sysdeps/ieee754/dbl-64/mptan.c

   1
   2 /*
   3  * IBM Accurate Mathematical Library
   4  * written by International Business Machines Corp.
   5  * Copyright (C) 2001 Free Software Foundation
   6  *
   7  * This program 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
  10  * (at your option) any later version.
  11  *
  12  * This program is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15  * GNU Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public License
  18  * along with this program; if not, write to the Free Software
  19  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  20  */
  21 /**********************************************************************/
  22 /* MODULE_NAME:mptan.c                                                */
  23 /*                                                                    */
  24 /* FUNCTION: mptan                                                    */
  25 /*                                                                    */
  26 /* FILES NEEDED: endian.h  mpa.h                                      */
  27 /*               mpa.c  sincos32.c branred.c                          */
  28 /*                                                                    */
  29 /* Multi-Precision tan() function subroutine, for p=32.  It is based  */
  30 /* on the routines mpranred() and c32(). mpranred() performs range    */
  31 /* reduction of a double number x into a multiple precision number    */
  32 /* y, such that y=x-n*pi/2, abs(y)<pi/4, n=0,+-1,+-2,....  c32()      */
  33 /* computes both sin(y), cos(y).  tan(x) is either sin(y)/cos(y)      */
  34 /* or -cos(y)/sin(y). The precision of the result is of about 559     */
  35 /* significant bits.                                                  */
  36 /*                                                                    */
  37 /**********************************************************************/
  38 #include "endian.h"
  39 #include "mpa.h"
  40
  41 int __mpranred(double, mp_no *, int);
  42 void __c32(mp_no *, mp_no *, mp_no *, int);
  43
  44 void __mptan(double x, mp_no *mpy, int p) {
  45
  46   static const double MONE = -1.0;
  47
  48   int n;
  49   mp_no mpw, mpc, mps;
  50
  51   n = __mpranred(x, &mpw, p) & 0x00000001; /* negative or positive result */
  52   __c32(&mpw, &mpc, &mps, p);              /* computing sin(x) and cos(x) */
  53   if (n)                     /* second or fourth quarter of unit circle */
  54   { __dvd(&mpc,&mps,mpy,p);
  55     mpy->d[0] *= MONE;
  56   }                          /* tan is negative in this area */
  57   else  __dvd(&mps,&mpc,mpy,p);
  58
  59   return;
  60 }