Annotation of OpenXM/src/kan96xx/gmp-2.0.2-ssh-2/mpf/set_q.c, Revision 1.1
1.1 ! takayama 1: /* mpf_set_q (mpf_t rop, mpq_t op) -- Convert the rational op to the float rop.
! 2:
! 3: Copyright (C) 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: void
! 52: #if __STDC__
! 53: mpf_set_q (mpf_t r, mpq_srcptr q)
! 54: #else
! 55: mpf_set_q (r, q)
! 56: mpf_t r;
! 57: mpq_srcptr q;
! 58: #endif
! 59: {
! 60: mp_ptr np, dp;
! 61: mp_ptr rp;
! 62: mp_size_t nsize, dsize;
! 63: mp_size_t qsize, rsize;
! 64: mp_size_t sign_quotient;
! 65: unsigned normalization_steps;
! 66: mp_limb_t qlimb;
! 67: mp_ptr qp;
! 68: mp_size_t prec;
! 69: mp_exp_t exp;
! 70: TMP_DECL (marker);
! 71:
! 72: nsize = SIZ (&q->_mp_num);
! 73: dsize = SIZ (&q->_mp_den);
! 74:
! 75: if (nsize == 0)
! 76: {
! 77: SIZ (r) = 0;
! 78: EXP (r) = 0;
! 79: return;
! 80: }
! 81:
! 82: prec = PREC (r) + 1;
! 83:
! 84: TMP_MARK (marker);
! 85:
! 86: qp = PTR (r);
! 87:
! 88: sign_quotient = nsize ^ dsize;
! 89: nsize = ABS (nsize);
! 90: dsize = ABS (dsize);
! 91: np = PTR (&q->_mp_num);
! 92: dp = PTR (&q->_mp_den);
! 93:
! 94: exp = nsize - dsize;
! 95:
! 96: if (nsize > prec)
! 97: {
! 98: np += nsize - prec;
! 99: nsize = prec;
! 100: }
! 101: if (dsize > prec)
! 102: {
! 103: dp += dsize - prec;
! 104: dsize = prec;
! 105: }
! 106:
! 107: rsize = MAX (nsize, dsize);
! 108: rp = (mp_ptr) TMP_ALLOC ((rsize + 1) * BYTES_PER_MP_LIMB);
! 109:
! 110: count_leading_zeros (normalization_steps, dp[dsize - 1]);
! 111:
! 112: /* Normalize the denominator, i.e. make its most significant bit set by
! 113: shifting it NORMALIZATION_STEPS bits to the left. Also shift the
! 114: numerator the same number of steps (to keep the quotient the same!). */
! 115: if (normalization_steps != 0)
! 116: {
! 117: mp_ptr tp;
! 118: mp_limb_t nlimb;
! 119:
! 120: /* Shift up the denominator setting the most significant bit of
! 121: the most significant limb. Use temporary storage not to clobber
! 122: the original contents of the denominator. */
! 123: tp = (mp_ptr) TMP_ALLOC (dsize * BYTES_PER_MP_LIMB);
! 124: mpn_lshift (tp, dp, dsize, normalization_steps);
! 125: dp = tp;
! 126:
! 127: if (rsize != nsize)
! 128: {
! 129: MPN_ZERO (rp, rsize - nsize);
! 130: nlimb = mpn_lshift (rp + (rsize - nsize),
! 131: np, nsize, normalization_steps);
! 132: }
! 133: else
! 134: {
! 135: nlimb = mpn_lshift (rp, np, nsize, normalization_steps);
! 136: }
! 137: if (nlimb != 0)
! 138: {
! 139: rp[rsize] = nlimb;
! 140: rsize++;
! 141: exp++;
! 142: }
! 143: }
! 144: else
! 145: {
! 146: if (rsize != nsize)
! 147: {
! 148: MPN_ZERO (rp, rsize - nsize);
! 149: MPN_COPY (rp + (rsize - nsize), np, nsize);
! 150: }
! 151: else
! 152: {
! 153: MPN_COPY (rp, np, rsize);
! 154: }
! 155: }
! 156:
! 157: qlimb = mpn_divrem (qp, prec - 1 - (rsize - dsize), rp, rsize, dp, dsize);
! 158: qsize = prec - 1;
! 159: if (qlimb)
! 160: {
! 161: qp[qsize] = qlimb;
! 162: qsize++;
! 163: exp++;
! 164: }
! 165:
! 166: EXP (r) = exp;
! 167: SIZ (r) = qsize;
! 168:
! 169: TMP_FREE (marker);
! 170: }
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