Function curvePointAt

pj_plotting/widget/src/CurveTracker.cpp:233–263

}

std::optional<QPointF> curvePointAt(const QwtPlotCurve* curve, double x) {
  if (curve == nullptr || curve->dataSize() == 0) {
    return std::nullopt;
  }
  if (curve->dataSize() == 1) {
    return curve->sample(0);
  }

  std::size_t low = 0;
  std::size_t high = curve->dataSize();
  while (low < high) {
    const std::size_t mid = low + (high - low) / 2;
    if (curve->sample(mid).x() <= x) {
      low = mid + 1;
    } else {
      high = mid;
    }
  }

  if (low == 0) {
    return curve->sample(0);
  }
  if (low >= curve->dataSize()) {
    return curve->sample(curve->dataSize() - 1);
  }

  const QPointF left = curve->sample(low - 1);
  const QPointF right = curve->sample(low);
  const double middle_x = (left.x() + right.x()) * 0.5;
  return x < middle_x ? left : right;
}

}  // namespace PJ