
//@HEADER
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// 
//               HPCCG: Simple Conjugate Gradient Benchmark Code
//                 Copyright (2006) Sandia Corporation
// 
// Under terms of Contract DE-AC04-94AL85000, there is a non-exclusive
// license for use of this work by or on behalf of the U.S. Government.
// 
// This library is free software; you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as
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// Lesser General Public License for more details.
//  
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
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// Questions? Contact Michael A. Heroux (maherou@sandia.gov) 
// 
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/////////////////////////////////////////////////////////////////////////

// Routine to compute the 1-norm difference between two vectors where:

// n - number of vector elements (on this processor)

// v1, v2 - input vectors

// residual - pointer to scalar value, on exit will contain result.

/////////////////////////////////////////////////////////////////////////

#include <cmath>  // needed for fabs
using std::fabs;
#include "compute_residual.hpp"

int compute_residual(const int n, const double * const v1, 
		     const double * const v2, double * const residual)
{
  double local_residual = 0.0;
  
  for (int i=0; i<n; i++) {
    double diff = fabs(v1[i] - v2[i]);
    if (diff > local_residual) local_residual = diff;
  }
#ifdef USING_MPI
  // Use MPI's reduce function to collect all partial sums

  double global_residual = 0;
  
  MPI_Allreduce(&local_residual, &global_residual, 1, MPI_DOUBLE, MPI_MAX,
                MPI_COMM_WORLD);
  *residual = global_residual;
#else
  *residual = local_residual;
#endif

  return(0);
}