plotter/adapters/qt6/qt_pointer.hpp

// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: 2026 Martino Ferrari

#pragma once

#include <plot/core/types.hpp>
#include <plot/input/events.hpp>
#include <plot/math/vec2.hpp>

namespace plot::qt {

/// The cached pointer position a move event's movement is measured from, in
/// **logical** widget pixels.
///
/// Logical, not device, pixels on purpose. Qt reports every widget position in
/// logical pixels, and the device-pixel ratio can change between two move events
/// -- the window being dragged onto a monitor with another scale factor is the
/// case -- so an anchor cached in device pixels would compare a position computed
/// at the old ratio with one computed at the new one. At 1.0 -> 1.25 a logical
/// position of 500 would then look like a jump of 125 device pixels, and
/// `Controller::pan` applies a delta as a fraction of the range span (and clears
/// auto scale), so a scale change mid-drag would pan the plot by 125 pixels' worth
/// of data. Keeping the anchor logical and scaling the *difference* by the ratio
/// in effect now makes the same gesture map to the same device distance whatever
/// the window's scale is.
///
/// A plain value with no toolkit dependency: the widget owns one, and this is the
/// part of the adapter's translation a test can drive directly (see
/// tests/test_qt_pointer.cpp).
struct PointerAnchor {
  bool have = false;  ///< false until the first sample, which has no movement
  f32 x = 0.0f;       ///< last logical position, widget pixels
  f32 y = 0.0f;

  /// Movement from the cached position to `logical`, in device pixels at `dpr`.
  /// Zero before the first sample, so the first motion of a drag never includes
  /// the jump to where the drag began.
  Vec2 device_delta(Vec2 logical, f64 dpr) const noexcept {
    if (!have) return Vec2{0.0f, 0.0f};
    return Vec2{static_cast<f32>((static_cast<f64>(logical.x) - static_cast<f64>(x)) * dpr),
                static_cast<f32>((static_cast<f64>(logical.y) - static_cast<f64>(y)) * dpr)};
  }

  /// Caches `logical` as the position the next movement is measured from. Always
  /// logical pixels, whatever the ratio is: see the type's comment.
  void set(Vec2 logical) noexcept {
    x = logical.x;
    y = logical.y;
    have = true;
  }
};

/// The state one pinch gesture is folded into `input::GestureEvent`s from.
///
/// Qt reports a pinch as **cumulative** values with no delta of its own:
/// `QPinchGesture::totalScaleFactor()` is the zoom since the gesture began, and
/// `centerPoint()` the focal point now, both absolute in the gesture's life. The
/// controller wants the opposite of both -- `GestureEvent::scale` is *this frame's*
/// zoom factor (1.0 = no change) and `(dx, dy)` the focal point's movement since the
/// previous gesture event -- so the widget owns one of these and folds each update:
/// the zoom arrives as the ratio of the two cumulative scales, the pan as the
/// difference of two focal points.
///
/// The focal point is cached in **logical** widget pixels, like `PointerAnchor`
/// above, for the same reason: `devicePixelRatioF()` can change mid-gesture (the
/// window dragged onto another display), and a cache in device pixels would compare
/// a point taken at the old ratio with one taken at the new one -- a phantom zoom
/// anchor movement, delivered to the controller as a pan. The difference of two
/// logical points, scaled by the ratio in force now, is the movement the fingers
/// actually made.
///
/// A plain value with no toolkit dependency: the widget owns one, and this is the
/// part of the adapter's pinch translation a test can drive directly (see
/// tests/test_qt_pointer.cpp).
struct PinchTracker {
  bool active = false;  ///< true between the gesture's begin and its end/cancel
  f32 last_scale = 1.0f;  ///< the cumulative scale the previous update folded
  f32 x = 0.0f;           ///< last focal point, logical widget pixels
  f32 y = 0.0f;

  /// Starts a gesture at Qt's first cumulative `scale` and logical focal point.
  /// The first event must carry no zoom and no movement -- a pinch that begins has
  /// zoomed nothing yet -- so this only records where the folding starts from.
  void begin(f32 scale, Vec2 logical_center) noexcept {
    active = true;
    last_scale = scale;
    x = logical_center.x;
    y = logical_center.y;
  }

  /// Folds one update into an incremental `GestureEvent`, in device pixels at
  /// `dpr`: the zoom about the focal point, and the focal point's movement since
  /// the previous update. Zero zoom and zero movement before `begin()`, so an
  /// update that arrives without a begin -- a host that missed the start -- is a
  /// no-op rather than a zoom from a made-up anchor.
  input::GestureEvent update(f32 scale, Vec2 logical_center, f64 dpr) noexcept {
    input::GestureEvent g;
    if (!active) return g;
    // The frame's zoom factor: Qt's cumulative ratio over the one this tracker
    // last folded. The first update after begin() divides the scale by itself,
    // which is 1.0 -- a pinch that has not moved has zoomed nothing.
    g.scale = last_scale != 0.0f ? scale / last_scale : 1.0f;
    g.x = static_cast<f32>(static_cast<f64>(logical_center.x) * dpr);
    g.y = static_cast<f32>(static_cast<f64>(logical_center.y) * dpr);
    g.dx = static_cast<f32>((static_cast<f64>(logical_center.x) - static_cast<f64>(x)) * dpr);
    g.dy = static_cast<f32>((static_cast<f64>(logical_center.y) - static_cast<f64>(y)) * dpr);
    last_scale = scale;
    x = logical_center.x;
    y = logical_center.y;
    return g;
  }

  /// Ends the gesture: an update after this (a straggler `GestureUpdated` Qt
  /// delivers out of order, a cancel that still reports) is again a no-op.
  void end() noexcept { active = false; }
};

}  // namespace plot::qt