Annotation of OpenXM_contrib/gmp/mpq/get_d.c, Revision 1.1
1.1 ! maekawa 1: /* double mpq_get_d (mpq_t src) -- Return the double approximation to SRC.
! 2:
! 3: Copyright (C) 1995, 1996 Free Software Foundation, Inc.
! 4:
! 5: This file is part of the GNU MP Library.
! 6:
! 7: The GNU MP Library is free software; you can redistribute it and/or modify
! 8: it under the terms of the GNU Library General Public License as published by
! 9: the Free Software Foundation; either version 2 of the License, or (at your
! 10: option) any later version.
! 11:
! 12: The GNU MP Library is distributed in the hope that it will be useful, but
! 13: WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
! 14: or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public
! 15: License for more details.
! 16:
! 17: You should have received a copy of the GNU Library General Public License
! 18: along with the GNU MP Library; see the file COPYING.LIB. If not, write to
! 19: the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
! 20: MA 02111-1307, USA. */
! 21:
! 22: #include "gmp.h"
! 23: #include "gmp-impl.h"
! 24: #include "longlong.h"
! 25:
! 26: /* Algorithm:
! 27: 1. Develop >= n bits of src.num / src.den, where n is the number of bits
! 28: in a double. This (partial) division will use all bits from the
! 29: denominator.
! 30: 2. Use the remainder to determine how to round the result.
! 31: 3. Assign the integral result to a temporary double.
! 32: 4. Scale the temporary double, and return the result.
! 33:
! 34: An alternative algorithm, that would be faster:
! 35: 0. Let n be somewhat larger than the number of significant bits in a double.
! 36: 1. Extract the most significant n bits of the denominator, and an equal
! 37: number of bits from the numerator.
! 38: 2. Interpret the extracted numbers as integers, call them a and b
! 39: respectively, and develop n bits of the fractions ((a + 1) / b) and
! 40: (a / (b + 1)) using mpn_divrem.
! 41: 3. If the computed values are identical UP TO THE POSITION WE CARE ABOUT,
! 42: we are done. If they are different, repeat the algorithm from step 1,
! 43: but first let n = n * 2.
! 44: 4. If we end up using all bits from the numerator and denominator, fall
! 45: back to the first algorithm above.
! 46: 5. Just to make life harder, The computation of a + 1 and b + 1 above
! 47: might give carry-out... Needs special handling. It might work to
! 48: subtract 1 in both cases instead.
! 49: */
! 50:
! 51: double
! 52: #if __STDC__
! 53: mpq_get_d (const MP_RAT *src)
! 54: #else
! 55: mpq_get_d (src)
! 56: const MP_RAT *src;
! 57: #endif
! 58: {
! 59: mp_ptr np, dp;
! 60: mp_ptr rp;
! 61: mp_size_t nsize = src->_mp_num._mp_size;
! 62: mp_size_t dsize = src->_mp_den._mp_size;
! 63: mp_size_t qsize, rsize;
! 64: mp_size_t sign_quotient = nsize ^ dsize;
! 65: unsigned normalization_steps;
! 66: mp_limb_t qlimb;
! 67: #define N_QLIMBS (1 + (sizeof (double) + BYTES_PER_MP_LIMB-1) / BYTES_PER_MP_LIMB)
! 68: mp_limb_t qp[N_QLIMBS + 1];
! 69: TMP_DECL (marker);
! 70:
! 71: if (nsize == 0)
! 72: return 0.0;
! 73:
! 74: TMP_MARK (marker);
! 75: nsize = ABS (nsize);
! 76: dsize = ABS (dsize);
! 77: np = src->_mp_num._mp_d;
! 78: dp = src->_mp_den._mp_d;
! 79:
! 80: rsize = dsize + N_QLIMBS;
! 81: rp = (mp_ptr) TMP_ALLOC ((rsize + 1) * BYTES_PER_MP_LIMB);
! 82:
! 83: count_leading_zeros (normalization_steps, dp[dsize - 1]);
! 84:
! 85: /* Normalize the denominator, i.e. make its most significant bit set by
! 86: shifting it NORMALIZATION_STEPS bits to the left. Also shift the
! 87: numerator the same number of steps (to keep the quotient the same!). */
! 88: if (normalization_steps != 0)
! 89: {
! 90: mp_ptr tp;
! 91: mp_limb_t nlimb;
! 92:
! 93: /* Shift up the denominator setting the most significant bit of
! 94: the most significant limb. Use temporary storage not to clobber
! 95: the original contents of the denominator. */
! 96: tp = (mp_ptr) TMP_ALLOC (dsize * BYTES_PER_MP_LIMB);
! 97: mpn_lshift (tp, dp, dsize, normalization_steps);
! 98: dp = tp;
! 99:
! 100: if (rsize > nsize)
! 101: {
! 102: MPN_ZERO (rp, rsize - nsize);
! 103: nlimb = mpn_lshift (rp + (rsize - nsize),
! 104: np, nsize, normalization_steps);
! 105: }
! 106: else
! 107: {
! 108: nlimb = mpn_lshift (rp, np + (nsize - rsize),
! 109: rsize, normalization_steps);
! 110: }
! 111: if (nlimb != 0)
! 112: {
! 113: rp[rsize] = nlimb;
! 114: rsize++;
! 115: }
! 116: }
! 117: else
! 118: {
! 119: if (rsize > nsize)
! 120: {
! 121: MPN_ZERO (rp, rsize - nsize);
! 122: MPN_COPY (rp + (rsize - nsize), np, nsize);
! 123: }
! 124: else
! 125: {
! 126: MPN_COPY (rp, np + (nsize - rsize), rsize);
! 127: }
! 128: }
! 129:
! 130: qlimb = mpn_divmod (qp, rp, rsize, dp, dsize);
! 131: qsize = rsize - dsize;
! 132: if (qlimb)
! 133: {
! 134: qp[qsize] = qlimb;
! 135: qsize++;
! 136: }
! 137:
! 138: {
! 139: double res;
! 140: mp_size_t i;
! 141:
! 142: res = qp[qsize - 1];
! 143: for (i = qsize - 2; i >= 0; i--)
! 144: res = res * MP_BASE_AS_DOUBLE + qp[i];
! 145:
! 146: res = __gmp_scale2 (res, BITS_PER_MP_LIMB * (nsize - dsize - N_QLIMBS));
! 147:
! 148: TMP_FREE (marker);
! 149: return sign_quotient >= 0 ? res : -res;
! 150: }
! 151: }
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