bits from the
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2. Use the remainder to determine how to round the result.
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3. Assign the integral result to a temporary double.
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4. Scale the temporary double, and return the result.
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An alternative algorithm, that would be faster:
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0. Let n be somewhat larger than the number of significant bits in a double.
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1. Extract the most significant n bits of the denominator, and an equal
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number of bits from the numerator.
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2. Interpret the extracted numbers as integers, call them a and b
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respectively, and develop n bits of the fractions ((a + 1) / b) and
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(a / (b + 1)) using mpn_divrem.
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3. If the computed values are identical UP TO THE POSITION WE CARE ABOUT,
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we are done. If they are different, repeat the algorithm from step 1,
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but first let n = n * 2.
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4. If we end up using all bits from the numerator and denominator, fall
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back to the first algorithm above.
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5. Just to make life harder, The computation of a + 1 and b + 1 above
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might give carry-out... Needs special handling. It might work to
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subtract 1 in both cases instead.
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mpq_get_d (const MP_RAT *src)
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mp_size_t nsize = src->_mp_num._mp_size;
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mp_size_t dsize = src->_mp_den._mp_size;
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mp_size_t qsize, rsize;
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mp_size_t sign_quotient = nsize ^ dsize;
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#define N_QLIMBS (1 + (sizeof (double) + BYTES_PER_MP_LIMB-1) / BYTES_PER_MP_LIMB)
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mp_limb_t qarr[N_QLIMBS + 1];
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np = src->_mp_num._mp_d;
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dp = src->_mp_den._mp_d;
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rsize = dsize + N_QLIMBS;
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rp = (mp_ptr) TMP_ALLOC ((rsize + 1) * BYTES_PER_MP_LIMB);
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/* Normalize the denominator, i.e. make its most significant bit set by
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shifting it NORMALIZATION_STEPS bits to the left. Also shift the
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numerator the same number of steps (to keep the quotient the same!). */
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if ((dp[dsize - 1] & GMP_NUMB_HIGHBIT) == 0)
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unsigned normalization_steps;
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count_leading_zeros (normalization_steps, dp[dsize - 1]);
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normalization_steps -= GMP_NAIL_BITS;
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/* Shift up the denominator setting the most significant bit of
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the most significant limb. Use temporary storage not to clobber
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the original contents of the denominator. */
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tp = (mp_ptr) TMP_ALLOC (dsize * BYTES_PER_MP_LIMB);
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mpn_lshift (tp, dp, dsize, normalization_steps);
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MPN_ZERO (rp, rsize - nsize);
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nlimb = mpn_lshift (rp + (rsize - nsize),
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np, nsize, normalization_steps);
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nlimb = mpn_lshift (rp, np + (nsize - rsize),
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rsize, normalization_steps);
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MPN_ZERO (rp, rsize - nsize);
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MPN_COPY (rp + (rsize - nsize), np, nsize);
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MPN_COPY (rp, np + (nsize - rsize), rsize);
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qlimb = mpn_divmod (qp, rp, rsize, dp, dsize);
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qsize = rsize - dsize;
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mp_size_t scale = nsize - dsize - N_QLIMBS;
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#if defined (__vax__)
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/* Ignore excess quotient limbs. This is necessary on a vax
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with its small double exponent, since we'd otherwise get
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exponent overflow while forming RES. */
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if (qsize > N_QLIMBS)
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qp += qsize - N_QLIMBS;
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scale += qsize - N_QLIMBS;
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for (i = qsize - 2; i >= 0; i--)
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res = res * MP_BASE_AS_DOUBLE + qp[i];
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res = __gmp_scale2 (res, GMP_NUMB_BITS * scale);
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return sign_quotient >= 0 ? res : -res;