atl_lcm.c

基于Blas CLapck的.用过的人知道是干啥的

/*
 *             Automatically Tuned Linear Algebra Software v3.8.0
 *                    (C) Copyright 1999 R. Clint Whaley
 *
 * Code contributers : R. Clint Whaley, Antoine P. Petitet
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *   1. Redistributions of source code must retain the above copyright
 *      notice, this list of conditions and the following disclaimer.
 *   2. Redistributions in binary form must reproduce the above copyright
 *      notice, this list of conditions, and the following disclaimer in the
 *      documentation and/or other materials provided with the distribution.
 *   3. The name of the ATLAS group or the names of its contributers may
 *      not be used to endorse or promote products derived from this
 *      software without specific written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE ATLAS GROUP OR ITS CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 */

int ATL_lcm(const int M, const int N)
/*
 * Returns least common multiple (LCM) of two positive integers M & N by
 * computing greatest common divisor (GCD) and using the property that
 * M*N = GCD*LCM.
 */
{
   register int tmp, max, min, gcd=0;

   if (M != N)
   {
      if (M > N) { max = M; min = N; }
      else { max = N; min = M; }
      if (min > 0)  /* undefined for negative numbers */
      {
         do  /* while (min) */
         {
            if ( !(min & 1) ) /* min is even */
            {
               if ( !(max & 1) ) /* max is also even */
               {
                  do
                  {
                     min >>= 1;
                     max >>= 1;
                     gcd++;
                     if (min & 1) goto MinIsOdd;
                  }
                  while ( !(max & 1) );
               }
               do min >>=1 ; while ( !(min & 1) );
            }
/*
 *          Once min is odd, halve max until it too is odd.  Then, use
 *          property that gcd(max, min) = gcd(max, (max-min)/2)
 *          for odd max & min
 */
MinIsOdd:
            if (min != 1)
            {
               do  /* while (max >= min */
               {
                  max -= (max & 1) ? min : 0;
                  max >>= 1;
               }
               while (max >= min);
            }
            else return( (M*N) / (1<<gcd) );
            tmp = max;
            max = min;
            min = tmp;
         }
         while(tmp);
      }
      return( (M*N) / (max<<gcd) );
   }
   else return(M);
}