Annotation of OpenXM_contrib/gmp/mpfr/cmp2.c, Revision 1.1
1.1 ! ohara 1: /* mpfr_cmp2 -- exponent shift when subtracting two numbers.
! 2:
! 3: Copyright 1999, 2000, 2001 Free Software Foundation.
! 4:
! 5: This file is part of the MPFR Library.
! 6:
! 7: The MPFR Library is free software; you can redistribute it and/or modify
! 8: it under the terms of the GNU Lesser General Public License as published by
! 9: the Free Software Foundation; either version 2.1 of the License, or (at your
! 10: option) any later version.
! 11:
! 12: The MPFR 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 Lesser General Public
! 15: License for more details.
! 16:
! 17: You should have received a copy of the GNU Lesser General Public License
! 18: along with the MPFR 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: #include "mpfr.h"
! 26: #include "mpfr-impl.h"
! 27:
! 28: /* Returns the number of canceled bits when one subtracts |c| from |b|
! 29: if |b| != |c|, and the sign.
! 30:
! 31: Assumes neither of b or c is NaN or +/- infinity.
! 32:
! 33: In other terms, if |b| != |c|, mpfr_cmp2 (b, c) returns
! 34: EXP(max(|b|,|c|)) - EXP(|b| - |c|).
! 35: */
! 36:
! 37: int
! 38: mpfr_cmp2 (mpfr_srcptr b, mpfr_srcptr c, mp_prec_t *cancel)
! 39: {
! 40: mp_limb_t *bp, *cp, bb, cc = 0, lastc = 0, dif, high_dif = 0;
! 41: mp_size_t bn, cn;
! 42: mp_exp_unsigned_t diff_exp;
! 43: mp_prec_t res = 0;
! 44: int sign;
! 45:
! 46: MPFR_ASSERTN(MPFR_IS_FP(b));
! 47: MPFR_ASSERTN(MPFR_IS_FP(c));
! 48:
! 49: /* Optimized case x - x */
! 50: if (b == c)
! 51: return 0;
! 52:
! 53: if (MPFR_IS_ZERO(b))
! 54: {
! 55: if (MPFR_IS_ZERO(c))
! 56: return 0;
! 57:
! 58: *cancel = 0;
! 59: return -1;
! 60: }
! 61:
! 62: if (MPFR_IS_ZERO(c))
! 63: {
! 64: *cancel = 0;
! 65: return 1;
! 66: }
! 67:
! 68: if (MPFR_EXP(b) >= MPFR_EXP(c))
! 69: {
! 70: sign = 1;
! 71: diff_exp = (mp_exp_unsigned_t) MPFR_EXP(b) - MPFR_EXP(c);
! 72:
! 73: bp = MPFR_MANT(b);
! 74: cp = MPFR_MANT(c);
! 75:
! 76: bn = (MPFR_PREC(b) - 1) / BITS_PER_MP_LIMB;
! 77: cn = (MPFR_PREC(c) - 1) / BITS_PER_MP_LIMB;
! 78:
! 79: if (diff_exp == 0)
! 80: {
! 81: while (bn >= 0 && cn >= 0 && bp[bn] == cp[cn])
! 82: {
! 83: bn--;
! 84: cn--;
! 85: res += BITS_PER_MP_LIMB;
! 86: }
! 87:
! 88: if (bn < 0)
! 89: {
! 90: if (cn < 0) /* b = c */
! 91: return 0;
! 92:
! 93: bp = cp;
! 94: bn = cn;
! 95: cn = -1;
! 96: sign = -1;
! 97: }
! 98:
! 99: if (cn < 0) /* c discards exactly the upper part of b */
! 100: {
! 101: unsigned int z;
! 102:
! 103: MPFR_ASSERTN(bn >= 0);
! 104:
! 105: while (bp[bn] == 0)
! 106: {
! 107: if (--bn < 0) /* b = c */
! 108: return 0;
! 109: res += BITS_PER_MP_LIMB;
! 110: }
! 111:
! 112: count_leading_zeros(z, bp[bn]); /* bp[bn] <> 0 */
! 113: *cancel = res + z;
! 114: return sign;
! 115: }
! 116:
! 117: MPFR_ASSERTN(bn >= 0);
! 118: MPFR_ASSERTN(cn >= 0);
! 119: MPFR_ASSERTN(bp[bn] != cp[cn]);
! 120: if (bp[bn] < cp[cn])
! 121: {
! 122: mp_limb_t *tp;
! 123: mp_size_t tn;
! 124:
! 125: tp = bp; bp = cp; cp = tp;
! 126: tn = bn; bn = cn; cn = tn;
! 127: sign = -1;
! 128: }
! 129: }
! 130: } /* MPFR_EXP(b) >= MPFR_EXP(c) */
! 131: else /* MPFR_EXP(b) < MPFR_EXP(c) */
! 132: {
! 133: sign = -1;
! 134: diff_exp = (mp_exp_unsigned_t) MPFR_EXP(c) - MPFR_EXP(b);
! 135:
! 136: bp = MPFR_MANT(c);
! 137: cp = MPFR_MANT(b);
! 138:
! 139: bn = (MPFR_PREC(c) - 1) / BITS_PER_MP_LIMB;
! 140: cn = (MPFR_PREC(b) - 1) / BITS_PER_MP_LIMB;
! 141: }
! 142:
! 143: /* now we have removed the identical upper limbs of b and c
! 144: (can happen only when diff_exp = 0), and after the possible
! 145: swap, we have |b| > |c|: bp[bn] > cc, bn >= 0, cn >= 0 */
! 146:
! 147: if (diff_exp < BITS_PER_MP_LIMB)
! 148: {
! 149: cc = cp[cn] >> diff_exp;
! 150: /* warning: a shift by BITS_PER_MP_LIMB may give wrong results */
! 151: if (diff_exp)
! 152: lastc = cp[cn] << (BITS_PER_MP_LIMB - diff_exp);
! 153: cn--;
! 154: }
! 155: else
! 156: diff_exp -= BITS_PER_MP_LIMB;
! 157:
! 158: dif = bp[bn--] - cc; /* necessarily dif >= 1 */
! 159:
! 160: while ((cn >= 0 || lastc != 0) && (high_dif == 0) && (dif == 1))
! 161: { /* dif=1 implies diff_exp = 0 or 1 */
! 162: bb = (bn >= 0) ? bp[bn] : 0;
! 163: cc = lastc;
! 164: if (cn >= 0)
! 165: {
! 166: if (diff_exp == 0)
! 167: {
! 168: cc += cp[cn];
! 169: }
! 170: else /* diff_exp = 1 */
! 171: {
! 172: cc += cp[cn] >> 1;
! 173: lastc = cp[cn] << (BITS_PER_MP_LIMB - 1);
! 174: }
! 175: }
! 176: else
! 177: lastc = 0;
! 178: high_dif = 1 - mpn_sub_n (&dif, &bb, &cc, 1);
! 179: bn--;
! 180: cn--;
! 181: res += BITS_PER_MP_LIMB;
! 182: }
! 183:
! 184: /* (cn<0 and lastc=0) or (high_dif,dif)<>(0,1) */
! 185:
! 186: if (high_dif != 0) /* necessarily high_dif = 1 */
! 187: {
! 188: res--;
! 189: if (dif != 0)
! 190: {
! 191: *cancel = res;
! 192: return sign;
! 193: }
! 194: }
! 195: else /* high_dif = 0 */
! 196: {
! 197: unsigned int z;
! 198:
! 199: count_leading_zeros(z, dif); /* dif > 1 here */
! 200: res += z;
! 201: if (dif != (MP_LIMB_T_ONE << (BITS_PER_MP_LIMB - z - 1)))
! 202: { /* dif is not a power of two */
! 203: *cancel = res;
! 204: return sign;
! 205: }
! 206: }
! 207:
! 208: /* now result is res + (low(b) < low(c)) */
! 209: while (bn >= 0 && (cn >= 0 || lastc != 0))
! 210: {
! 211: if (diff_exp >= BITS_PER_MP_LIMB)
! 212: diff_exp -= BITS_PER_MP_LIMB;
! 213: else
! 214: {
! 215: cc = lastc;
! 216: if (cn >= 0)
! 217: {
! 218: cc += cp[cn] >> diff_exp;
! 219: if (diff_exp != 0)
! 220: lastc = cp[cn] << (BITS_PER_MP_LIMB - diff_exp);
! 221: }
! 222: else
! 223: lastc = 0;
! 224: cn--;
! 225: }
! 226: if (bp[bn] != cc)
! 227: {
! 228: *cancel = res + (bp[bn] < cc);
! 229: return sign;
! 230: }
! 231: bn--;
! 232: }
! 233:
! 234: if (bn < 0)
! 235: {
! 236: if (lastc != 0)
! 237: res++;
! 238: else
! 239: {
! 240: while (cn >= 0 && cp[cn] == 0)
! 241: cn--;
! 242: if (cn >= 0)
! 243: res++;
! 244: }
! 245: }
! 246:
! 247: *cancel = res;
! 248: return sign;
! 249: }
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