program main

!*****************************************************************************80
!
!! MAIN is the main program for QUAD.
!
!  Discussion:
!
!    QUAD is a FORTRAN90 program to estimate the integral of
!      f = 50.0 / ( pi * ( 2500.0 * x(i) * x(i) + 1.0 ) )
!    from 0 to 10.
!
!    If you modify the program in 4 places, it should run correctly
!    under OpenMP.
!
!    If you can do that, then compare the execution time when you
!    use 1, 2, 4 or 8 threads.
!
!  Licensing:
!
!    This code is distributed under the GNU LGPL license.
!
!  Modified:
!
!    03 September 2013
!
!  Author:
!
!    John Burkardt
!

!
!  CHANGE 1) Activate the following USE statement.
!
!     use omp_lib

  double precision a
  double precision b
  double precision error
  double precision exact
  integer i
  integer n
  double precision pi
  double precision total
  double precision wtime
  double precision x(1000000)

  a = 0.0D+00
  b = 10.0D+00
  exact = 0.49936338107645674464D+00
  n = 1000000
  pi = 3.141592653589793D+00
  wtime = 0.0D+00

  write ( *, * ) ' '
  write ( *, * ) 'QUAD:'
  write ( *, * ) '  FORTRAN90 version'
  write ( *, * ) ' '
  write ( *, * ) '  Estimate the integral of f(x) from A to B.'
  write ( *, * ) '  f(x) = 50 / ( pi * ( 2500 * x * x + 1 ) ).'
  write ( *, * ) '  A = ', a
  write ( *, * ) '  B = ', b
  write ( *, * ) '  Exact integral from 0 to 10 is 0.49936338107645674464...'
!
!  Load the array X with evenly spaced values between A and B.
!
  do i = 1, n
    x(i) =  ( dble ( n - i     ) * a   &
            + dble (     i - 1 ) * b ) &
            / dble ( n     - 1 )
  end do
!
!  Evaluate F at each node, sum and average.
!
!  CHANGE 2) Use the OpenMP timer here to initialize WTIME.
!
!  wtime = ?

   total = 0.0D+00
!
!  CHANGE 3) Fill in these two OpenMP directives here
!
!$omp parallel private ( ??? ) shared ( ??? )
!$omp do ???
  do i = 1, n
    total = total + 50.0D+00 &
      / ( pi * ( 2500.0D+00 * x(i) * x(i) + 1.0D+00 ) )
  end do
!
!  CHANGE 3.1) Insert "closing" OpenMP directives after the loop
!
!$omp end do
!$omp end parallel
!
  total = ( b - a ) * total / dble ( n )
!
!  CHANGE 4) Call the OpenMP timer here to update WTIME.
!
! wtime = ?
!
!  Print quadrature estimate, error, time
!
  error = abs ( total - exact )

  write ( *, * ) ' '
  write ( *, * ) '  Estimate = ', total
  write ( *, * ) '  Error    = ', error
  write ( *, * ) '  Wallclock time   = ', wtime

  stop
end