tests: port HyperIdealVisualizationPlugin circle-projection tests

Implements getEuclideanCircleFromHyperbolic() in C++ (hyperboloid model
→ Poincaré disk via Lorentz boost + circumcircle). Ports the 2 Java tests
that were the only remaining candidates not requiring HDS or a solver.

All other unported tests (DataTypesTest, SchottkyIOTest,
UniformizationDataTest, BranchedCoverTorusTest) are blocked by XML
serialization or HDS – stubs remain for Phase 4.

Test totals: 36 registered | 23 passed | 13 skipped | 0 failed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

code/include/hyper_ideal_visualization_utility.hpp

@@ -0,0 +1,129 @@
+#pragma once
+// Port of the static helper
+//   HyperIdealVisualizationPlugin.getEuclideanCircleFromHyperbolic()
+// from de.varylab.discreteconformal.plugin.
+//
+// Converts a hyperbolic circle (center + radius in the hyperboloid model)
+// to its Euclidean representation (cx, cy, r) in the Poincaré disk model.
+//
+// Mathematical background
+// -----------------------
+// Hyperboloid model: points (x,y,z,w) with w²-x²-y²-z²=1, w>0.
+// Metric signature: g = diag(+1,+1,+1,−1)  (spatial-first, time-last).
+//
+// Hyperbolic translation from the origin e₄=(0,0,0,1) to p=(a,b,c,d):
+//   T = [ I₃ + p'·p'ᵀ/(d+1)   p' ]      p' = (a,b,c)
+//       [        p'ᵀ            d  ]
+// This is the standard Lorentz boost; it is in O(3,1) and maps e₄ → p.
+//
+// Poincaré disk projection (jReality convention):
+//   (x,y,z,w)  →  (x,y) / (w+1)
+//
+// The three reference points on the unit hyperbolic circle (at origin) are
+//   p1 = (sinh r, 0, 0, cosh r)
+//   p2 = (0, sinh r, 0, cosh r)
+//   p3 = (-sinh r, 0, 0, cosh r)
+// After translation and projection to the Poincaré disk their circumcircle
+// equals the image of the original hyperbolic circle.
+
+#include <Eigen/Dense>
+#include <array>
+#include <cmath>
+
+namespace conformallab {
+
+// ---------------------------------------------------------------------------
+// Circumcenter of three 2-D points
+// ---------------------------------------------------------------------------
+inline Eigen::Vector2d circumcenter2d(
+    const Eigen::Vector2d& a,
+    const Eigen::Vector2d& b,
+    const Eigen::Vector2d& c)
+{
+    double ax = a.x(), ay = a.y();
+    double bx = b.x(), by = b.y();
+    double cx = c.x(), cy = c.y();
+
+    double D = 2.0 * (ax*(by - cy) + bx*(cy - ay) + cx*(ay - by));
+    double a2 = ax*ax + ay*ay;
+    double b2 = bx*bx + by*by;
+    double c2 = cx*cx + cy*cy;
+
+    double ux = (a2*(by - cy) + b2*(cy - ay) + c2*(ay - by)) / D;
+    double uy = (a2*(cx - bx) + b2*(ax - cx) + c2*(bx - ax)) / D;
+    return {ux, uy};
+}
+
+// ---------------------------------------------------------------------------
+// 4×4 Lorentz boost: maps the hyperboloid origin e₄=(0,0,0,1) to `center`.
+// `center` must lie on the hyperboloid: center[3]² - ‖center.head<3>()‖² = 1.
+// ---------------------------------------------------------------------------
+inline Eigen::Matrix4d hyperboloidTranslation(const Eigen::Vector4d& center)
+{
+    Eigen::Vector3d p = center.head<3>();
+    double          d = center(3);
+
+    Eigen::Matrix4d T = Eigen::Matrix4d::Identity();
+    // Upper-left 3×3 block: I + p'·p'ᵀ / (d+1)
+    T.block<3,3>(0,0) += p * p.transpose() / (d + 1.0);
+    // Right column and bottom row
+    T.block<3,1>(0,3) = p;
+    T.block<1,3>(3,0) = p.transpose();
+    T(3,3) = d;
+    return T;
+}
+
+// ---------------------------------------------------------------------------
+// Project a hyperboloid point to the Poincaré disk (jReality convention:
+// add 1 to the w-coordinate, then dehomogenize the spatial part).
+// ---------------------------------------------------------------------------
+inline Eigen::Vector2d toPoincareDisk(const Eigen::Vector4d& x)
+{
+    double w = x(3) + 1.0;
+    return {x(0) / w, x(1) / w};
+}
+
+// ---------------------------------------------------------------------------
+// getEuclideanCircleFromHyperbolic
+//
+// Inputs
+//   center  – point on the hyperboloid, e.g. (0,0,0,1) for the origin
+//   radius  – hyperbolic radius (real number > 0)
+//
+// Output
+//   { euclidean_cx, euclidean_cy, euclidean_radius }
+//   describing the circle in the Poincaré disk that corresponds to the
+//   given hyperbolic circle.
+//
+// Port of HyperIdealVisualizationPlugin.getEuclideanCircleFromHyperbolic()
+// ---------------------------------------------------------------------------
+inline std::array<double,3> getEuclideanCircleFromHyperbolic(
+    const Eigen::Vector4d& center, double radius)
+{
+    const double s  = std::sinh(radius);
+    const double ch = std::cosh(radius);
+
+    // Three points on the hyperbolic circle centered at the origin
+    Eigen::Vector4d p1( s, 0.0, 0.0, ch);
+    Eigen::Vector4d p2(0.0,  s, 0.0, ch);
+    Eigen::Vector4d p3(-s, 0.0, 0.0, ch);
+
+    // Apply the hyperbolic translation to the target center
+    const Eigen::Matrix4d T = hyperboloidTranslation(center);
+    p1 = T * p1;
+    p2 = T * p2;
+    p3 = T * p3;
+
+    // Project to the Poincaré disk
+    const Eigen::Vector2d q1 = toPoincareDisk(p1);
+    const Eigen::Vector2d q2 = toPoincareDisk(p2);
+    const Eigen::Vector2d q3 = toPoincareDisk(p3);
+
+    // Euclidean circumcircle of the three projected points
+    const Eigen::Vector2d ec = circumcenter2d(q1, q2, q3);
+    const double           r  = (ec - q1).norm();
+
+    return {ec.x(), ec.y(), r};
+}
+
+} // namespace conformallab

code/tests/CMakeLists.txt

@@ -6,6 +6,7 @@ add_executable(conformallab_tests
     test_surface_curve_utility.cpp
     test_discrete_elliptic_utility.cpp
     test_p2_utility.cpp
+    test_hyper_ideal_visualization_utility.cpp
 
     # ── Stubs: blocked until HDS port (Phase 4) ──────────────────────────────
     # All tests call GTEST_SKIP() with a clear explanation.

code/tests/test_hyper_ideal_visualization_utility.cpp

@@ -0,0 +1,50 @@
+// Port of de.varylab.discreteconformal.plugin.HyperIdealVisualizationPluginTest (Java/JUnit).
+//
+// Tests the conversion from a hyperbolic circle (hyperboloid model)
+// to its Euclidean representation (Poincaré disk model).
+//
+// Java test: static method HyperIdealVisualizationPlugin
+//              .getEuclideanCircleFromHyperbolic(double[] center, double radius)
+// C++ port:  conformallab::getEuclideanCircleFromHyperbolic(Vector4d, double)
+//              in hyper_ideal_visualization_utility.hpp
+
+#include "hyper_ideal_visualization_utility.hpp"
+#include <gtest/gtest.h>
+#include <cmath>
+
+using namespace conformallab;
+
+// Corresponds to Java testGetEuclideanCircleFromHyperbolic_Centered()
+//
+// A hyperbolic circle centered at the origin (0,0,0,1) with radius 1.
+// In the Poincaré disk this maps to a Euclidean circle centered at (0,0)
+// with radius  sinh(1) / (cosh(1) + 1).
+TEST(HyperIdealVisualizationUtilityTest, EuclideanCircleFromHyperbolic_Centered)
+{
+    Eigen::Vector4d center(0.0, 0.0, 0.0, 1.0);
+    const double radius = 1.0;
+
+    auto result = getEuclideanCircleFromHyperbolic(center, radius);
+
+    const double expected_r = std::sinh(1.0) / (std::cosh(1.0) + 1.0);
+    EXPECT_NEAR(0.0,        result[0], 1E-12) << "Euclidean cx should be 0";
+    EXPECT_NEAR(0.0,        result[1], 1E-12) << "Euclidean cy should be 0";
+    EXPECT_NEAR(expected_r, result[2], 1E-12) << "Euclidean radius mismatch";
+}
+
+// Corresponds to Java testGetEuclideanCircleFromHyperbolic_OffCenter()
+//
+// A hyperbolic circle centered at (sinh(1),0,0,cosh(1)) with radius 1.
+// By symmetry (center is on the x-axis, circle is symmetric about it):
+//   • the Euclidean center lies on the x-axis  → cy = 0
+//   • the Euclidean center equals the radius   → cx = r  (the circle passes through the Poincaré origin)
+TEST(HyperIdealVisualizationUtilityTest, EuclideanCircleFromHyperbolic_OffCenter)
+{
+    Eigen::Vector4d center(std::sinh(1.0), 0.0, 0.0, std::cosh(1.0));
+    const double radius = 1.0;
+
+    auto result = getEuclideanCircleFromHyperbolic(center, radius);
+
+    EXPECT_NEAR(result[0], result[2], 1E-12) << "cx should equal Euclidean radius";
+    EXPECT_NEAR(0.0, result[1],       1E-12) << "cy should be 0 (x-axis symmetry)";
+}