fix(plot): a degenerate range cannot strand a live axis, and the demo's follow check fails the run

examples/glfw_demo.cpp

@@ -20,7 +20,9 @@
 //    (`XFollow::Live`), driving three wheel notches and a pan through the host's own
 //    controller and printing, at each step, the range's width, its distance behind
 //    the newest sample and the follow state -- the interaction spec §12 is about,
-//    measured on a real stream rather than in a unit test.
+//    measured on a real stream rather than in a unit test. It is a *check*, not a
+//    readout: the run exits non-zero if a measured property fails to hold, or if the
+//    sequence did not get far enough to measure all four.
 //
 // The producer is joined before the stream it writes into is destroyed, and
 // before the host closes the GL context (main's destruction order does that).
@@ -183,10 +185,17 @@ struct Meter {
 /// Each measurement prints the range's width, how far its right edge sits behind the
 /// newest sample, and whether following is still armed. Only the zoom may change the
 /// width, only the pan may change the distance behind, and following must be armed
-/// throughout; the last line of the run says whether that held, so the run is a check
-/// rather than a printout to be read.
+/// throughout. The verdict is carried out of the run in `failed`, so `main()` exits
+/// non-zero on a failed check -- a check whose result only stdout knows is not a check
+/// -- and also when the sequence never completed, since a run that measured nothing
+/// has not passed.
 struct FollowScript {
   bool enabled = false;
+  /// Set by report(); read by main(). False until the checks have all held.
+  bool failed = false;
+  /// Set by report(): the sequence reached a verdict at all. A run that ended before
+  /// the fourth measurement has not passed, it just has not finished.
+  bool completed = false;
   plot::u32 step = 0;
   plot::f64 width[4]{};
   plot::f64 behind[4]{};
@@ -250,7 +259,8 @@ struct FollowScript {
     const plot::Rect axes = p.layout().axes;
     const f32 x = axes.x + 0.5f * axes.w;
     const f32 y = axes.y + 0.5f * axes.h;
-    host.controller().on_pointer({x, y, 0.0f, 0.0f, plot::input::Button::Left, 0, true, true, false});
+    host.controller().on_pointer(
+        {x, y, 0.0f, 0.0f, plot::input::Button::Left, 0, true, true, false});
     host.controller().on_pointer(
         {x + 100.0f, y, 100.0f, 0.0f, plot::input::Button::Left, 0, true, false, false});
     host.controller().on_pointer(
@@ -282,6 +292,8 @@ struct FollowScript {
     const bool pan_only_behind =
         at_edge(behind[0]) && at_edge(behind[1]) && behind[2] > 1e-6 && same(behind[2], behind[3]);
     const bool following = live[0] && live[1] && live[2] && live[3];
+    failed = !(zoom_only_width && pan_only_behind && following);
+    completed = true;
     std::printf(
         "[glfw_demo] follow checks: width changed only by the zoom: %s; distance behind only"
         " by the pan: %s; following armed throughout: %s\n",
@@ -404,6 +416,25 @@ int main() {
     return 1;
   }
 
+  // The live-follow script's verdict, before the summary: a run that asked for the
+  // verification and did not get a passing one fails, whether a check said NO or the
+  // sequence never got far enough to say anything (a deadline too short to reach the
+  // fourth measurement -- a run that measured nothing has not passed).
+  if (script.enabled) {
+    if (script.failed) {
+      std::fprintf(stderr,
+                   "plot_glfw_demo: the live-follow checks failed (see the 'follow checks'"
+                   " line above)\n");
+      return 1;
+    }
+    if (!script.completed) {
+      std::fprintf(stderr,
+                   "plot_glfw_demo: the live-follow script did not complete (raise"
+                   " PLOT_DEMO_EXIT_AFTER_MS)\n");
+      return 1;
+    }
+  }
+
   const plot::Renderer* r = host.renderer();
   const plot::FrameStats stats = r != nullptr ? r->stats() : plot::FrameStats{};
   const plot::Rect axes = std::as_const(p).layout().axes;

include/plot/plot/axis.hpp

@@ -57,8 +57,11 @@ struct Axis {
   /// units, under `XFollow::Live`. Stored rather than implied by the range,
   /// because the range is recomputed from the series' newest x on every frame:
   /// this is the part of the view the user chose, so it is the part that survives
-  /// the recompute. `Plot::set_x_offset()` is the only way in, and clamps it to
-  /// what the traces still retain.
+  /// the recompute. Three things write it, all of them through one clamp: a pan or
+  /// a zoom through `Plot::set_x_range()`, which derives it from the range the
+  /// gesture asked for, `Plot::set_x_offset()` for a caller that states it
+  /// directly, and `Plot::layout()`'s placement, which re-clamps it against the
+  /// retained window every frame. `Plot::x_offset()` reads it back.
   f64 offset = 0.0;
   /// Ticks for the last `refresh()`, ascending, majors and (when asked) minors.
   std::vector<Tick> ticks;

include/plot/plot/plot.hpp

@@ -170,6 +170,13 @@ class Plot {
   void set_y_range(u32 axis, Range r);
 
   /// Re-arms (or disarms) the per-layout auto scaling of one axis.
+  ///
+  /// On an x axis that is armed `XFollow::Live` and has a live edge, this switch is
+  /// inert: `layout()` leaves that axis to the follow path and never fits it (see
+  /// `set_x_follow()`), so an axis that reads `auto_scale == true` -- including the
+  /// `AxisDesc` default -- can still be one whose window is not being fitted. The
+  /// follow state wins because following is the more specific statement about that
+  /// axis, and `auto_fit_x()` is what frames such an axis.
   void set_x_auto_scale(u32 axis, bool on);
   void set_y_auto_scale(u32 axis, bool on);
 
@@ -184,7 +191,11 @@ class Plot {
   /// Arming an axis that has a live edge places its window at the live edge at
   /// once, so `x_range()` reports the window the next layout will produce; the span
   /// it starts from is the span the axis already had, so a caller that wants the
-  /// retained window framed calls `auto_fit_x()`. Turning `Live` off leaves the
+  /// retained window framed calls `auto_fit_x()`. An axis whose stored span is not
+  /// a span -- a range a caller left degenerate while the axis was `Off`, which this
+  /// setter does not second-guess -- has no window to place and nothing to follow
+  /// with; arm it and call `auto_fit_x()`, and note that a range set *under* `Live`
+  /// cannot leave it in that state (see `set_x_range()`). Turning `Live` off leaves the
   /// axis exactly where it is: the last live window becomes an ordinary stored
   /// range, and `auto_scale` is untouched.
   ///

src/plot/plot.cpp

@@ -167,8 +167,7 @@ void Plot::set_x_range(u32 axis, Range r) {
   if (axis >= x_axes_.size()) return;
   Axis& a = x_axes_[axis];
   Range live{};
-  if (a.follow == XFollow::Live && live_x_bounds(axis, live) && r.span() > 0.0 &&
-      std::isfinite(r.min) && std::isfinite(r.max)) {
+  if (a.follow == XFollow::Live && live_x_bounds(axis, live)) {
     // Under Live the caller is stating a *span* and where the window sits relative
     // to the live edge, not a pair of absolute positions: the next layout()
     // recomputes the window from the series' newest x, so storing `r` verbatim
@@ -178,8 +177,22 @@ void Plot::set_x_range(u32 axis, Range r) {
     // drag's data delta -- and place_live_x_window() clamps it and stores the
     // window that follows from it, so the state is consistent before the next
     // layout rather than only after it.
-    a.range = r;
-    a.offset = live.max - r.max;
+    //
+    // This branch is taken unconditionally on a live axis, and what it refuses to
+    // store is what would strand it: `place_live_x_window()` needs a positive span,
+    // and every gesture keeps a non-positive one out of its arithmetic, so a
+    // degenerate range stored here would leave the axis following nothing, movable
+    // by nothing, while `x_follow()` still said Live. An inverted range is a
+    // caller's ordering slip, not a request to break the axis' `min <= max`
+    // invariant -- the rest of the library reads a range as a range -- so it is
+    // normalised; a span that is not a span is ignored outright, leaving the span
+    // and offset the axis holds. `auto_fit_x()` is how a window is framed either
+    // way.
+    const f64 lo = std::min(r.min, r.max);
+    const f64 hi = std::max(r.min, r.max);
+    if (!std::isfinite(lo) || !std::isfinite(hi) || !(hi > lo)) return;
+    a.range = Range{lo, hi};
+    a.offset = live.max - hi;
     place_live_x_window(axis);
     a.auto_scale = false;  // as ever: a range a caller chose is not an auto-fit
     return;

tests/test_controller.cpp

@@ -681,8 +681,8 @@ PLOT_TEST(drag_zoom_is_logical_the_wheel_is_not_scaled) {
     const f32 drag_px = 20.0f * scale;  // 20 *logical* px at both scales
     c.on_pointer({strip_x, strip_y, 0, 0, plot::input::Button::Middle, 0, true, true, false});
     PLOT_CHECK(c.drag_mode() == plot::input::DragMode::AxisZoomX);
-    c.on_pointer({strip_x, strip_y + drag_px, 0, drag_px, plot::input::Button::Middle, 0, true, false,
-                  false});
+    c.on_pointer({strip_x, strip_y + drag_px, 0, drag_px, plot::input::Button::Middle, 0, true,
+                  false, false});
     c.on_pointer({strip_x, strip_y + drag_px, 0, 0, plot::input::Button::Middle, 0, false, false,
                   true});
     PLOT_CHECK_NEAR(p.x_range(ax).span(), 100.0 * std::pow(10.0, -20.0 / 200.0), 1e-6);
@@ -692,3 +692,32 @@ PLOT_TEST(drag_zoom_is_logical_the_wheel_is_not_scaled) {
     PLOT_CHECK_NEAR(p.x_range(ax).span(), span_before * std::exp(-0.15), 1e-9);
   }
 }
+
+/// The consequences of a degenerate range on a live axis, from the gesture side: the
+/// wheel, the pan and the axis zoom all refuse a span that is not a span, so the
+/// frozen axis a stored zero-span left behind was one no interaction could move --
+/// while `x_follow()` said `Live`. With the guard in `set_x_range` the slips a caller
+/// can actually make leave the gestures working.
+PLOT_TEST(live_gestures_survive_a_degenerate_range) {
+  LiveFixture f(200, 40);
+  f.live();
+  const plot::Rect axes = std::as_const(f.graph).layout().axes;
+  const f64 span = f.xr().span();
+  const f64 newest = f.newest();
+  const plot::input::WheelEvent wheel = strip_wheel(f.graph, 1.0f);
+
+  f.graph.set_x_range(f.ax, {newest, newest});  // the programmatic slip
+  PLOT_CHECK_NEAR(f.xr().span(), span, 1e-9);
+  f.graph.set_x_range(f.ax, {newest - 5.0, newest - 15.0});  // and an inverted one
+  PLOT_CHECK_NEAR(f.xr().span(), 10.0, 1e-9);                // normalised, not stored inverted
+
+  f.controller.on_wheel(wheel);
+  PLOT_CHECK_NEAR(f.xr().span(), 10.0 * std::exp(-0.15), 1e-9);  // the wheel still zooms
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+  const f64 span_after_wheel = f.xr().span();
+  drag(f.controller, axes, 60.0f);  // a left drag: the pan gesture
+  PLOT_CHECK(f.graph.x_offset(f.ax) > 0.0);  // the pan still pans
+  PLOT_CHECK_NEAR(f.xr().span(), span_after_wheel, 1e-9);
+  PLOT_CHECK_NEAR(f.xr().max, f.newest() - f.graph.x_offset(f.ax), 1e-9);
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+}

tests/test_plot.cpp

@@ -1365,3 +1365,49 @@ PLOT_TEST(a_static_plot_is_unaffected) {
   PLOT_CHECK_NEAR(f.xr().min, frozen.min, 1e-9);
   PLOT_CHECK_NEAR(f.xr().max, frozen.max, 1e-9);
 }
+
+/// A degenerate range must not be able to strand a live axis. `set_x_range` under
+/// `Live` is read as a span and an offset, and a span that is not a span is not a
+/// window: storing it would leave the axis with a range nothing can place -- every
+/// layout rejects a non-positive span -- and, because every gesture refuses one too,
+/// with a range nothing can move either, while `x_follow()` still said `Live`. So the
+/// one setter that could produce that state refuses to.
+PLOT_TEST(a_degenerate_range_cannot_freeze_a_live_axis) {
+  LiveFixture f(200, 40);
+  f.live();
+  const f64 span = f.xr().span();
+  const f64 newest = f.newest();
+
+  // A zero-width range is ignored: the axis keeps the span and the offset it holds.
+  f.graph.set_x_range(f.ax, {newest, newest});
+  PLOT_CHECK_NEAR(f.xr().span(), span, 1e-9);
+  PLOT_CHECK_NEAR(f.xr().max, newest, 1e-9);
+  PLOT_CHECK(f.graph.x_axis(f.ax).range.max > f.graph.x_axis(f.ax).range.min);
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+  // ...and the stream carries on being followed, which is what the state was for.
+  f.feed(10);
+  f.graph.layout();
+  PLOT_CHECK_NEAR(f.xr().max, f.newest(), 1e-9);
+
+  // A non-finite endpoint is ignored for the same reason.
+  f.graph.set_x_range(f.ax, {f.newest() - 5.0, std::numeric_limits<f64>::quiet_NaN()});
+  PLOT_CHECK_NEAR(f.xr().span(), span, 1e-9);
+  PLOT_CHECK_NEAR(f.xr().max, f.newest(), 1e-9);
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+
+  // An inverted range is a caller's ordering slip, not a request to break the axis'
+  // `min <= max` invariant -- every consumer of a range reads it as one -- so it is
+  // normalised, and the window still spans what the two endpoints enclose.
+  const f64 edge = f.newest();
+  f.graph.set_x_range(f.ax, {edge - 5.0, edge - 15.0});
+  PLOT_CHECK_NEAR(f.xr().span(), 10.0, 1e-9);
+  PLOT_CHECK_NEAR(f.xr().max, edge - 5.0, 1e-9);
+  PLOT_CHECK_NEAR(f.graph.x_offset(f.ax), 5.0, 1e-9);
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+
+  // The offset that came with it is the one the next frames keep.
+  f.feed(20);
+  f.graph.layout();
+  PLOT_CHECK_NEAR(f.xr().max, f.newest() - 5.0, 1e-9);
+  PLOT_CHECK_NEAR(f.xr().span(), 10.0, 1e-9);
+}