program main

!*****************************************************************************80
!
!! grid_surface() draws a surface from a table Z(X,Y).
!
!  Discussion:
!
!    Here, the grid is a 41x41 array of equally spaced points in [-2,2]x[-2,2].
!
!    and Z(X,Y) is a function which can be evaluated by:
!
!      Z = exp ( - ( X^2 + Y^2 ) ) * cos ( 0.25 * X )
!        * sin ( Y ) * cos ( 2 * (X^2 + Y^2 ) )
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    16 April 2020
!
!  Author:
!
!    John Burkardt
!
  implicit none
  
  integer, parameter :: rk = kind ( 1.0D+00 )

  integer, parameter :: m = 41
  integer, parameter :: n = 41

  character ( len = 255 ) command_filename
  integer command_unit
  character ( len = 255 ) data_filename
  integer data_unit
  character ( len = 12 ) :: header = 'grid_surface'
  integer i
  integer j
  real ( kind = rk ) r2
  real ( kind = rk ) x
  real ( kind = rk ) y
  real ( kind = rk ) z

  write ( *, '(a)' ) ''
  write ( *, '(a)' ) 'grid_surface():'
  write ( *, '(a)' ) '  Fortran90 version'
  write ( *, '(a)' ) '  Make a surface plot Z(X,Y) for a 41x41 table.'
!
!  Create the data file.
!
  call get_unit ( data_unit )

  data_filename = header // '_data.txt'

  open ( unit = data_unit, file = data_filename, status = 'replace' )

  do i = 1, m
    if ( 1 < i ) then
      write ( data_unit, '(a)' )
    end if
    x = ( real ( m - i,     kind = rk ) * ( -2.0 ) & 
        + real (     i - 1, kind = rk ) * (  2.0 ) ) & 
        / real ( m     - 1, kind = rk )
    do j = 1, n
      y = ( real ( n - j,     kind = rk ) * ( -2.0 ) & 
          + real (     j - 1, kind = rk ) * (  2.0 ) ) & 
          / real ( n     - 1, kind = rk )
      r2 = x**2 + y**2
      z = exp ( - r2 ) * cos ( 0.25 * x ) * sin ( y ) * cos ( 2 * r2 )
      write ( data_unit, '(3(2x,g14.6))' ) x, y, z
    end do
  end do

  close ( unit = data_unit )

  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) '  grid_surface: data stored in "' &
    // trim ( data_filename ) // '".'
!
!  Create the command file.
!
  call get_unit ( command_unit )

  command_filename = trim ( header ) // '_commands.txt'

  open ( unit = command_unit, file = command_filename, status = 'replace' )

  write ( command_unit, '(a)' ) '# ' // trim ( command_filename )
  write ( command_unit, '(a)' ) '#'
  write ( command_unit, '(a)' ) '# Usage:'
  write ( command_unit, '(a)' ) '#  gnuplot < ' // trim ( command_filename )
  write ( command_unit, '(a)' ) '#'
  write ( command_unit, '(a)' ) 'set term png'
  write ( command_unit, '(a)' ) 'set output "' // trim ( header ) // '.png"'
  write ( command_unit, '(a)' ) 'set xlabel "X"'
  write ( command_unit, '(a)' ) 'set ylabel "Y"'
  write ( command_unit, '(a)' ) 'set zlabel "Z"'
  write ( command_unit, '(a)' ) 'set title "Grid Surface"'
  write ( command_unit, '(a)' ) 'set view 77, 77'
  write ( command_unit, '(a)' ) 'set xyplane -0.5'
  write ( command_unit, '(a)' ) 'set hidden3d'
  write ( command_unit, '(a)' ) 'set timestamp'
  write ( command_unit, '(a)' ) 'set grid'
  write ( command_unit, '(a)' ) 'set style data lines'
  write ( command_unit, '(a)' ) 'splot "' // trim ( data_filename ) // &
    '" with lines'
  write ( command_unit, '(a)' ) 'quit'

  close ( unit = command_unit )

  write ( *, '(a)' ) '  grid_surface: plot commands stored in "' &
    // trim ( command_filename ) // '".'
!
!  Terminate.
!
  write ( *, '(a)' ) ''
  write ( *, '(a)' ) 'grid_surface():'
  write ( *, '(a)' ) '  Normal end of execution.'

  stop ( 0 )
end
subroutine get_unit ( iunit )

!*****************************************************************************80
!
!! GET_UNIT returns a free FORTRAN unit number.
!
!  Discussion:
!
!    A "free" FORTRAN unit number is a value between 1 and 99 which
!    is not currently associated with an I/O device.  A free FORTRAN unit
!    number is needed in order to open a file with the OPEN command.
!
!    If IUNIT = 0, then no free FORTRAN unit could be found, although
!    all 99 units were checked (except for units 5, 6 and 9, which
!    are commonly reserved for console I/O).
!
!    Otherwise, IUNIT is a value between 1 and 99, representing a
!    free FORTRAN unit.  Note that GET_UNIT assumes that units 5 and 6
!    are special, and will never return those values.
!
!  Licensing:
!
!    This code is distributed under the MIT license. 
!
!  Modified:
!
!    26 October 2008
!
!  Author:
!
!    John Burkardt
!
!  Parameters:
!
!    Output, integer IUNIT, the free unit number.
!
  implicit none

  integer i
  integer ios
  integer iunit
  logical lopen
 
  iunit = 0
 
  do i = 1, 99
 
    if ( i /= 5 .and. i /= 6 .and. i /= 9 ) then

      inquire ( unit = i, opened = lopen, iostat = ios )
 
      if ( ios == 0 ) then
        if ( .not. lopen ) then
          iunit = i
          return
        end if
      end if

    end if
 
  end do

  return
end