Add Oregonator example with reference verification and work-precision output (#40)

Copilot · ivan-pi · web-flow · commit 2f41df8c6ddf · 2026-05-11T22:22:27.000+02:00
* Add Oregonator example skeleton Agent-Logs-Url: https://github.com/ivan-pi/stiff3/sessions/f7b0e04d-83b2-4b6d-9b36-32da6a997174 Co-authored-by: ivan-pi <21085643+ivan-pi@users.noreply.github.com> * Finish Oregonator example implementation Agent-Logs-Url: https://github.com/ivan-pi/stiff3/sessions/f7b0e04d-83b2-4b6d-9b36-32da6a997174 Co-authored-by: ivan-pi <21085643+ivan-pi@users.noreply.github.com> --------- Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: ivan-pi <21085643+ivan-pi@users.noreply.github.com>
diff --git a/README.md b/README.md
@@ -74,14 +74,16 @@ stiff3.git = "https://github.com/ivan-pi/stiff3"
 
 ### Running the examples
 
-Six examples called `robertson`, `vanpol`, `lorenz`, `predator_prey`, `pendant_drop`, and `three_equation_system` are provided. They can be run with the command
+Seven examples called `robertson`, `vanpol`, `lorenz`, `predator_prey`, `pendant_drop`, `three_equation_system`, and `oregonator` are provided. They can be run with the command
 
 ```sh
 fpm run --example <name>
 ```
 
 With CMake, the compiled executables are placed in the `build/example/` directory and can be run directly or via CTest.
 
+The `oregonator` example verifies a published reference solution at `t = 360` and writes `oregonator_work_precision.csv` together with an `oregonator_work_precision.svg` plot in the current working directory.
+
 ## Usage
 
 Basic use of the solver is demonstrated using the [Van der Pol oscillator](https://en.wikipedia.org/wiki/Van_der_Pol_oscillator):
diff --git a/example/CMakeLists.txt b/example/CMakeLists.txt
@@ -1,4 +1,4 @@
-foreach(example robertson vanpol lorenz predator_prey pendant_drop three_equation_system)
+foreach(example robertson vanpol lorenz predator_prey pendant_drop three_equation_system oregonator)
   add_executable(${example} ${example}.f90)
   target_link_libraries(${example} PRIVATE stiff3)
   add_test(NAME ${example} COMMAND ${example})
diff --git a/example/oregonator.f90 b/example/oregonator.f90
@@ -0,0 +1,239 @@
+program oregonator
+
+  use stiff3_solver, only: stiff3, wp => stiff3_wp
+
+  implicit none
+
+  integer, parameter :: n = 3
+  integer, parameter :: ncases = 6
+  real(wp), parameter :: y0(n) = [1.0_wp, 2.0_wp, 3.0_wp]
+  real(wp), parameter :: yref(n) = [ &
+    1.000814870318523_wp, 1228.178521549917_wp, 132.0554942846706_wp ]
+  real(wp), parameter :: x0 = 0.0_wp
+  real(wp), parameter :: x1 = 360.0_wp
+  real(wp), parameter :: h0_initial = 1.0e-3_wp
+  real(wp), parameter :: verification_tol = 1.0e-8_wp
+  real(wp), parameter :: eps_values(ncases) = [ &
+    1.0e-4_wp, 1.0e-5_wp, 1.0e-6_wp, 1.0e-7_wp, 1.0e-8_wp, 1.0e-9_wp ]
+  character(len=*), parameter :: csv_file = 'oregonator_work_precision.csv'
+  character(len=*), parameter :: svg_file = 'oregonator_work_precision.svg'
+
+  integer :: i
+  integer :: verification_stats(6)
+  integer :: stats(6,ncases)
+  real(wp) :: y(n), w(n), errors(ncases), work(ncases), verification_error
+
+  w = 1.0_wp
+
+  call integrate_case(1.0e-8_wp, y, verification_stats)
+  verification_error = max_relative_error(y, yref)
+  if (verification_error > verification_tol) then
+    print '(A,ES12.4,A,ES12.4)', &
+      'reference verification failed: max relative error=', verification_error, &
+      ' exceeds ', verification_tol
+    error stop 1
+  end if
+
+  print '(A,ES12.4)', 'verified reference solution with max relative error ', verification_error
+  print '(A,3(1X,ES24.16))', 'y(360) =', y
+  print '(A,6(I0,1X))', 'verification stats [nacc nrej nfev njev nlu nsol]: ', verification_stats
+
+  do i = 1, ncases
+    call integrate_case(eps_values(i), y, stats(:,i))
+    errors(i) = max_relative_error(y, yref)
+    work(i) = real(stats(3,i), wp)
+  end do
+
+  call write_csv(csv_file, eps_values, errors, stats)
+  call write_svg(svg_file, eps_values, errors, work)
+
+  print '(A)', 'work-precision data:'
+  print '(A)', '  eps         error        nfev   njev   nlu   nsol'
+  do i = 1, ncases
+    print '(2(1X,ES10.2),4(1X,I6))', eps_values(i), errors(i), &
+      stats(3,i), stats(4,i), stats(5,i), stats(6,i)
+  end do
+  print '(A,1X,A)', 'wrote', csv_file
+  print '(A,1X,A)', 'wrote', svg_file
+
+contains
+
+  subroutine integrate_case(eps, y, stats)
+    real(wp), intent(in) :: eps
+    real(wp), intent(out) :: y(n)
+    integer, intent(out) :: stats(6)
+
+    real(wp) :: h0
+
+    y = y0
+    h0 = h0_initial
+    call stiff3(n, fun, x0, y, x1, jac, h0, eps, w, stats=stats)
+  end subroutine
+
+  function max_relative_error(y, y_reference) result(err)
+    real(wp), intent(in) :: y(n), y_reference(n)
+    real(wp) :: err
+
+    err = maxval(abs(y - y_reference)/max(1.0_wp, abs(y_reference)))
+  end function
+
+  subroutine write_csv(filename, eps, err, stats)
+    character(len=*), intent(in) :: filename
+    real(wp), intent(in) :: eps(:), err(:)
+    integer, intent(in) :: stats(:,:)
+
+    integer :: i, ios, unit
+
+    open(newunit=unit, file=filename, status='replace', action='write', iostat=ios)
+    if (ios /= 0) error stop 'failed to open CSV output file'
+
+    write(unit,'(A)') 'eps,max_relative_error,nfev,njev,nlu,nsol,nacc,nrej'
+    do i = 1, size(eps)
+      write(unit,'(ES24.16,'','',ES24.16,'','',I0,'','',I0,'','',I0,'','',I0,'','',I0,'','',I0)') &
+        eps(i), err(i), stats(3,i), stats(4,i), stats(5,i), stats(6,i), stats(1,i), stats(2,i)
+    end do
+
+    close(unit)
+  end subroutine
+
+  subroutine write_svg(filename, eps, err, work)
+    character(len=*), intent(in) :: filename
+    real(wp), intent(in) :: eps(:), err(:), work(:)
+
+    integer, parameter :: width = 800
+    integer, parameter :: height = 600
+    real(wp), parameter :: left = 100.0_wp
+    real(wp), parameter :: right = 30.0_wp
+    real(wp), parameter :: top = 50.0_wp
+    real(wp), parameter :: bottom = 80.0_wp
+
+    character(len=32) :: label
+    integer :: i, ios, unit
+    real(wp) :: log_work(size(work)), log_err(size(err))
+    real(wp) :: xmin, xmax, ymin, ymax, xpad, ypad
+    real(wp) :: x1p, x2p, y1p, y2p, xtick, ytick, tick_value
+
+    log_work = log10(max(work, tiny(1.0_wp)))
+    log_err = log10(max(err, tiny(1.0_wp)))
+
+    xmin = minval(log_work)
+    xmax = maxval(log_work)
+    ymin = minval(log_err)
+    ymax = maxval(log_err)
+    xpad = 0.05_wp*max(xmax - xmin, 1.0_wp)
+    ypad = 0.08_wp*max(ymax - ymin, 1.0_wp)
+    xmin = xmin - xpad
+    xmax = xmax + xpad
+    ymin = ymin - ypad
+    ymax = ymax + ypad
+
+    open(newunit=unit, file=filename, status='replace', action='write', iostat=ios)
+    if (ios /= 0) error stop 'failed to open SVG output file'
+
+    write(unit,'(A)') '<?xml version="1.0" encoding="UTF-8"?>'
+    write(unit,'(A,I0,A,I0,A)') '<svg xmlns="http://www.w3.org/2000/svg" width="', width, &
+      '" height="', height, '" viewBox="0 0 800 600">'
+    write(unit,'(A)') '<rect width="100%" height="100%" fill="white"/>'
+    write(unit,'(A)') &
+      '<text x="400" y="28" font-family="sans-serif" font-size="22" ' // &
+      'text-anchor="middle">Oregonator work-precision diagram</text>'
+    write(unit,'(A,F0.3,A,F0.3,A,F0.3,A,F0.3,A)') &
+      '<rect x="', left, '" y="', top, '" width="', real(width,wp) - left - right, &
+      '" height="', real(height,wp) - top - bottom, '" fill="none" stroke="black" stroke-width="1.5"/>'
+
+    do i = 0, 4
+      xtick = left + real(i,wp)*(real(width,wp) - left - right)/4.0_wp
+      tick_value = 10.0_wp**(xmin + real(i,wp)*(xmax - xmin)/4.0_wp)
+      write(label,'(ES10.2)') tick_value
+      write(unit,'(A,F0.3,A,F0.3,A)') '<line x1="', xtick, '" y1="520" x2="', xtick, &
+        '" y2="526" stroke="#666"/>'
+      write(unit,'(A,F0.3,A)') '<text x="', xtick, '" y="548" font-family="sans-serif" font-size="12" text-anchor="middle">'
+      write(unit,'(A)') trim(adjustl(label))//'</text>'
+    end do
+
+    do i = 0, 4
+      ytick = real(height,wp) - bottom - real(i,wp)*(real(height,wp) - top - bottom)/4.0_wp
+      tick_value = 10.0_wp**(ymin + real(i,wp)*(ymax - ymin)/4.0_wp)
+      write(label,'(ES10.2)') tick_value
+      write(unit,'(A,F0.3,A,F0.3,A)') '<line x1="94" y1="', ytick, '" x2="100" y2="', ytick, &
+        '" stroke="#666"/>'
+      write(unit,'(A,F0.3,A)') '<text x="84" y="', ytick + 4.0_wp, '" font-family="sans-serif" font-size="12" text-anchor="end">'
+      write(unit,'(A)') trim(adjustl(label))//'</text>'
+    end do
+
+    write(unit,'(A)') &
+      '<text x="435" y="585" font-family="sans-serif" font-size="16" ' // &
+      'text-anchor="middle">rhs evaluations (nfev)</text>'
+    write(unit,'(A)') &
+      '<text x="24" y="285" font-family="sans-serif" font-size="16" ' // &
+      'text-anchor="middle" transform="rotate(-90 24 285)">' // &
+      'max relative error</text>'
+
+    do i = 1, size(work) - 1
+      x1p = xcoord(log_work(i), xmin, xmax, width, left, right)
+      y1p = ycoord(log_err(i), ymin, ymax, height, top, bottom)
+      x2p = xcoord(log_work(i + 1), xmin, xmax, width, left, right)
+      y2p = ycoord(log_err(i + 1), ymin, ymax, height, top, bottom)
+      write(unit,'(A,F0.3,A,F0.3,A,F0.3,A,F0.3,A)') '<line x1="', x1p, '" y1="', y1p, &
+        '" x2="', x2p, '" y2="', y2p, '" stroke="#1f77b4" stroke-width="2"/>'
+    end do
+
+    do i = 1, size(work)
+      x1p = xcoord(log_work(i), xmin, xmax, width, left, right)
+      y1p = ycoord(log_err(i), ymin, ymax, height, top, bottom)
+      write(label,'(ES9.1)') eps(i)
+      write(unit,'(A,F0.3,A,F0.3,A)') '<circle cx="', x1p, '" cy="', y1p, '" r="4.5" fill="#d62728"/>'
+      write(unit,'(A,F0.3,A,F0.3,A)') '<text x="', x1p + 8.0_wp, '" y="', y1p - 8.0_wp, &
+        '" font-family="sans-serif" font-size="12">'
+      write(unit,'(A)') 'eps='//trim(adjustl(label))//'</text>'
+    end do
+
+    write(unit,'(A)') '</svg>'
+
+    close(unit)
+
+  end subroutine
+
+  function xcoord(value, xmin, xmax, width, left, right) result(xplot)
+    real(wp), intent(in) :: value, xmin, xmax, left, right
+    integer, intent(in) :: width
+    real(wp) :: xplot
+
+    xplot = left + (value - xmin)*(real(width,wp) - left - right)/(xmax - xmin)
+  end function
+
+  function ycoord(value, ymin, ymax, height, top, bottom) result(yplot)
+    real(wp), intent(in) :: value, ymin, ymax, top, bottom
+    integer, intent(in) :: height
+    real(wp) :: yplot
+
+    yplot = real(height,wp) - bottom - &
+      (value - ymin)*(real(height,wp) - top - bottom)/(ymax - ymin)
+  end function
+
+  subroutine fun(n, y, f)
+    integer, intent(in) :: n
+    real(wp), intent(in) :: y(n)
+    real(wp), intent(inout) :: f(n)
+
+    f(1) = 77.27_wp*(y(2) + y(1)*(1.0_wp - 8.375e-6_wp*y(1) - y(2)))
+    f(2) = (y(3) - (1.0_wp + y(1))*y(2))/77.27_wp
+    f(3) = 0.161_wp*(y(1) - y(3))
+  end subroutine
+
+  subroutine jac(n, y, df)
+    integer, intent(in) :: n
+    real(wp), intent(in) :: y(n)
+    real(wp), intent(inout) :: df(n,n)
+
+    df = 0.0_wp
+    df(1,1) = 77.27_wp*(1.0_wp - 2.0_wp*8.375e-6_wp*y(1) - y(2))
+    df(1,2) = 77.27_wp*(1.0_wp - y(1))
+    df(2,1) = -y(2)/77.27_wp
+    df(2,2) = -(1.0_wp + y(1))/77.27_wp
+    df(2,3) = 1.0_wp/77.27_wp
+    df(3,1) = 0.161_wp
+    df(3,3) = -0.161_wp
+  end subroutine
+
+end program

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-foreach(example robertson vanpol lorenz predator_prey pendant_drop three_equation_system)`
	`1`	`+foreach(example robertson vanpol lorenz predator_prey pendant_drop three_equation_system oregonator)`
`2`	`2`	`add_executable(${example} ${example}.f90)`
`3`	`3`	`target_link_libraries(${example} PRIVATE stiff3)`
`4`	`4`	`add_test(NAME ${example} COMMAND ${example})`