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

include/plot/input/events.hpp

@@ -0,0 +1,65 @@
+#pragma once
+
+#include <plot/core/types.hpp>
+
+namespace plot::input {
+
+/// Mouse buttons a host can report. The controller never assumes a fixed
+/// mapping: right-drag pans as well as left, for hosts that reserve left-drag.
+enum class Button { Left, Middle, Right };
+
+/// Keyboard modifier bits for `PointerEvent::mods` / `WheelEvent::mods`.
+enum class Mod { None = 0, Shift = 1, Ctrl = 2, Alt = 4, Super = 8 };
+
+/// True when `mods` holds `m`. The single place modifier bits are tested, so a
+/// call site never hand-rolls a mask.
+constexpr bool has_mod(u32 mods, Mod m) noexcept {
+  return (mods & static_cast<u32>(m)) != 0u;
+}
+
+/// A pointer event in viewport pixels (origin top-left, the same space as
+/// `Rect`). `dx`/`dy` are the movement since the previous event; the controller
+/// reads them for panning and the absolute `x`/`y` for anchoring and hit tests.
+/// `pressed` and `released` are the transitions; `down` is the state after the
+/// event. A plain move has all three false/true as appropriate (down only).
+struct PointerEvent {
+  f32 x = 0.0f;
+  f32 y = 0.0f;
+  f32 dx = 0.0f;
+  f32 dy = 0.0f;
+  Button button = Button::Left;
+  u32 mods = 0;
+  bool down = false;      ///< the button is held after this event
+  bool pressed = false;   ///< a press transition happened on this event
+  bool released = false;  ///< a release transition happened on this event
+};
+
+/// A wheel/scroll event at a pixel position. A positive `dy` (scroll up) zooms
+/// in; a positive `dx` scrolls right. Only `dy` drives the controller's zoom.
+struct WheelEvent {
+  f32 x = 0.0f;
+  f32 y = 0.0f;
+  f32 dx = 0.0f;
+  f32 dy = 0.0f;
+  u32 mods = 0;
+};
+
+/// A key event. `key` is an ASCII codepoint for printable keys; hosts that only
+/// have scancodes translate them before calling. `down` distinguishes the press
+/// from the release -- the controller's actions are press-only.
+struct KeyEvent {
+  int key = 0;
+  u32 mods = 0;
+  bool down = false;
+};
+
+/// The printable keys the controller reacts to, named so no call site spells a
+/// bare character literal. `kKeyZoomIn`/`kKeyZoomOut` zoom about the plot centre,
+/// `kKeyFit` re-fits every axis to its traces.
+inline constexpr int kKeyZoomIn = '+';
+inline constexpr int kKeyZoomOut = '-';
+inline constexpr int kKeyFit = 'f';
+/// Hosts that report shifted keys uppercase can pass this unchanged.
+inline constexpr int kKeyFitUpper = 'F';
+
+}  // namespace plot::input