plotter/examples/gtk_demo.cpp

// The GTK4 application: the same 1 MS/s stream the other demos draw, as the
// content of a GtkApplicationWindow whose header bar carries its own "Fit"
// button, with GTK drawing the window chrome around a plot the area renders into
// an FBO of its own.
//
// What it demonstrates:
//
//  - `plot::gtk4::PlotArea`: a GtkGLArea (GTK's own framebuffer, no texture, no
//    offscreen pass of this project's) driven by GTK's gestures and event
//    controllers -- drag to pan, shift+drag to box zoom, ctrl+drag to move a
//    cursor, middle-drag on a label strip to zoom one axis, wheel to zoom about
//    the pointer, `+` / `-` / `f` on the keyboard;
//  - the device-pixel mapping: GTK allocates and reports in logical pixels, the
//    area's framebuffer is in device pixels, and the area converts both the
//    viewport and every input coordinate with the same factor (see the header);
//  - the demo-side controls the area knows nothing about -- the header bar's "Fit";
//  - a scripted resize before the exit frame, so the headless run proves the area
//    re-lays the plot out for the new geometry rather than scaling a stale one;
//  - the headless smoke mode every demo here supports
//    (PLOT_DEMO_EXIT_AFTER_MS / PLOT_DEMO_SCREENSHOT). The screenshot is the GL
//    area's *own framebuffer*, read back by `dump_area_ppm` in the render
//    callback, so the PPM holds the plot surface: the header bar and the window
//    frame are GTK's and are not in that framebuffer. (GTK composites the area's
//    FBO into the window as a texture, so a whole-window shot would need GTK's
//    snapshot machinery; this one is honest about what it shows.)
//
// The producer is joined before the stream it writes into is destroyed, and
// before the area's GL resources are released with the widget.

// This library's GL surface first, then GTK -- the order every adapter here keeps.
// GTK's headers pull in no `<GL/gl.h>`, so unlike Qt there is no macro to dodge.
#include <gtk4/gtk_plot_area.hpp>

#include <plot/data/stream_series.hpp>
#include <plot/gl/api.hpp>
#include <plot/plot/ticks.hpp>

#include <gtk/gtk.h>

#include <atomic>
#include <chrono>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <numbers>
#include <thread>
#include <utility>
#include <vector>

namespace {

using plot::f32;
using plot::f64;
using plot::u32;
using plot::u64;

/// One sample per microsecond: 1 MS/s.
constexpr f64 kDx = 1.0e-6;
/// The retained window: 1 << 20 samples = 1.048576 s.
constexpr u64 kCapacity = 1u << 20;
/// Producer wakeup: about a millisecond of samples per push batch.
constexpr u64 kBatch = 1u << 10;
/// Cap on one push batch, so a process that was stopped and resumed catches up
/// over several iterations instead of allocating a batch for the whole gap.
constexpr u64 kMaxBatch = 1u << 16;

constexpr f64 kSineHz = 437.0;
constexpr f64 kNoiseAmp = 0.02;
constexpr f64 kSpikePeriodS = 0.1;
constexpr f64 kSpikeWidthS = 2.0e-6;
constexpr f64 kSpikeAmp = 3.0;

/// The trace colour the render smoke test's blue-dominance predicate expects, so
/// the same predicate reads this demo's screenshot.
constexpr plot::Color kTraceColor = plot::Color::hex(0x40C0FF);
constexpr plot::Color kCursorAColor = plot::Color::hex(0xFFB020);
constexpr plot::Color kCursorBColor = plot::Color::hex(0x35D0A8);

/// Logical pixels. The area's framebuffer -- the area's GL viewport -- is in
/// device pixels.
constexpr int kWindowWidth = 1200;
constexpr int kWindowHeight = 800;
/// The size the scripted resize before the exit frame goes to: different on both
/// axes, so a screenshot that kept the old layout is visibly the old size.
constexpr int kExitWidth = 820;
constexpr int kExitHeight = 620;

/// y(t) for sample index `i`: sine + deterministic noise + a narrow spike, the
/// signal the other demos use.
f32 sample_at(u64 i) noexcept {
  const f64 t = static_cast<f64>(i) * kDx;
  f64 y = std::sin(2.0 * std::numbers::pi * kSineHz * t);

  u32 h = static_cast<u32>(i) * 2654435761u;
  h ^= h >> 15;
  h *= 2246822519u;
  h ^= h >> 13;
  h *= 3266489917u;
  h ^= h >> 16;
  y += kNoiseAmp * (static_cast<f64>(h & 0xFFFFFFu) / 8388608.0 - 1.0);

  const f64 phase = std::fmod(t, kSpikePeriodS);
  if (phase < kSpikeWidthS) {
    y += kSpikeAmp * (1.0 - phase / kSpikeWidthS);
  }
  return static_cast<f32>(y);
}

/// Feeds a `StreamSeries` from its own thread at 1 MS/s on average, paced by
/// absolute deadlines off a steady clock: every wakeup produces every sample
/// whose time has already come, so a late wakeup is caught up instead of pushing
/// the phase back.
class Producer {
 public:
  explicit Producer(plot::data::StreamSeries& series)
      : series_(&series), thread_([this] { run(); }) {}
  ~Producer() { stop(); }

  Producer(const Producer&) = delete;
  Producer& operator=(const Producer&) = delete;

  /// Sets the stop flag and joins the thread. Idempotent; the destructor calls it,
  /// so the thread is never detached and never outlives the stream.
  void stop() noexcept {
    stop_.store(true, std::memory_order_relaxed);
    if (thread_.joinable()) thread_.join();
  }

  u64 produced() const noexcept { return produced_.load(std::memory_order_relaxed); }

 private:
  void run() {
    using clock = std::chrono::steady_clock;
    const auto start = clock::now();
    u64 next = 0;
    while (!stop_.load(std::memory_order_relaxed)) {
      const f64 elapsed = std::chrono::duration<f64>(clock::now() - start).count();
      const u64 due = static_cast<u64>(elapsed / kDx) + 1;
      if (due > next) {
        u64 n = due - next;
        if (n > kMaxBatch) n = kMaxBatch;
        batch_.resize(static_cast<plot::usize>(n));
        for (u64 k = 0; k < n; ++k) {
          batch_[static_cast<plot::usize>(k)] = sample_at(next + k);
        }
        series_->push(batch_.data(), n);
        next += n;
        produced_.store(next, std::memory_order_relaxed);
      }
      const f64 wake_s = static_cast<f64>(next + kBatch) * kDx;
      std::this_thread::sleep_until(
          start + std::chrono::duration_cast<clock::duration>(std::chrono::duration<f64>(wake_s)));
    }
  }

  plot::data::StreamSeries* series_;
  std::vector<f32> batch_;
  std::atomic<u64> produced_{0};
  std::atomic<bool> stop_{false};
  std::thread thread_;  ///< declared last: the lambda runs once everything is up
};

/// The live readout, refreshed a few times a second.
struct Meter {
  f64 elapsed = 0.0;
  f64 last_report = 0.0;
  u64 last_produced = 0;
  f64 rate = 0.0;  ///< samples/s over the last metered interval
  u64 frames = 0;  ///< frame callbacks run (one per staged frame)
};

f64 now_s() noexcept {
  return std::chrono::duration<f64>(std::chrono::steady_clock::now().time_since_epoch()).count();
}

/// `PLOT_DEMO_EXIT_AFTER_MS` as seconds: 0.0 when unset, unparsable or not
/// positive, which is "run until the window is closed".
f64 exit_after_seconds() noexcept {
  const char* v = std::getenv("PLOT_DEMO_EXIT_AFTER_MS");
  if (v == nullptr || v[0] == '\0') return 0.0;
  char* end = nullptr;
  const f64 ms = std::strtod(v, &end);
  if (end == v || !(ms > 0.0)) return 0.0;
  return ms * 1.0e-3;
}

/// Writes the GL area's own framebuffer out as a binary PPM (P6, 8 bits per
/// channel, rows top-down).
///
/// Demo-side, not library API -- hence the name: this is the smoke mode's picture
/// of the *area's* framebuffer, which is what GTK composites into the window.
/// `glReadPixels`'s origin is the framebuffer's bottom-left and a PPM's is its
/// top-left, so the rows are written in reverse; the alpha channel is dropped,
/// and the plot clears its whole viewport to an opaque background, so the RGB is
/// final. An `GL_RGBA` row is `width * 4` bytes -- a multiple of four -- so the
/// default `GL_PACK_ALIGNMENT` holds and no pixel-store state has to be touched.
bool dump_area_ppm(const char* path, int w, int h) {
  if (path == nullptr || path[0] == '\0' || w <= 0 || h <= 0) return false;
  const plot::usize width = static_cast<plot::usize>(w);
  const plot::usize height = static_cast<plot::usize>(h);
  std::vector<plot::u8> pixels(width * height * 4u);
  plot::gl::glReadPixels(0, 0, w, h, plot::gl::GL_RGBA, plot::gl::GL_UNSIGNED_BYTE,
                         pixels.data());

  std::FILE* file = std::fopen(path, "wb");
  if (file == nullptr) return false;
  if (std::fprintf(file, "P6\n%d %d\n255\n", w, h) < 0) {
    std::fclose(file);
    return false;
  }
  const plot::usize row_bytes = width * 3u;
  std::vector<plot::u8> row(row_bytes);
  bool ok = true;
  for (plot::usize y = height; y-- > 0u && ok;) {
    const plot::u8* src = pixels.data() + y * width * 4u;
    for (plot::usize x = 0; x < width; ++x) {
      row[x * 3u + 0u] = src[x * 4u + 0u];
      row[x * 3u + 1u] = src[x * 4u + 1u];
      row[x * 3u + 2u] = src[x * 4u + 2u];
    }
    ok = std::fwrite(row.data(), 1u, row_bytes, file) == row_bytes;
  }
  if (!ok) {
    std::fclose(file);
    return false;
  }
  return std::fclose(file) == 0;
}

/// The run summary's numbers, taken while the widget -- and with it the area and
/// its renderer -- is still alive. GTK frees the wrapper when the widget is
/// finalized, so the summary prints this instead of reading through a pointer
/// that may already be gone.
struct Snapshot {
  bool have_renderer = false;
  plot::ErrorCode init_code = plot::ErrorCode::Ok;
  const char* init_message = "";
  u64 frames = 0;
  f32 mean_ms = 0.0f;
  f32 last_ms = 0.0f;
  plot::FrameStats stats{};
  plot::Rect axes{};
  f32 dense_spp = 0.0f;
  bool persistent = false;
  bool gl_error = false;
};

/// Everything the demo owns, reachable from the signal callbacks and from the
/// exit path.
struct Demo {
  plot::data::StreamSeries* stream = nullptr;
  Producer* producer = nullptr;
  plot::Plot* graph = nullptr;
  plot::gtk4::PlotArea* area = nullptr;  ///< null once GTK has destroyed the widget
  GtkWindow* window = nullptr;
  GtkApplication* app = nullptr;
  u32 axis_x = 0;
  u32 axis_y = 0;
  u32 cursor_a = 0;
  u32 cursor_b = 0;
  u32 measure = 0;
  /// A weak pointer on the area's widget: GTK frees the wrapper with it, so this
  /// is how the callbacks that outlive the window (the pump, the summary) know
  /// whether the widget can still be touched.
  GtkWidget* widget = nullptr;

  bool automated = false;
  f64 exit_after_s = 0.0;
  f64 run_start = 0.0;
  const char* screenshot = nullptr;
  /// Set by the exit path, read by the after-render handler that takes the
  /// screenshot and ends the run. Non-null means "this render is the exit frame".
  const char* dump_path = nullptr;
  bool exit_pending = false;
  bool summary_printed = false;
  /// The two exit sequences, each guarded so it runs exactly once: the timer's
  /// block (which pumps the main context and can be re-entered by its own timeout)
  /// and the teardown (scheduled as an idle source).
  bool exit_started = false;
  bool torn_down = false;

  bool cursors_follow = true;
  Meter meter;
  Snapshot snapshot;
};

void capture_snapshot(Demo& demo) {
  Snapshot& s = demo.snapshot;
  s.init_code = demo.area->init_status().code;
  s.init_message = demo.area->init_status().message;
  plot::Renderer* const renderer = demo.area->renderer();
  s.have_renderer = renderer != nullptr;
  s.frames = demo.area->frames_painted();
  s.mean_ms = demo.area->mean_frame_ms();
  s.last_ms = demo.area->last_frame_ms();
  s.axes = std::as_const(*demo.graph).layout().axes;
  if (renderer != nullptr) {
    s.stats = renderer->stats();
    s.dense_spp = renderer->style().dense_threshold_spp;
    s.persistent = renderer->using_persistent_upload();
    s.gl_error = renderer->last_frame_had_gl_error();
  }
}

/// A window closed by hand -- or by the window manager -- takes the ordering the
/// automated path takes: the producer is joined *before* GTK destroys the widget,
/// and with it the renderer whose frames read the stream. GTK emits this before its
/// own handler destroys the window, and returning FALSE lets that default happen.
gboolean on_close_request(GtkWindow*, gpointer user_data) {
  Demo& demo = *static_cast<Demo*>(user_data);
  demo.producer->stop();
  return FALSE;
}

/// Runs the window's destruction and the quit outside any signal emission: see
/// `on_render_after`.
gboolean on_teardown(gpointer user_data) {
  Demo& demo = *static_cast<Demo*>(user_data);
  // The teardown runs once, whatever scheduled it (the exit frame's idle source, or
  // a second one if two renders were pending): the window is destroyed and the
  // application quit here, and both of those must not happen twice.
  if (demo.torn_down) return G_SOURCE_REMOVE;
  demo.torn_down = true;
  // The producer is joined *first*, while the widget and the stream it feeds are
  // both still alive: the join is what makes "no thread pushes into the stream
  // after this point" true for the destruction below, and for the frames the
  // widget may still stage while it is being torn down.
  demo.producer->stop();
  // Destroyed explicitly rather than left to the application's shutdown, so an
  // automated run exercises the widget's teardown -- the renderer's GL objects are
  // released in `::unrealize`, with the context still current -- and a fault in it
  // shows up in the run's exit status rather than after the output has been read.
  gtk_window_destroy(demo.window);
  g_application_quit(G_APPLICATION(demo.app));
  return G_SOURCE_REMOVE;
}

/// The repaint pump. GTK only redraws the area when something asks it to, so this
/// is the frame rate the demo runs at (a real application drives the same timer,
/// or its toolkit's equivalent, from whatever it animates). Nothing here is
/// library API: `gtk_gl_area_queue_render` is GTK's.
gboolean on_repaint(gpointer user_data) {
  Demo& demo = *static_cast<Demo*>(user_data);
  if (demo.widget == nullptr) {  // the window was closed: the wrapper is gone too
    demo.area = nullptr;
    return G_SOURCE_REMOVE;
  }
  capture_snapshot(demo);
  gtk_gl_area_queue_render(GTK_GL_AREA(demo.area->widget()));
  return G_SOURCE_CONTINUE;
}

/// Connected with `g_signal_connect_after`, so it runs once the adapter's own
/// handler has staged the frame and the area's FBO holds it. It is also the one
/// place that sees a render after a failed init -- the adapter's handler returns
/// early then, but GTK still emits the signal -- which keeps the snapshot's
/// status meaningful.
gboolean on_render_after(GtkGLArea*, GdkGLContext*, gpointer user_data) {
  Demo& demo = *static_cast<Demo*>(user_data);
  if (demo.area != nullptr) capture_snapshot(demo);
  if (!demo.exit_pending) return FALSE;
  demo.exit_pending = false;
  if (demo.dump_path != nullptr && demo.dump_path[0] != '\0') {
    const plot::Rect viewport = demo.area->device_viewport();
    const bool written = dump_area_ppm(demo.dump_path, static_cast<int>(viewport.w),
                                       static_cast<int>(viewport.h));
    std::printf("[gtk_demo] screenshot  %s (%dx%d GL px): %s\n", demo.dump_path,
                static_cast<int>(viewport.w), static_cast<int>(viewport.h),
                written ? "written" : "FAILED");
    std::fflush(stdout);
  }
  // The teardown is *deferred*: closing the window destroys this very widget, and
  // the render signal is still being emitted, so doing it on this stack would free
  // the area GTK is in the middle of calling back into. An idle source runs on the
  // next main-loop iteration, after the emission has returned.
  g_idle_add(&on_teardown, &demo);
  return FALSE;
}

/// The header bar's "Fit": one-shot framing of the data. `auto_fit_*` re-arms
/// auto scale, which the frame callback then follows, so this is also how a pan
/// is undone.
void on_fit_clicked(GtkButton*, gpointer user_data) {
  Demo& demo = *static_cast<Demo*>(user_data);
  demo.graph->auto_fit_x();
  demo.graph->auto_fit_y(demo.axis_y);
  // A click moves focus to the button, which would take the controller's keys away
  // from the plot; hand it back, or `+`/`-`/`f` would stop reaching the plot.
  gtk_widget_grab_focus(demo.area->widget());
  gtk_gl_area_queue_render(GTK_GL_AREA(demo.area->widget()));
}

/// The headless exit path: at the deadline, resize the window (scripted), check
/// that the layout was recomputed for the new geometry, then queue the exit frame
/// and let `on_render_after` take the screenshot and end the run.
gboolean on_exit_timer(gpointer user_data) {
  Demo& demo = *static_cast<Demo*>(user_data);
  if (!demo.automated || now_s() - demo.run_start < demo.exit_after_s) return G_SOURCE_CONTINUE;
  // The pump below iterates the main context, so this very timeout can fire from
  // inside this call; the guard is what keeps the exit sequence -- the scripted
  // resize, its check, the exit frame -- to one run.
  if (demo.exit_started) return G_SOURCE_REMOVE;
  demo.exit_started = true;
  if (demo.widget == nullptr) return G_SOURCE_REMOVE;  // the window is already gone

  const plot::PlotLayout before = std::as_const(*demo.graph).layout();
  const plot::Rect viewport_before = demo.area->device_viewport();
  const int width_before = gtk_widget_get_width(demo.area->widget());
  const int height_before = gtk_widget_get_height(demo.area->widget());

  // The resize goes through the window system, so pump the main context until the
  // area's allocation follows, with a deadline. The pump is what lets the resize
  // and the frames in between (which re-derive the viewport) actually happen.
  gtk_window_set_default_size(demo.window, kExitWidth, kExitHeight);
  const f64 deadline = now_s() + 1.0;
  while (gtk_widget_get_width(demo.area->widget()) == width_before && now_s() < deadline) {
    while (g_main_context_pending(nullptr)) g_main_context_iteration(nullptr, FALSE);
    g_usleep(1000);
  }
  const plot::PlotLayout after = std::as_const(*demo.graph).layout();
  const plot::Rect viewport = demo.area->device_viewport();

  // The evidence that the layout was *recomputed and not scaled*: the y label
  // strip is measured from the labels' font metrics -- a logical size, converted to
  // device pixels at *this* display's scale (see `Style::ui_scale`) -- so at a
  // constant scale it stays the same size while the axes takes the rest of the new
  // allocation. Two things are checked, and neither is "the strip is the same
  // number":
  //
  //   * the axes and its strips tile the *current* viewport exactly -- a layout
  //     left over from the old geometry would still tile the old one;
  //   * the strip is nearer the previous strip than that strip scaled by the resize
  //     ratio -- a layout whose strips were *scaled* would land on the second.
  //
  // The strip is not invariant, and the check must not pretend it is: it is the
  // widest major tick label plus padding, and the tick *step* follows the axes
  // length, which the resize changes. Measured at scale 2: 18 majors over 1440
  // device px with "-800m" the widest label, 8 over 1080 with "-500m" -- 78 and
  // 76 px in this bitmap font, whose '8' is the wider glyph. That is 2 px of
  // recomputed strip, against 174 * 1620/2380 = 118 px of scaled strip.
  const f32 resize_ratio = viewport_before.w > 0.0f ? viewport.w / viewport_before.w : 1.0f;
  const f32 scaled_strip = before.axes.x * resize_ratio;
  const f32 drift = std::fabs(after.axes.x - before.axes.x);
  const bool tiles = std::fabs(after.axes.w - (viewport.w - after.axes.x)) <= 0.5f &&
                     std::fabs(after.axes.h - (viewport.h - after.axes.y - after.x_axis_height -
                                               after.x_title_height)) <= 0.5f;
  const bool metric_strip = drift < std::fabs(after.axes.x - scaled_strip);
  const bool recomputed = after.axes.w != before.axes.w && tiles && metric_strip;
  std::printf("\n[gtk_demo] ---- exit frame ----\n");
  std::printf("[gtk_demo] resize      logical %dx%d -> %dx%d, device viewport %ux%u, scale %g\n",
              width_before, height_before, gtk_widget_get_width(demo.area->widget()),
              gtk_widget_get_height(demo.area->widget()), static_cast<unsigned>(viewport.w),
              static_cast<unsigned>(viewport.h), static_cast<double>(demo.area->device_scale()));
  std::printf("[gtk_demo] layout      axes %.0fx%.0f at (%.0f,%.0f) -> %.0fx%.0f at (%.0f,%.0f): %s\n",
              static_cast<double>(before.axes.w), static_cast<double>(before.axes.h),
              static_cast<double>(before.axes.x), static_cast<double>(before.axes.y),
              static_cast<double>(after.axes.w), static_cast<double>(after.axes.h),
              static_cast<double>(after.axes.x), static_cast<double>(after.axes.y),
              recomputed ? "recomputed for the new viewport (strips metric, not scaled)"
                         : "FAIL: stale or rescaled layout");
  std::printf("[gtk_demo] layout      y label strip %.0f -> %.0f px of a %.0f -> %.0f px wide axes,"
              " %.0f px of drift against a scaled %.0f\n",
              static_cast<double>(before.axes.x), static_cast<double>(after.axes.x),
              static_cast<double>(before.axes.w), static_cast<double>(after.axes.w),
              static_cast<double>(drift), static_cast<double>(scaled_strip));
  std::printf("[gtk_demo] init        %s: %s\n", plot::to_string(demo.area->init_status().code),
              demo.area->init_status().message);
  std::fflush(stdout);

  demo.dump_path = demo.screenshot;
  demo.exit_pending = true;
  gtk_gl_area_queue_render(GTK_GL_AREA(demo.area->widget()));
  return G_SOURCE_REMOVE;
}

void on_activate(GtkApplication* app, gpointer user_data) {
  Demo& demo = *static_cast<Demo*>(user_data);
  demo.app = app;

  GtkWidget* window = gtk_application_window_new(app);
  demo.window = GTK_WINDOW(window);
  gtk_window_set_title(GTK_WINDOW(window), "plot gtk4 demo");
  gtk_window_set_default_size(GTK_WINDOW(window), kWindowWidth, kWindowHeight);
  // The hand-close path's half of the shutdown ordering (see `on_close_request`);
  // the automated path stops the producer in `on_teardown` instead, because
  // `gtk_window_destroy()` does not emit this signal.
  g_signal_connect(window, "close-request", G_CALLBACK(on_close_request), &demo);

  // The plot area is the window's content. GTK owns the widget from here on: the
  // wrapper is freed with it, which is also what releases the plot's GL resources.
  demo.area = plot::gtk4::PlotArea::create(*demo.graph);
  gtk_window_set_child(GTK_WINDOW(window), demo.area->widget());
  // The weak pointer has to be seeded with the widget it watches: GLib only
  // remembers the *address* and clears it at finalization, it does not read or
  // store the object (this is why the demo's copy is NULL until the two lines
  // below -- and why a missing assignment would leave every callback believing
  // the window was already gone).
  demo.widget = demo.area->widget();
  g_object_add_weak_pointer(G_OBJECT(demo.widget), reinterpret_cast<gpointer*>(&demo.widget));

  GtkWidget* header = gtk_header_bar_new();
  gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header), gtk_label_new("plot gtk4 demo"));
  GtkWidget* fit = gtk_button_new_with_label("Fit");
  g_signal_connect(fit, "clicked", G_CALLBACK(on_fit_clicked), &demo);
  gtk_header_bar_pack_start(GTK_HEADER_BAR(header), fit);
  gtk_window_set_titlebar(GTK_WINDOW(window), header);

  // The exit frame's screenshot is taken from the render signal, after the
  // adapter's handler has drawn the frame: `_after` is what guarantees the order.
  g_signal_connect_after(demo.area->widget(), "render", G_CALLBACK(on_render_after), &demo);

  // The frame callback: the same host-side work the other demos do, on the thread
  // that owns the GL context (the GTK main thread), never on the producer's.
  demo.area->set_frame_callback([&demo](plot::Plot& p, plot::f64 dt) {
    demo.stream->acquire();  // once per frame, on the render thread

    // Re-frame the live stream while auto scale is armed. `auto_fit_*` re-arms it,
    // so this may only run while it is still on: calling it after a pan would undo
    // the pan every frame.
    if (p.x_axis(demo.axis_x).auto_scale) p.auto_fit_x();
    if (p.y_axis(demo.axis_y).auto_scale) p.auto_fit_y(demo.axis_y);

    if (demo.cursors_follow) {
      if (demo.area->controller().drag_mode() == plot::input::DragMode::MoveCursor ||
          !p.x_axis(demo.axis_x).auto_scale) {
        demo.cursors_follow = false;
      } else {
        const plot::Range xr = p.x_range(demo.axis_x);
        p.set_cursor_x(demo.cursor_a, xr.min + 0.35 * xr.span());
        p.set_cursor_x(demo.cursor_b, xr.min + 0.65 * xr.span());
      }
    }

    demo.meter.frames += 1;
    demo.meter.elapsed += dt;
    if (demo.meter.elapsed - demo.meter.last_report >= 0.5) {
      const u64 produced = demo.producer->produced();
      demo.meter.rate = static_cast<f64>(produced - demo.meter.last_produced) /
                        (demo.meter.elapsed - demo.meter.last_report);
      demo.meter.last_produced = produced;
      demo.meter.last_report = demo.meter.elapsed;
      const plot::Rect viewport = demo.area->last_device_viewport();
      const plot::Rect axes = std::as_const(p).layout().axes;
      const plot::Range xr = p.x_range(demo.axis_x);
      std::printf(
          "[gtk_demo] t=%.1fs  %.3f MS/s  %.2f ms/frame  scale %g  fbo %ux%u  axes %.0fx%.0f"
          "  x [%.6f, %.6f] span %.6f auto %d  %llu frames\n",
          demo.meter.elapsed, demo.meter.rate * 1.0e-6,
          static_cast<double>(demo.area->mean_frame_ms()),
          static_cast<double>(demo.area->device_scale()), static_cast<unsigned>(viewport.w),
          static_cast<unsigned>(viewport.h),
          static_cast<double>(axes.w), static_cast<double>(axes.h), xr.min, xr.max, xr.span(),
          p.x_axis(demo.axis_x).auto_scale ? 1 : 0,
          static_cast<unsigned long long>(demo.meter.frames));
      std::fflush(stdout);
    }
  });

  gtk_window_present(GTK_WINDOW(window));
  // So the controller's keys reach the plot: a key event goes to the focused
  // widget, and a `GtkGLArea` is focusable because the adapter says so.
  gtk_window_set_focus(GTK_WINDOW(window), demo.area->widget());

  // The repaint pump (~60 Hz, the display rate this demo runs at) and -- in the
  // automated mode -- the deadline that runs the exit path.
  g_timeout_add(16, &on_repaint, &demo);
  if (demo.automated) g_timeout_add(20, &on_exit_timer, &demo);
}

/// The run summary, printed after the main loop: the stream, the plot and the
/// producer are the demo's, and outlive the window, so only the renderer's
/// numbers come from the snapshot.
void print_summary(Demo& demo) {
  const Snapshot& s = demo.snapshot;
  const plot::Measurement m = demo.graph->measurement(demo.measure);
  const f64 spp = s.axes.w > 0.0f ? static_cast<f64>(s.stats.visible_samples) /
                                        static_cast<f64>(s.axes.w)
                                  : 0.0;

  std::printf("\n[gtk_demo] ---- run summary ----\n");
  std::printf("[gtk_demo] producer    %llu samples pushed, last metered %.3f MS/s\n",
              static_cast<unsigned long long>(demo.producer->produced()), demo.meter.rate * 1.0e-6);
  std::printf("[gtk_demo] frames      %llu staged (%llu frame callbacks) in %.3f s, mean %.2f"
              " ms/frame (%.1f fps), last %.2f ms\n",
              static_cast<unsigned long long>(s.frames),
              static_cast<unsigned long long>(demo.meter.frames), demo.meter.elapsed,
              static_cast<double>(s.mean_ms),
              s.mean_ms > 0.0f ? 1.0e3 / static_cast<double>(s.mean_ms) : 0.0,
              static_cast<double>(s.last_ms));
  std::printf("[gtk_demo] retained    %llu samples, x [%.6f, %.6f] s\n",
              static_cast<unsigned long long>(demo.stream->size()),
              demo.stream->x_bounds().min, demo.stream->x_bounds().max);
  std::printf("[gtk_demo] last frame  %.1f samples/px (dense threshold %.1f) -> %s, visible %llu,"
              " axes %.0fx%.0f, %u segments, %u draw calls\n",
              spp, static_cast<double>(s.dense_spp),
              s.stats.lod_traces > 0 ? "LOD pyramid" : "raw samples",
              static_cast<unsigned long long>(s.stats.visible_samples),
              static_cast<double>(s.axes.w), static_cast<double>(s.axes.h), s.stats.staged_segments,
              s.stats.draw_calls);
  std::printf("[gtk_demo] measurement %s dx %g s dy %g rms %g mean %g\n",
              m.valid ? "valid:" : "INVALID", m.dx, m.dy, m.rms, m.mean);
  std::printf("[gtk_demo] gl          %s upload, gl error on the last frame %d\n",
              s.persistent ? "persistent-mapped ring" : "orphan", s.gl_error ? 1 : 0);
  std::printf("[gtk_demo] init        %s: %s\n", plot::to_string(s.init_code), s.init_message);
  std::fflush(stdout);
}

}  // namespace

int main(int argc, char** argv) {
  const f64 exit_after_s = exit_after_seconds();
  const bool automated = exit_after_s > 0.0;
  const char* const screenshot = std::getenv("PLOT_DEMO_SCREENSHOT");

  // --- the stream and the plot the area draws -----------------------------
  plot::data::StreamSeries stream(kCapacity, 0.0, kDx);
  Producer producer(stream);

  plot::Plot graph;
  // Time axis: clock labels, which `TickFormat::Time` routes through
  // `format_time`. The value axis keeps SI labels, which is `format_si`.
  const u32 axis_x = graph.add_x_axis(
      {"time", {-0.5, 0.5}, true, plot::AxisPlace::Bottom, false, plot::TickFormat::Time});
  const u32 axis_y = graph.add_y_axis(
      {"signal", {-4.0, 4.0}, true, plot::AxisPlace::Left, false, plot::TickFormat::Si});
  const u32 trace = graph.add_trace({&stream, kTraceColor, 1.5f, axis_x, axis_y});

  // Snapped cursors sit exactly on a sample, so the readout measures samples and
  // not a rounded position between two of them.
  const u32 cursor_a = graph.add_cursor(0.0, kCursorAColor);
  const u32 cursor_b = graph.add_cursor(0.0, kCursorBColor);
  graph.set_cursor_snap(cursor_a, plot::SnapMode::NearestSample);
  graph.set_cursor_snap(cursor_b, plot::SnapMode::NearestSample);
  const u32 measure = graph.add_measure(cursor_a, cursor_b, trace);

  Demo demo;
  demo.stream = &stream;
  demo.producer = &producer;
  demo.graph = &graph;
  demo.axis_x = axis_x;
  demo.axis_y = axis_y;
  demo.cursor_a = cursor_a;
  demo.cursor_b = cursor_b;
  demo.measure = measure;
  demo.automated = automated;
  demo.exit_after_s = exit_after_s;
  demo.run_start = now_s();
  demo.screenshot = screenshot;

  GtkApplication* app = gtk_application_new("org.plot.gtk_demo", G_APPLICATION_DEFAULT_FLAGS);
  g_signal_connect(app, "activate", G_CALLBACK(on_activate), &demo);
  const int app_result = g_application_run(G_APPLICATION(app), argc, argv);

  // The window is gone by now -- `on_render_after` destroyed it before quitting --
  // so only what the demo owns is read below.
  producer.stop();  // already stopped by on_teardown; idempotent, and this is the
                    // path a window closed by hand takes (no on_teardown then)

  print_summary(demo);
  std::printf("[gtk_demo] teardown    the area widget is %s after the main loop\n",
              demo.widget != nullptr ? "still alive" : "destroyed (the wrapper and its GL"
                                                      " resources went with it)");

  g_object_unref(app);
  return app_result;
}