plotter/include/plot/input/controller.hpp

#pragma once

#include <plot/core/rect.hpp>
#include <plot/core/types.hpp>
#include <plot/input/events.hpp>
#include <plot/plot/region.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, ///< a drag is moving the cursor marker it grabbed
  MoveRegion, ///< a drag is translating a whole region, width preserved
  ResizeRegionLeft,  ///< a drag is moving the bound that was `x0` when it opened
  ResizeRegionRight, ///< a drag is moving the bound that was `x1` when it opened
  AxisZoomX,  ///< middle drag inside the x label strip; zooms x only
  AxisZoomY,  ///< middle drag inside the y label strip; zooms y only
};

/// The grab zone of a region, named with the primitive it describes
/// (`plot/plot/region.hpp`, which this header imports it from): the input layer
/// reports it and the renderer colours by it -- through `Plot::hot_handles()`,
/// which carries the zone the pointer is on -- so both layers speak of one type
/// rather than of two that would have to be kept in step.
using plot::RegionGrab;

/// 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 |
/// |---|---|---|
/// | shift + left drag                    | `BoxZoom`    | both ranges from the selection rect |
/// | left drag on a handle                | `MoveCursor` / `MoveRegion` / `ResizeRegion*` | move the handle the press grabbed |
/// | left drag elsewhere                  | `Pan`        | translate both ranges |
/// | right drag                           | `Pan`        | same, for hosts that reserve left |
/// | 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 |
///
/// The right button pans *and* grabs: both are buttons a host drives drags with, so
/// neither one is singled out for handles. Middle keeps its own role on the label
/// strips, and a middle drag anywhere else still does nothing.
///
/// Both axis-zoom drags read the vertical `dy` in pointer pixels with the
/// wheel's sign convention (dragging down zooms in), anchored at the pointer's
/// data coordinate on the axis they zoom. The drag has its own per-pixel
/// sensitivity -- a pixel is not a wheel notch -- of one decade per 200 **logical**
/// px, so a 200 px drag scales the span by 10 whatever the display's scale.
///
/// 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.
///
/// ## Direct manipulation (spec §5)
///
/// Cursors and regions are *grabbable*: a press on one enters a drag that moves it,
/// with no modifier, and a press anywhere else pans exactly as it always has -- so
/// the new gesture is a superset of the old one and every host that drives a drag
/// keeps panning. `Ctrl` is no longer needed for a cursor move and does not change
/// what a press on one does; it only means the same thing it always did (a press
/// away from a handle pans, with or without it).
///
/// One hit test runs for both, in this order, and that precedence is the whole rule:
///
///  1. **Cursor markers first**, at the radius `hovered_cursor()` already uses, the
///     nearest one winning. A marker is drawn on top of everything and is hit-tested
///     the same way: the shaded band between two measurement cursors is grabbed by
///     dragging either of the cursors that bound it, because its edges *are* those
///     cursors -- no second path, no second tolerance.
///  2. **Regions next**, resolved nearest-first against the geometry `Plot::layout()`
///     measured: a bound within the grab radius beats the body (so the bound of a
///     wide region is grabbable even though the pointer is inside the fill), the
///     nearer of the two bounds wins, and with no bound in range the topmost region
///     whose interior holds the pointer is the body. The radius is the one above, cut
///     back when a region is too narrow to hold a band at each end *and* an interior:
///     a region narrower than twice it would otherwise have every interior point
///     within a bound's reach, and could be resized but never moved.
///  3. **Nothing hot**, and the press pans.
///
/// A region narrower than a few logical pixels is a click rather than an edge grab:
/// its two bounds are under the pointer together, so an edge would always win and
/// the region could never be moved again once collapsed. Such a region is
/// grabbed by its body instead, widened to that same minimum, and it stays movable --
/// and between that minimum and twice the grab radius, where the two bounds would
/// still cover every interior point, the edge bands are cut back until the middle is
/// left, so a region in that band of widths can be moved as well as resized.
/// The *body* drag translates both bounds by one delta -- the width is preserved
/// exactly -- while a bound drag moves one end and leaves the other where it was,
/// so a drag that crosses one bound over the other swaps them rather than inverting
/// the region (`Plot::set_region()` orders on assignment).
///
/// A region that asks to snap is put on the sample grid once, *on release*, not on
/// every motion: a body dragged a fraction of a sample apart would otherwise snap
/// both ends back to where they started and never move at all.
///
/// The hovered and grabbed handles are published to the plot
/// (`Plot::set_hot_handles()`) after every event that can move the geometry under the
/// pointer -- a pointer event, a wheel notch, a key stroke -- so the renderer draws them
/// highlighted without knowing what a pointer is; `hovered_cursor()`,
/// `grabbed_cursor()`, `hovered_region()`, `grabbed_region()` and `region_grab()` are
/// the same answer for a host that wants it directly -- a resize cursor, a tooltip,
/// a status line.
///
/// ## Zoom and pan on a live x axis (spec §12)
///
/// An x axis armed with `XFollow::Live` keeps following through every gesture
/// below. Nothing here reads or writes the follow state, and the ranges these
/// gestures set are read by `Plot::set_x_range` as a span and an offset behind the
/// live edge rather than as absolute positions, so both halves of the requirement
/// fall out of the paths that already existed:
///
///  - the wheel *multiplies* the span, so notches compound and the zoom is
///    scale-free. Its anchor is the data coordinate under the pointer while the
///    pointer is inside `axes`; outside it there is no such coordinate, and on a
///    live axis the live edge is used instead (see `x_zoom_anchor()`), which leaves
///    the window ending where it ended. Either way the offset is adjusted to keep
///    the anchor still, and following stays armed;
///  - a pan shifts the offset by the drag's data delta and leaves the span alone, so
///    a user can walk back through the retained window and stops at the live edge
///    rather than past it.
///
/// An `Off` axis -- every axis by default -- behaves exactly as it always has.
///
/// ## Tolerances are logical, the coordinates they are compared against are not
///
/// The plot geometry the controller hit-tests in is device pixels (see
/// `PlotLayout`), and each adapter converts its pointer coordinates into the same
/// space before forwarding them. A tolerance written as a device constant would
/// therefore change its *physical* size with the display: a 5 px grab radius is
/// half the hand movement on a 2x panel, so the same cursor becomes harder to
/// catch there. Both tolerances -- the cursor grab radius and the box-zoom click
/// threshold -- are therefore logical constants, converted through
/// `Style::device()` at the scale of the plot the controller already holds: that is
/// the single place the logical -> device rule lives, and the plot is the one that
/// knows what its device pixel is worth, so the controller keeps no copy of the scale
/// for a host to keep in step.
///
/// The middle-drag axis zoom's per-pixel gain is logical for the same reason, by a
/// different route: it is stated in *logical* pixels (200 of them is one decade),
/// and the drag delta arrives in device pixels like every other coordinate here, so
/// `axis_zoom()` divides it by `Plot::ui_scale()` before applying the gain. Without
/// that the same physical drag would zoom twice as much at 2x. The wheel needs no
/// such conversion: a notch is already device-independent, and the wheel path is
/// left 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);

  /// Events are sanitised before they are acted on: a non-finite coordinate or
  /// delta -- a host's "no pointer" sentinel (ImGui reports `-FLT_MAX`), a NaN
  /// from a degenerate mapping, an infinity from a lost device -- becomes the last
  /// known position and no movement. Garbage must not reach the ranges: a gesture
  /// that sets a range also clears auto scale, so a range driven to 1e35 is one the
  /// user can neither pan nor zoom back. The buttons in the same event are still
  /// acted on, so a drag whose release arrives with a bad position still ends.
  void on_pointer(const PointerEvent& e);
  /// Same, and a wheel amount that is not a number is *refused* rather than
  /// zeroed: a zero-notch zoom changes no range but still goes through the range
  /// setters, which clear auto scale -- so a sanitised amount would quietly disarm
  /// auto-fit. An unusable amount leaves the plot exactly as it was; an unusable
  /// *position* only loses its anchor, falling back to the last known one.
  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;
  /// The cursor a drag is moving, or `kInvalid` when no cursor drag is open. The two
  /// are the same answer the plot publishes for the renderer (see HotHandles), for a
  /// host that wants it directly.
  u64 grabbed_cursor() const noexcept { return drag_cursor_; }
  /// The region a drag is moving or resizing, or `kInvalid` when no region drag is
  /// open.
  u64 grabbed_region() const noexcept { return drag_region_; }
  /// The region the pointer is on, or `kInvalid`. `region_grab()` says which zone.
  u64 hovered_region() const;
  /// Which zone of `region` the pointer is on: the bound nearest within the grab
  /// radius (`Left`/`Right`) -- cut back for a narrow region, see `zone_hit()` -- else
  /// `Body` when the pointer is inside the interval --
  /// widened to the minimum a collapsed region keeps -- and `None` when it is on
  /// neither, when the pointer is outside the plot area, or when no pointer has been
  /// seen yet. What a host sets its resize cursor from; an id that names no region
  /// reports `None`.
  RegionGrab region_grab(u32 region) const;

 private:
  void begin_drag(const PointerEvent& e);
  void update_drag(const PointerEvent& e);
  void end_drag(const PointerEvent& e);

  void pan(const PointerEvent& e);
  void box_zoom_update(const PointerEvent& e);
  void apply_box_zoom();
  void move_cursor(const PointerEvent& e);
  void move_region(const PointerEvent& e);
  void resize_region(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;

  /// The region and zone a pointer position is on. Both fields are "nothing" when
  /// the pointer is outside `axes` or no region is hot; `zone` is `Body` for an
  /// interior hit and never `None` for a region that is named.
  struct RegionHot {
    u64 region = kInvalid;
    RegionGrab zone = RegionGrab::None;
  };
  /// One region's zone under a pointer position, with the distance to the bound the
  /// zone names: 0 for a body, and meaningless for `None`. The one place the region
  /// hit test lives -- `region_hot()` compares what it returns and `region_grab()`
  /// reports it -- so the nearest-first rule is written once.
  struct ZoneHit {
    RegionGrab zone = RegionGrab::None;
    f32 distance = 0.0f;
  };
  /// The band a bound answers to is the controller's grab radius, cut back so that a
  /// narrow region keeps an interior a body drag can reach (see the class comment):
  /// `min(radius, (width - minimum interior) / 2)`, and no band at all when the two
  /// bounds are closer together than the minimum width that separates them.
  ZoneHit zone_hit(const RegionLayout& region, f32 px, f32 py) const;
  /// Resolves that hit against the geometry `Plot::layout()` measured -- the same
  /// rects the renderer paints, so what is drawn and what can be grabbed cannot
  /// drift apart -- with the precedence the class comment states: nearest grabbable
  /// bound first, topmost body after it.
  RegionHot region_hot(f32 px, f32 py) const;
  /// Opens the drag a press asks for: the cursor hit first, then the region zones.
  /// False when the press is on no handle at all, i.e. when the caller should pan.
  /// Records what the drag needs to keep hold of -- the bound an edge resize must
  /// leave alone, the bounds and the grabbed data x a body move needs to preserve
  /// the width -- so a changed pointer motion never accumulates error.
  bool begin_handle_drag(const PointerEvent& e);
  /// Publishes what the pointer is on to the plot, for the renderer to highlight
  /// (see HotHandles). Called after every event that can move the geometry under the
  /// pointer -- a pointer event, a wheel notch, a key stroke -- so hover and grab are
  /// always the answer the hit test gives for the geometry of this frame rather than
  /// the geometry of the last time the pointer moved.
  void publish_hot();

  /// The data x a zoom of `axis` holds fixed, for a gesture at (px, py). The
  /// coordinate under the pointer while the pointer is inside `axes`, which is what
  /// makes a zoom feel anchored to the data being pointed at; outside `axes` there
  /// is no such coordinate, and a live axis (`XFollow::Live`) uses its live edge
  /// instead, so a zoom made from the label strip or off the plot zooms the window
  /// about the newest sample and leaves it ending where it ended.
  f64 x_zoom_anchor(u32 axis, 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;
  /// under `XFollow::Live` the range is read by `Plot::set_x_range` as a span and an
  /// offset behind the live edge, which is what keeps following armed.
  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;
  /// The region drag's state. `drag_region_` is the region and `drag_zone_` the zone
  /// the press resolved, kept for the whole gesture so the highlight a grab started
  /// follows the grab rather than the pointer. The three numbers are what keeps a
  /// motion from accumulating error: an edge resize holds `drag_fixed_x_`, the bound
  /// it must leave alone, and a body move holds the bounds as they were at the press
  /// (`drag_x0_`/`drag_x1_`) plus the data x the press grabbed (`drag_grab_x_`), so
  /// each motion is a fresh function of the pointer rather than a step from the last
  /// one.
  u64 drag_region_ = kInvalid;
  RegionGrab drag_zone_ = RegionGrab::None;
  f64 drag_fixed_x_ = 0.0;
  f64 drag_grab_x_ = 0.0;
  f64 drag_x0_ = 0.0;
  f64 drag_x1_ = 0.0;
  bool has_pointer_ = false;
  f32 pointer_x_ = 0.0f;
  f32 pointer_y_ = 0.0f;
};

}  // namespace plot::input