c
c     file resc2cr.f
c
c  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c  .                                                             .
c  .                  copyright (c) 2008 by UCAR                 .
c  .                                                             .
c  .       UNIVERSITY CORPORATION for ATMOSPHERIC RESEARCH       .
c  .                                                             .
c  .                      all rights reserved                    .
c  .                                                             .
c  .                                                             .
c  .                      MUDPACK version 5.0.1                  .
c  .                                                             .
c  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c
c     subroutine resc2cr(nx,ny,work,res)
c
c ... purpose
c
c
c     subroutine resc2cr computes the fine grid residual in the nx by ny array
c     res after calling cud2cr or cuh2cr.  if
c
c          l * p = f
c
c     is the n by n (n = nx*ny) block tri-diagonal linear system resulting
c     from the pde discretization (done internally in cud2cr or cuh2cr) and
c     phi is the approximation to p obtained by calling cud2cr or cuh2cr,
c     then resc2cr computes the nx by ny residual array
c
c          res = f - l * phi.
c
c     one of the vector norms of res,
c
c          || res ||
c
c     can be computed as a "measure" of how well phi satisfies the
c     discretization equations.  for example, the following statements
c     will compute the location and size of the maximum residual in res
c     on cray computers:
c
c          ij = isamax(nx*ny,res,1)
c
c          jmax = (ij-1)/nx + 1
c
c          imax = ij - (jmax-1)*nx
c
c          resmax = abs(res(imax,jmax))
c
c
c *** please note:
c
c          let pe be the exact continuous solution to the elliptic pde
c          evaluated on the nx by ny discretization grid
c
c          let p be the exact solution to the linear discretization
c
c          let phi be the approximation to p generated by the mudpack solver
c
c     then discretization level error is defined by the condition
c
c          || phi - p || < || p - pe ||.
c                        =
c
c     a common measure of multigrid efficieny is that discretization level
c     error is reached in one full multigrid cycle (see references [2,9] in
c     the mudpack file "readme").  this can happen before the residual is
c     reduced to the level of roundoff error.  consequently, || res || is
c     a conservative measure of accuracy which can be wasteful if multi-
c     grid cycles are executed until it reaches the level of roundoff error.
c
c     || res || can be used to estimate the convergence rate of multigrid
c     iteration.  let r(n) be the residual and e(n) be the error after
c     executing n cycles.  they are related by the residual equation
c
c          l * e(n) = r(n).
c
c     it follows that the ratio
c
c          || r(n+1) || / || r(n) ||
c
c     estimates
c
c          || e(n+1) || / || e(n) ||
c
c     which in turn estimates the convergence rate
c
c          c = max || e(k+1) || / || e(k) ||.
c               k
c
c     notice
c                         n
c          || e(n) || <  c  || e(0) ||.
c
c
c ... assumptions (see cuh2cr.d or cud2cr.d)
c
c     (1) nx,ny have the same values as iparm(10),iparm(11) (used
c         to set the fine grid resolution when calling cud2cr or cuh2cr)
c
c     (2) work is the same argument used in calling cud2cr or cuh2cr.
c
c     (3) work has not changed since the last call to cud2cr or cuh2cr.
c
c     if these assumptions are not true then resc2cr cannot compute the
c     residual in res. (3) assures a copy of the last approximation phi
c     is contained in work.
c
      subroutine resc2cr(nx,ny,work,res)
      implicit none
      integer nx,ny,ic
      real work(*),res(nx,ny)
c
c     set pointer for fine grid coefficients in work
c
      ic = 1+(nx+2)*(ny+2)
      call rec2cr(nx,ny,work,work(ic),res)
      return
      end

      subroutine rec2cr(nx,ny,phi,cof,res)
c
c     compute residual
c
      implicit none
      integer nx,ny,i,j
      real phi(0:nx+1,0:ny+1),cof(nx,ny,10),res(nx,ny)
      do j=1,ny
	do i=1,nx
	  res(i,j) = cof(i,j,10) - (cof(i,j,9)*phi(i,j) +
     +               cof(i,j,1)*phi(i+1,j) + cof(i,j,2)*phi(i+1,j+1) +
     +               cof(i,j,3)*phi(i,j+1) + cof(i,j,4)*phi(i-1,j+1) +
     +               cof(i,j,5)*phi(i-1,j) + cof(i,j,6)*phi(i-1,j-1) +
     +               cof(i,j,7)*phi(i,j-1) + cof(i,j,8)*phi(i+1,j-1) )
	end do
      end do
      return
      end