feat(input): host-agnostic events and pan/zoom/cursor controller

include/plot/input/controller.hpp

@@ -0,0 +1,101 @@
+#pragma once
+
+#include <plot/core/rect.hpp>
+#include <plot/core/types.hpp>
+#include <plot/input/events.hpp>
+
+namespace plot {
+class Plot;
+}
+
+namespace plot::input {
+
+/// What a drag in progress is doing.
+enum class DragMode {
+  None,
+  Pan,        ///< translate every range (left or right drag)
+  BoxZoom,    ///< a shift+left selection is open; the ranges change on release
+  MoveCursor, ///< ctrl+left drag is moving `hovered_cursor()`
+  AxisZoomX,  ///< middle drag inside the x label strip; zooms x only
+  AxisZoomY,  ///< middle drag inside the y label strip; zooms y only
+};
+
+/// Turns host events into plot gestures, knowing nothing about Qt, GTK, GLFW or
+/// ImGui -- Tasks 8-11 translate their native events into the value types above.
+///
+/// It is pure CPU and needs no GL context and no font: everything it reads comes
+/// from `Plot`'s layout, ranges and transforms. The controller owns no viewport
+/// of its own: set_viewport() forwards to `Plot::set_viewport` (and refreshes the
+/// layout) so the two can never disagree, and every gesture runs through
+/// `Plot`'s range setters, which clear `auto_scale` in one place.
+///
+/// Gesture table (grab-and-drag: the data follows the cursor):
+/// | gesture                              | mode         | effect |
+/// |---|---|---|
+/// | left drag                            | `Pan`        | translate both ranges |
+/// | right drag                           | `Pan`        | same, for hosts that reserve left |
+/// | shift + left drag                    | `BoxZoom`    | both ranges from the selection rect |
+/// | ctrl + left drag near a cursor       | `MoveCursor` | move that cursor |
+/// | middle drag on the x label strip     | `AxisZoomX`  | x-only zoom |
+/// | middle drag on the y label strip     | `AxisZoomY`  | y-only zoom |
+/// | wheel (shift: x only, ctrl: y only)  | --           | zoom about the pointer |
+/// | `+` / `-`                            | --           | zoom about the plot centre |
+/// | `f`                                  | --           | auto-fit every axis |
+///
+/// Both axis-zoom drags read the vertical `dy` with the wheel's sign convention
+/// (dragging down zooms in), anchored at the pointer's data coordinate on the
+/// axis they zoom.
+///
+/// A drag that changes no range is harmless: a box-zoom selection under 4 px on
+/// either side is a click, and leaves the ranges alone.
+class Controller {
+ public:
+  explicit Controller(Plot& plot) noexcept : plot_(&plot) {}
+
+  /// Sets the plot's viewport (the controller keeps none of its own) and
+  /// refreshes the layout, so gesture geometry and the host agree.
+  void set_viewport(Rect px);
+
+  void on_pointer(const PointerEvent& e);
+  void on_wheel(const WheelEvent& e);
+  void on_key(const KeyEvent& e);
+
+  DragMode drag_mode() const noexcept { return mode_; }
+  /// The current box-zoom selection in pixels; meaningful while BoxZoom.
+  Rect selection_rect() const noexcept { return selection_; }
+  /// The cursor under the pointer, or `kInvalid` when the pointer is outside the
+  /// plot area or no cursor is within a few pixels.
+  u64 hovered_cursor() const;
+
+ private:
+  void begin_drag(const PointerEvent& e);
+  void update_drag(const PointerEvent& e);
+  void end_drag();
+
+  void pan(const PointerEvent& e);
+  void box_zoom_update(const PointerEvent& e);
+  void apply_box_zoom();
+  void move_cursor(const PointerEvent& e);
+  void axis_zoom(const PointerEvent& e);
+
+  /// The nearest visible cursor within the hit radius of (px, py), or kInvalid.
+  /// kInvalid when (px, py) is outside the plot axes.
+  u64 cursor_at(f32 px, f32 py) const;
+
+  /// Zoom one axis about `anchor` (a data coordinate) by `factor`, recomputing
+  /// the range so the anchor keeps its screen position. Leaves `auto_scale` false.
+  void zoom_x_axis(u32 axis, f64 anchor, f64 factor);
+  void zoom_y_axis(u32 axis, f64 anchor, f64 factor);
+
+  Plot* plot_ = nullptr;
+  DragMode mode_ = DragMode::None;
+  Rect selection_{};
+  f32 origin_x_ = 0.0f;
+  f32 origin_y_ = 0.0f;
+  u64 drag_cursor_ = kInvalid;
+  bool has_pointer_ = false;
+  f32 pointer_x_ = 0.0f;
+  f32 pointer_y_ = 0.0f;
+};
+
+}  // namespace plot::input