program main

!*****************************************************************************80
!
!! clausen_test() tests clausen().
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    06 September 2021
!
!  Author:
!
!    John Burkardt
!
  implicit none

  call timestamp ( )
  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) 'clausen_test():'
  write ( *, '(a)' ) '  Fortran90 version'
  write ( *, '(a)' ) '  Test clausen().'

  call clausen_values_test ( )
  call clausen_plot_test ( )
!
!  Terminate.
!
  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) 'clausen_test():'
  write ( *, '(a)' ) '  Normal end of execution.'
  write ( *, '(a)' ) ' '
  call timestamp ( )

  stop 0
end
subroutine clausen_values_test ( )

!*****************************************************************************80
!
!! clausen_values_test() tests clausen() against a table of values.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    06 September 2021
!
!  Author:
!
!    John Burkardt
!
  implicit none

  integer, parameter :: rk8 = kind ( 1.0D+00 )

  real ( kind = rk8 ) clausen
  real ( kind = rk8 ) diff
  real ( kind = rk8 ) fx1
  real ( kind = rk8 ) fx2
  integer n_data
  real ( kind = rk8 ) x

  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) 'clausen_values_test():'
  write ( *, '(a)' ) '  clausen() evaluates the Clausen function.'
  write ( *, '(a)' ) '  Compare its results to tabulated data.'
  write ( *, '(a)' ) ''
  write ( *, '(a)' ) '                               Tabulated               Computed'
  write ( *, '(a)' ) '           X                          FX                     FX        Diff'
  write ( *, '(a)' ) ''

  n_data = 0

  do

    call clausen_values ( n_data, x, fx1 )

    if ( n_data == 0 ) then
      exit
    end if

    fx2 = clausen ( x )

    diff = abs ( fx1 - fx2 )

    write ( *, '(2x,g14.6,2x,g24.16,2x,g24.16,2x,e7.1)' ) x, fx1, fx2, diff

  end do

  return
end

subroutine clausen_plot_test ( )

!*****************************************************************************80
!
!! clausen_plot_test() plots the clausen function.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    30 March 2025
!
!  Author:
!
!    John Burkardt
!
  implicit none

  integer, parameter :: rk8 = kind ( 1.0D+00 )

  real ( kind = rk8 ) a
  real ( kind = rk8 ) b
  real ( kind = rk8 ) clausen
  real ( kind = rk8 ), allocatable :: fx(:)
  character ( len = 50 ) header
  integer i
  integer nx
  real ( kind = rk8 ), parameter :: r8_pi = 3.141592653589793D+00
  real ( kind = rk8 ), allocatable :: x(:)

  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) 'clausen_plot_test():'
  write ( *, '(a)' ) '  Plot the clausen() function.'

  nx = 201
  a = 0.0D+00
  b = 4.0D+00 * r8_pi
  allocate ( x(1:nx) )
  call r8vec_linspace ( nx, a, b, x )
  allocate ( fx(1:nx) )
  do i = 1, nx
    fx(i) = clausen ( x(i) )
  end do

  header = 'clausen_test'
  call gnuplot_fx ( nx, x, fx, trim ( header ) )

  deallocate ( fx )
  deallocate ( x )

  return
end
subroutine clausen_values ( n_data, x, fx )

!*****************************************************************************80
!
!! clausen_values() returns some values of the Clausen's integral.
!
!  Discussion:
!
!    The function is defined by:
!
!      CLAUSEN(x) = integral ( 0 <= t <= x ) -ln ( 2 * sin ( t / 2 ) ) dt
!
!    The data was reported by McLeod.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    06 September 2021
!
!  Author:
!
!    John Burkardt
!
!  Reference:
!
!    Milton Abramowitz, Irene Stegun,
!    Handbook of Mathematical Functions,
!    National Bureau of Standards, 1964,
!    ISBN: 0-486-61272-4,
!    LC: QA47.A34.
!
!    Allan McLeod,
!    Algorithm 757:
!    MISCFUN: A software package to compute uncommon special functions,
!    ACM Transactions on Mathematical Software,
!    Volume 22, Number 3, September 1996, pages 288-301.
!
!  Input:
!
!    integer N_DATA: the user sets N_DATA to 0 before the first call.
!
!  Output:
!
!    integer N_DATA.  On each call, the routine increments N_DATA by 1,
!    and returns the corresponding data; when there is no more data, the
!    output value of N_DATA will be 0 again.
!
!    real ( kind = rk8 ) X, the argument of the function.
!
!    real ( kind = rk8 ) FX, the value of the function.
!
  implicit none

  integer, parameter :: rk8 = kind ( 1.0D+00 )

  integer, parameter :: n_max = 20

  real ( kind = rk8 ) fx
  real ( kind = rk8 ), save, dimension ( n_max ) :: fx_vec = (/ &
     0.14137352886760576684D-01, &
     0.13955467081981281934D+00, &
    -0.38495732156574238507D+00, &
     0.84831187770367927099D+00, &
     0.10139591323607685043D+01, &
    -0.93921859275409211003D+00, &
     0.72714605086327924743D+00, &
     0.43359820323553277936D+00, &
    -0.98026209391301421161D-01, &
    -0.56814394442986978080D+00, &
    -0.70969701784448921625D+00, &
     0.99282013254695671871D+00, &
    -0.98127747477447367875D+00, &
    -0.64078266570172320959D+00, &
     0.86027963733231192456D+00, &
     0.39071647608680211043D+00, &
     0.47574793926539191502D+00, &
     0.10105014481412878253D+01, &
     0.96332089044363075154D+00, &
    -0.61782699481929311757D+00 /)
  integer n_data
  real ( kind = rk8 ) x
  real ( kind = rk8 ), save, dimension ( n_max ) :: x_vec = (/ &
      0.0019531250D+00, &
      0.0312500000D+00, &
     -0.1250000000D+00, &
      0.5000000000D+00, &
      1.0000000000D+00, &
     -1.5000000000D+00, &
      2.0000000000D+00, &
      2.5000000000D+00, &
     -3.0000000000D+00, &
      4.0000000000D+00, &
      4.2500000000D+00, &
     -5.0000000000D+00, &
      5.5000000000D+00, &
      6.0000000000D+00, &
      8.0000000000D+00, &
    -10.0000000000D+00, &
     15.0000000000D+00, &
     20.0000000000D+00, &
    -30.0000000000D+00, &
     50.0000000000D+00 /)

  if ( n_data < 0 ) then
    n_data = 0
  end if

  n_data = n_data + 1

  if ( n_max < n_data ) then
    n_data = 0
    x = 0.0D+00
    fx = 0.0D+00
  else
    x = x_vec(n_data)
    fx = fx_vec(n_data)
  end if

  return
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.  The code 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
!
!  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
subroutine gnuplot_fx ( n, x, y, header )

!*****************************************************************************80
!
!! gnuplot_fx() plots x, f(x) data.
!
!  Discussion:
!
!    Actually, we simply create two files for processing by gnuplot().
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    24 March 2025
!
!  Author:
!
!    John Burkardt
!
  implicit none

  integer, parameter :: rk8 = kind ( 1.0D+00 )

  integer n

  character ( len = 255 ) command_filename
  integer command_unit
  character ( len = 255 ) data_filename
  integer data_unit
  character ( len = * ) header
  integer i
  character ( len = 255 ) png_filename
  real ( kind = rk8 ) x(n)
  real ( kind = rk8 ) y(n)
!
!  Create a graphics data file.
!
  data_filename = header // '_data.txt'
  call get_unit ( data_unit )

  open ( unit = data_unit, file = data_filename, status = 'replace' )
  do i = 1, n
    write ( data_unit, '(2g14.6)' ) x(i), y(i)
  end do
  close ( unit = data_unit )
  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) '  Created graphics data file "' &
    // trim ( data_filename ) // '".'

  png_filename = header // '.png'
!
!  Create graphics command file.
!
  call get_unit ( command_unit )
  command_filename = 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 ( png_filename ) // '"'
  write ( command_unit, '(a)' ) 'set xlabel "<--- X --->"'
  write ( command_unit, '(a)' ) 'set ylabel "<-- Y(X) -->"'
  write ( command_unit, '(a)' ) 'set title "' // header // '"'
  write ( command_unit, '(a)' ) 'set grid'
  write ( command_unit, '(a)' ) 'set style data lines'
  write ( command_unit, '(a)' ) 'plot "' // trim ( data_filename ) // &
    '" using 1:2 lw 3 linecolor rgb "blue"'
  close ( unit = command_unit )
  write ( *, '(a)' ) &
    '  Created command file "' // trim ( command_filename ) // '".'

  return
end
subroutine r8vec_linspace ( n, a, b, x )

!*****************************************************************************80
!
!! r8vec_linspace() creates a vector of linearly spaced values.
!
!  Discussion:
!
!    An R8VEC is a vector of R8's.
!
!    4 points evenly spaced between 0 and 12 will yield 0, 4, 8, 12.
!
!    In other words, the interval is divided into N-1 even subintervals,
!    and the endpoints of intervals are used as the points.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    14 March 2011
!
!  Author:
!
!    John Burkardt
!
!  Input:
!
!    integer N, the number of entries in the vector.
!
!    real ( kind = rk8 ) A, B, the first and last entries.
!
!  Output:
!
!    real ( kind = rk8 ) X(N), a vector of linearly spaced data.
!
  implicit none

  integer, parameter :: rk8 = kind ( 1.0D+00 )

  integer n

  real ( kind = rk8 ) a
  real ( kind = rk8 ) b
  integer i
  real ( kind = rk8 ) x(n)

  if ( n == 1 ) then

    x(1) = ( a + b ) / 2.0D+00

  else

    do i = 1, n
      x(i) = ( real ( n - i,     kind = rk8 ) * a   &
             + real (     i - 1, kind = rk8 ) * b ) &
             / real ( n     - 1, kind = rk8 )
    end do

  end if

  return
end
subroutine timestamp ( )

!*****************************************************************************80
!
!! timestamp() prints the current YMDHMS date as a time stamp.
!
!  Example:
!
!    31 May 2001   9:45:54.872 AM
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    06 September 2021
!
!  Author:
!
!    John Burkardt
!
  implicit none

  character ( len = 8 ) ampm
  integer d
  integer h
  integer m
  integer mm
  character ( len = 9 ), parameter, dimension(12) :: month = (/ &
    'January  ', 'February ', 'March    ', 'April    ', &
    'May      ', 'June     ', 'July     ', 'August   ', &
    'September', 'October  ', 'November ', 'December ' /)
  integer n
  integer s
  integer values(8)
  integer y

  call date_and_time ( values = values )

  y = values(1)
  m = values(2)
  d = values(3)
  h = values(5)
  n = values(6)
  s = values(7)
  mm = values(8)

  if ( h < 12 ) then
    ampm = 'AM'
  else if ( h == 12 ) then
    if ( n == 0 .and. s == 0 ) then
      ampm = 'Noon'
    else
      ampm = 'PM'
    end if
  else
    h = h - 12
    if ( h < 12 ) then
      ampm = 'PM'
    else if ( h == 12 ) then
      if ( n == 0 .and. s == 0 ) then
        ampm = 'Midnight'
      else
        ampm = 'AM'
      end if
    end if
  end if

  write ( *, '(i2.2,1x,a,1x,i4,2x,i2,a1,i2.2,a1,i2.2,a1,i3.3,1x,a)' ) &
    d, trim ( month(m) ), y, h, ':', n, ':', s, '.', mm, trim ( ampm )

  return
end