// *****************************************************************
// Iterative template routine -- CHEBY
//
// CHEBY solves the symmetric positive definite linear
// system Ax = b using the Preconditioned Chebyshev Method
//
// CHEBY follows the algorithm described on p. 30 of the
// SIAM Templates book.
//
// The return value indicates convergence within max_iter (input)
// iterations (0), or no convergence within max_iter iterations (1).
//
// Upon successful return, output arguments have the following values:
//
//        x  --  approximate solution to Ax = b
// max_iter  --  the number of iterations performed before the
//               tolerance was reached
//      tol  --  the residual after the final iteration
//
// *****************************************************************

template <class Matrix, class Vector, class Preconditioner, class Real,
          class Type>
int
CHEBY(const Matrix & A, Vector & x, const Vector & b,
      const Preconditioner & M, int & max_iter, Real & tol,
      Type eigmin, Type eigmax)
{
  Real   resid;
  Type   alpha, beta, c, d;
  Vector p, q, z;

  Real   normb = norm(b);
  Vector r = b - A * x;

  if( normb == 0.0 )
    {
    normb = 1;
    }

  if( (resid = norm(r) / normb) <= tol )
    {
    tol = resid;
    max_iter = 0;
    return 0;
    }

  c = (eigmax - eigmin) / 2.0;
  d = (eigmax + eigmin) / 2.0;
  for( int i = 1; i <= max_iter; i++ )
    {
    z = M.solve(r);                 // apply preconditioner

    if( i == 1 )
      {
      p = z;
      alpha = 2.0 / d;
      }
    else
      {
      beta = c * alpha / 2.0;       // calculate new beta
      beta = beta * beta;
      alpha = 1.0 / (d - beta);     // calculate new alpha
      p = z + beta * p;             // update search direction
      }

    q = A * p;
    x += alpha * p;                 // update approximation vector
    r -= alpha * q;                 // compute residual

    if( (resid = norm(r) / normb) <= tol )
      {
      tol = resid;
      max_iter = i;
      return 0;                     // convergence
      }
    }

  tol = resid;
  return 1;                         // no convergence
}
