xref: /dflybsd-src/contrib/mpfr/src/isqrt.c (revision 2786097444a0124b5d33763854de247e230c6629)

/* __gmpfr_isqrt && __gmpfr_cuberoot -- Integer square root and cube root

Copyright 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
Contributed by the AriC and Caramel projects, INRIA.

This file is part of the GNU MPFR Library.

The GNU MPFR Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at your
option) any later version.

The GNU MPFR Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MPFR Library; see the file COPYING.LESSER.  If not, see
http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */

#include "mpfr-impl.h"

/* returns floor(sqrt(n)) */
unsigned long
__gmpfr_isqrt (unsigned long n)
{
  unsigned long i, s;

  /* First find an approximation to floor(sqrt(n)) of the form 2^k. */
  i = n;
  s = 1;
  while (i >= 2)
    {
      i >>= 2;
      s <<= 1;
    }

  do
    {
      s = (s + n / s) / 2;
    }
  while (!(s*s <= n && (s*s > s*(s+2) || n <= s*(s+2))));
  /* Short explanation: As mathematically s*(s+2) < 2*ULONG_MAX,
     the condition s*s > s*(s+2) is evaluated as true when s*(s+2)
     "overflows" but not s*s. This implies that mathematically, one
     has s*s <= n <= s*(s+2). If s*s "overflows", this means that n
     is "large" and the inequality n <= s*(s+2) cannot be satisfied. */
  return s;
}

/* returns floor(n^(1/3)) */
unsigned long
__gmpfr_cuberoot (unsigned long n)
{
  unsigned long i, s;

  /* First find an approximation to floor(cbrt(n)) of the form 2^k. */
  i = n;
  s = 1;
  while (i >= 4)
    {
      i >>= 3;
      s <<= 1;
    }

  /* Improve the approximation (this is necessary if n is large, so that
     mathematically (s+1)*(s+1)*(s+1) isn't much larger than ULONG_MAX). */
  if (n >= 256)
    {
      s = (2 * s + n / (s * s)) / 3;
      s = (2 * s + n / (s * s)) / 3;
      s = (2 * s + n / (s * s)) / 3;
    }

  do
    {
      s = (2 * s + n / (s * s)) / 3;
    }
  while (!(s*s*s <= n && (s*s*s > (s+1)*(s+1)*(s+1) ||
                          n < (s+1)*(s+1)*(s+1))));
  return s;
}