update src and test

tests/sla_print/sla_parabola_tests.cpp

@@ -0,0 +1,50 @@
+#include "sla_test_utils.hpp"
+
+#include <libslic3r/SLA/SupportIslands/ParabolaUtils.hpp>
+
+using namespace Slic3r;
+using namespace Slic3r::sla;
+
+void parabola_check_length(const ParabolaSegment &parabola)
+{
+    auto   diffPoint = parabola.to - parabola.from;
+    double min       = sqrt(diffPoint.x() * diffPoint.x() +
+                      diffPoint.y() * diffPoint.y());
+    double max       = static_cast<double>(diffPoint.x()) + diffPoint.y();
+    double len = ParabolaUtils::length(parabola);
+    double len2 = ParabolaUtils::length_by_sampling(parabola, 1.);
+    CHECK(fabs(len2 - len) < 1.);
+    CHECK(len >= min);
+    CHECK(len <= max);
+}
+
+// after generalization put to ParabolaUtils
+double getParabolaY(const Parabola &parabola, double x)
+{
+    double f    = ParabolaUtils::focal_length(parabola);
+    Vec2d  perp = parabola.directrix.normal().cast<double>();
+    // work only for test cases
+    if (perp.y() > 0.) perp *= -1.;
+    perp.normalize();
+    Vec2d v = parabola.focus.cast<double>() + perp * f;
+    return 1 / (4 * f) * (x - v.x()) * (x - v.x()) + v.y();
+}
+
+TEST_CASE("Parabola length", "[SupGen][Voronoi][Parabola]")
+{
+    using namespace Slic3r::sla;
+    double scale = 1e6;
+    // U shape parabola
+    Parabola parabola_x2(Line({-1. * scale, -.25 * scale},
+                              {1. * scale, -.25 * scale}),
+                         Point(0. * scale, .25 * scale));
+
+    double from_x = 1 * scale;
+    double to_x   = 3 * scale;
+    Point  from(from_x, getParabolaY(parabola_x2, from_x));
+    Point  to(to_x, getParabolaY(parabola_x2, to_x));
+    ParabolaSegment parabola_segment(parabola_x2, from, to);
+    parabola_check_length(parabola_segment);
+} 
+
+