sysdeps/generic/acos.c

   1 /* Copyright (C) 1991 Free Software Foundation, Inc.
   2 This file is part of the GNU C Library.
   3
   4 The GNU C Library is free software; you can redistribute it and/or
   5 modify it under the terms of the GNU Library General Public License as
   6 published by the Free Software Foundation; either version 2 of the
   7 License, or (at your option) any later version.
   8
   9 The GNU C Library is distributed in the hope that it will be useful,
  10 but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12 Library General Public License for more details.
  13
  14 You should have received a copy of the GNU Library General Public
  15 License along with the GNU C Library; see the file COPYING.LIB.  If
  16 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  17 Cambridge, MA 02139, USA.  */
  18
  19 #include <ansidecl.h>
  20 #include <errno.h>
  21 #include <math.h>
  22
  23 /* Return the inverse cosine of X.  */
  24 double
  25 DEFUN(acos, (x), double x)
  26 {
  27   double t;
  28
  29   if (__isnan(x))
  30     {
  31       errno = EDOM;
  32       return x;
  33     }
  34
  35   if (x == -1.0)
  36     /* If X is -1, the general formula blows up (zero divided by zero loses),
  37        but we know that acos(-1) = pi.  */
  38     t = atan2(1.0, 0.0);
  39   else
  40     t = atan2(sqrt((1.0 - x) / (1.0 + x)), 1.0);
  41   return t + t;
  42 }