trackpad_math.hpp

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#pragma once
 
#include <cmath>
#include <cstdint>
 
namespace toast::input {
 
struct TrackpadTransform {
  float scale;
  float x;
  float y;
};
 
struct TrackpadDelta {
  float dx;
  float dy;
  float zoom;
};
 
class TrackpadGesture {
public:
  static constexpr float pinch_scale = 125.0f;
  static constexpr float epsilon = 3e-5f;
 
  void reset() noexcept {
    m_kind = Kind::none;
    m_last = {pinch_scale, 0.0f, 0.0f};
  }
 
  [[nodiscard]]
  auto started() const noexcept -> bool {
    return m_kind != Kind::none;
  }
 
  auto accept(TrackpadTransform current) noexcept -> TrackpadDelta {
    if (current.scale == 0.0f || (std::abs(current.scale - m_last.scale) <= epsilon &&
                                  std::abs(current.x - m_last.x) <= epsilon && std::abs(current.y - m_last.y) <= epsilon)) {
      return {0.0f, 0.0f, 0.0f};
    }
 
    if (m_kind == Kind::none) {
      m_kind = Kind::pan;
    }
    if (m_kind == Kind::pan && std::abs(current.scale - pinch_scale) > epsilon) {
      m_kind = Kind::pinch;
    }
 
    if (m_kind == Kind::pinch) {
      const float zoom = current.scale - m_last.scale;
      m_last.scale = current.scale;
      return {0.0f, 0.0f, zoom};
    }
 
    const TrackpadDelta delta {current.x - m_last.x, current.y - m_last.y, 0.0f};
    m_last.x = current.x;
    m_last.y = current.y;
    return delta;
  }
 
private:
  enum class Kind : uint8_t {
    none,
    pan,
    pinch
  };
 
  Kind m_kind = Kind::none;
  TrackpadTransform m_last {pinch_scale, 0.0f, 0.0f};
};
 
}