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      1 /* Support routines for the intrinsic power (**) operator.
      2    Copyright (C) 2004-2024 Free Software Foundation, Inc.
      3    Contributed by Paul Brook
      4 
      5 This file is part of the GNU Fortran 95 runtime library (libgfortran).
      6 
      7 Libgfortran is free software; you can redistribute it and/or
      8 modify it under the terms of the GNU General Public
      9 License as published by the Free Software Foundation; either
     10 version 3 of the License, or (at your option) any later version.
     11 
     12 Libgfortran 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 General Public License for more details.
     16 
     17 Under Section 7 of GPL version 3, you are granted additional
     18 permissions described in the GCC Runtime Library Exception, version
     19 3.1, as published by the Free Software Foundation.
     20 
     21 You should have received a copy of the GNU General Public License and
     22 a copy of the GCC Runtime Library Exception along with this program;
     23 see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
     24 <http://www.gnu.org/licenses/>.  */
     25 
     26 #include "libgfortran.h"
     27 
     28 
     29 /* Use Binary Method to calculate the powi. This is not an optimal but
     30    a simple and reasonable arithmetic. See section 4.6.3, "Evaluation of
     31    Powers" of Donald E. Knuth, "Seminumerical Algorithms", Vol. 2, "The Art
     32    of Computer Programming", 3rd Edition, 1998.  */
     33 
     34 #if defined (HAVE_GFC_REAL_16) && defined (HAVE_GFC_INTEGER_16)
     35 
     36 GFC_REAL_16 pow_r16_i16 (GFC_REAL_16 a, GFC_INTEGER_16 b);
     37 export_proto(pow_r16_i16);
     38 
     39 GFC_REAL_16
     40 pow_r16_i16 (GFC_REAL_16 a, GFC_INTEGER_16 b)
     41 {
     42   GFC_REAL_16 pow, x;
     43   GFC_INTEGER_16 n;
     44   GFC_UINTEGER_16 u;
     45 
     46   n = b;
     47   x = a;
     48   pow = 1;
     49   if (n != 0)
     50     {
     51       if (n < 0)
     52 	{
     53 
     54 	  u = -n;
     55 	  x = pow / x;
     56 	}
     57       else
     58 	{
     59 	   u = n;
     60 	}
     61       for (;;)
     62 	{
     63 	  if (u & 1)
     64 	    pow *= x;
     65 	  u >>= 1;
     66 	  if (u)
     67 	    x *= x;
     68 	  else
     69 	    break;
     70 	}
     71     }
     72   return pow;
     73 }
     74 
     75 #endif
     76