From f9b7b6651b108136a16df44d91fb31735645f5a7 Mon Sep 17 00:00:00 2001
From: Sergiu Deitsch <sergiu.deitsch@gmail.com>
Date: Sun, 28 Sep 2025 08:35:06 +0200
Subject: [PATCH] Support Eigen3 5.0.0

Change-Id: I5cf476a6d8c090861f4ea4a254a35d39e36a6d68
---
 CMakeLists.txt                     | 27 ++++++++++++++++++++++++++-
 internal/ceres/covariance_impl.cc  |  5 +++++
 internal/ceres/invert_psd_matrix.h |  4 ++++
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 754f069bf..19ee2a864 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -232,9 +232,34 @@ endif (IOS)
 
 unset(CERES_COMPILE_OPTIONS)
 
+# We call find_package multiple time with different versioning scheme to support
+# both Eigen3 3.x and 5.x. If the first invocation fails and Eigen3_DIR was set
+# to a possible location, e.g., during cross-compilation, failure to locate the
+# package will unset the variable. To avoid losing the path hint, we save it and
+# reuse it in the second find_package call.
+set (_Ceres_Eigen3_DIR)
+
+if (Eigen3_DIR)
+  set (_Ceres_Eigen3_DIR "${Eigen3_DIR}")
+endif (Eigen3_DIR)
+
 # Eigen.
 # Eigen delivers Eigen3Config.cmake since v3.3.3
-find_package(Eigen3 3.3.4 REQUIRED NO_MODULE)
+find_package(Eigen3 3.3.4...5 NO_MODULE)
+
+if (NOT Eigen3_FOUND)
+  if (_Ceres_Eigen3_DIR)
+    set (Eigen3_DIR "${_Ceres_Eigen3_DIR}")
+  endif (_Ceres_Eigen3_DIR)
+  # Eigen3's CMake package config prior to 5.0.0 does not support version ranges
+  # with different major version components. To ensure backward compatibility,
+  # we locate the package using the previous version scheme as a fallback
+  # mechanism.
+  find_package(Eigen3 3.3.4 REQUIRED NO_MODULE)
+endif (NOT Eigen3_FOUND)
+
+unset (_Ceres_Eigen3_DIR)
+
 if (Eigen3_FOUND)
   message("-- Found Eigen version ${Eigen3_VERSION}: ${Eigen3_DIR}")
   if (EIGENSPARSE)
diff --git a/internal/ceres/covariance_impl.cc b/internal/ceres/covariance_impl.cc
index 56221945a..930711fd0 100644
--- a/internal/ceres/covariance_impl.cc
+++ b/internal/ceres/covariance_impl.cc
@@ -724,8 +724,13 @@ bool CovarianceImpl::ComputeCovarianceValuesUsingDenseSVD() {
   }
   event_logger.AddEvent("ConvertToDenseMatrix");
 
+#if EIGEN_VERSION_AT_LEAST(5, 0, 0)
+  Eigen::BDCSVD<Matrix, Eigen::ComputeThinU | Eigen::ComputeThinV> svd(
+      dense_jacobian);
+#else   // !EIGEN_VERSION_AT_LEAST(5, 0, 0)
   Eigen::BDCSVD<Matrix> svd(dense_jacobian,
                             Eigen::ComputeThinU | Eigen::ComputeThinV);
+#endif  // EIGEN_VERSION_AT_LEAST(5, 0, 0)
 
   event_logger.AddEvent("SingularValueDecomposition");
 
diff --git a/internal/ceres/invert_psd_matrix.h b/internal/ceres/invert_psd_matrix.h
index 21ba2dc8a..dd3a0a34b 100644
--- a/internal/ceres/invert_psd_matrix.h
+++ b/internal/ceres/invert_psd_matrix.h
@@ -67,7 +67,11 @@ typename EigenTypes<kSize, kSize>::Matrix InvertPSDMatrix(
 
   // For a thin SVD the number of columns of the matrix need to be dynamic.
   using SVDMType = typename EigenTypes<kSize, Eigen::Dynamic>::Matrix;
+#if EIGEN_VERSION_AT_LEAST(5, 0, 0)
+  Eigen::JacobiSVD<SVDMType, Eigen::ComputeThinU | Eigen::ComputeThinV> svd(m);
+#else   // !EIGEN_VERSION_AT_LEAST(5, 0, 0)
   Eigen::JacobiSVD<SVDMType> svd(m, Eigen::ComputeThinU | Eigen::ComputeThinV);
+#endif  // EIGEN_VERSION_AT_LEAST(5, 0, 0)
   return svd.solve(MType::Identity(size, size));
 }