All notable changes to this project will be documented in this file.

• JOSS badge and update CITATION.cff.
• Opt-in tidy CMake target running clang-tidy (bugprone-*/performance-*) over examples/ and tests/, not part of the default build. Configured via .clang-tidy, excluding the macro-generated wrapper_blas.hpp/wrapper_lapack.hpp from findings.

• Make std::execution optional via HTOOL_WITH_STD_EXECUTION_API macro to avoid linking with TBB. Default to false.
• Improve HMatrixBuilder interface.
• Lock down TreeNode's CRTP constructors and fold CRTPHelper into it.
• Replace std::endl/endl with '\n' across include/, tests/ and examples/.
• Fix unnecessary-value-param findings from clang-tidy: move sink parameters, take pure reads by const reference.

• Remove orphaned local_operators test and proto_ddm/wrapper_proto_ddm.
• Remove dead aspect_ratio stub.

• Fix uninitialized values in Partitioning_N.
• Fix bug in task-based HLU, PR#81 thanks to @ABoisneault.
• Fix number_of_generated_coefficient computation.
• Fix overflow in get_distributed_hmatrix_information.
• Fix float-point computation issue in create_cluster_tree.
• Fix Matrix move constructor/assignment to actually move m_pivots.
• Fix Matrix's int-overflow risk in allocation sizes (rows*cols computed in int then used as a new[]/read/write size), and a zero-length-array leak in resize().
• Fix real overflow/mistyping bugs surfaced by clang-tidy's widening/narrowing-conversion checks in matrix/utils/output.hpp and hmatrix_output.hpp.

• Fix execution header include, PR #77
• Fix wrong ordering of eigenvalues in solve_EVP_3 in some specific cases, PR #76
• Fix interface of add_hmatrix_matrix_product, PR#82.

• Fix wrong boolean tests in triangular_matrix_matrix_solve, PR #75 from @ABoisneault
• Avoid empty-sized gemv, PR #64 from @prj-

• HMatrix recompression with SVD.
• Generic recompressed low-rank compression with RecompressedLowRankGenerator.
• Checks about UPLO for hmatrix factorization.
• HMatrixBuilder for easier HMatrix creation (especially when using only the HMatrix component of Htool-DDM).
• add_hmatrix_vector_product and add_hmatrix_matrix_product for working in user numbering.
• For C++17 and onward, interfaces supporting execution policies (default being sequential execution) has been added for these functions:
  • HMatrixTreeBuilder::build
  • add_hmatrix_matrix_product
  • add_hmatrix_vector_product
  • lu_factorization
  • cholesky_factorization
• Mocking execution policies have been added when compiler does not define std::execution. See exec_compat.
• task_dependencies.hpp for miscellaneous functions used for task based approach.
• hmatrix_output_dot.hpp for L0 and block tree visualization.
• Task based parallelism support has been added via
  • HMatrixTreeBuilder::task_based_build for task based assembly.
  • task_based_internal_add_hmatrix_vector_product for task based alternative to {sequential,openmp}_internal_add_hmatrix_vector_product.
  • task_based_internal_add_hmatrix_hmatrix_product for task based alternative to {sequential,openmp}_internal_add_hmatrix_hmatrix_product.
  • task_based_internal_triangular_hmatrix_hmatrix_solve for task based alternative to internal_triangular_hmatrix_hmatrix_solve.
  • task_based_lu_factorization and task_based_cholesky_factorization for task based alternatives to {sequential,openmp}_lu_factorization and {sequential,openmp}_cholesky_factorization.
  • test_task_based_hmatrix_*.hpp for testing various task based features.
• internal_add_lrmat_hmatrix is now overloaded to handle the case where the HMatrix is larger than the LowRankMatrix.
• get_leaves_from is overloaded to return non const arguments.
• get_false_positive in a tree builder.
• left_hmatrix_ancestor_of_right_hmatrix and left_hmatrix_descendant_of_right_hmatrix for returning parent and children of a hmatrix.
• Partition_N is an alternative to Partition for defining the partition of a cluster. The latter only splits along the principal axis of the cluster, while the former tries to be smarter.

• VirtualInternalLowRankGenerator and VirtualLowRankGenerator's copy_low_rank_approximation function takes a LowRankMatrix as input to populate it and returns a boolean. The return value is true if the compression succeded, false otherwise.
• LowRankMatrix constructors changed. It only takes sizes and an epsilon or a required rank. Then, it is expected to call a VirtualInternalLowRankGenerator to populate it.
• ClusterTreeBuilder has now one strategy as VirtualPartitioning. Usual implementations are still available, for example using Partitioning<double,ComputeLargestExtent,RegularSplitting>.
• ClusterTreeBuilder parameter minclustersize was removed, and a parameter maximal_leaf_size has been added.
• DistributedOperator supports now both "global-to-local" and "local-to-local" operators, using respectively VirtualGlobalToLocalOperator and VirtualLocalToLocalOperator interfaces. The linear algebra associated has been updated to follow a more Blas-like interface.
• MatrixView has been added to ease the use of matrix product. Most public functions for matrix products have also new template arguments to accept, Matrix, MatrixView or any other type following the same interface.

• Fix inline definition of logging_level_to_string.
• Fix error when resizing Matrix.
• Fix error due to using int instead of size_t, thanks to @vdubos.
• Fix warnings with -Wold-style-cast.

• The old implementation of HMatrix was mixing the distributed operations and compression via hierarchical matrices. This is fixed by replacing HMatrix by:
  • DistributedOperator which contains a list of local operators and implements all the distributed operations,
  • VirtualLocalOperator which is the interface local operators must satisfy,
  • LocalDenseMatrix is an example of local operator consisting of a dense matrix Matrix,
  • and LocalHMatrix is an example of local operator consisting of a hierarchical matrix based on HMatrix (different from the previous HMatrix, see below).
• Utility classes that help build DistributedOperator and DDM objects are available, for example: DefaultApproximationBuilder and DDMSolverBuilder. They do all the wiring between the inner interfaces between Htool-DDM's objects, see include/htool/distributed_operator/utility.hpp and include/htool/solvers/utility.hpp.
• Formatter has been added, see .clang_format.
• A logger has been added with Logger. Its output can be customizerd via IObjectWriter.

• HMatrix is now a class representing a hierarchical matrix without distributed-memory parallelism (note that it can still use shared-memory parallelism):
  • It inherits from TreeNode, and it provides the algebra related to hierarchical matrices via free functions:
    • product with vector and matrix (threaded with OpenMP),
    • and with this new version, LU and Cholesky factorization (not threaded yet, WIP).
  • The algorithms for building the block cluster tree is contained in HMatrixTreeBuilder. Users can provide their own "factory".
• VirtualCluster is removed and the clustering part of the library has been rewritten:
  • Cluster now derives from TreeNode, whose template parameter corresponds to the precision of cluster nodes' radius and centre (previously only double).
  • Standards recursive build algorithms are provided via ClusterTreeBuilder. Users can provide their own "factory".
  • ClusterTreeBuilder is a class template and uses the policy pattern (a policy for computing direction, and another for splitting along the direction).
• DDM has been modified, one-level and two-level preconditioners can be customized now via VirtualLocalSolver, VirtualCoarseSpaceBuilder and VirtualCoarseOperatorBuilder.
  • Three one-level preconditioners are provided by Htool-DDM via DDMSolverBuilder:
    • block-jacobi with a local HMatrix
    • a DDM preconditioner where the local subdomain with overlap uses one HMatrix
    • a DDM preconditioner where the local subdomain with overlap uses one HMatrix for the local subdomain without overlap, and dense matrices for the overlap and its interaction with the subdomain without overlap.
  • A second-level (GenEO coarse space) is provided with GeneoCoarseSpaceDenseBuilder and GeneoCoarseOperatorBuilder.

• multilrmat and multihmatrix are removed.

• set_delay_dense_computation to HMatrix
• clustering with user partition requiring permutation

• improved CI with stricter flags
• moved version number definition from misc/define.hpp to htool_version.hpp

• Fix const-correctness for g++ 4.8.5
• Fix compatibility with c++20
• Fix MPI data type for complex float and double
• Fix issue with default MPI communicator

• doxygen documentation
• mvprod_transp_global_to_global and mvprod_transp_local_to_local added to VirtualHMatrix
• getters for clusters in VirtualHMatrix
• custom gmv in ddm

• Corner case with ACA resolved (first row/column only contains zeroes)
• Warnings from fujitsu compilers because of last line
• bug when using threading in mvprod

• CMakefile checks consistency of version number across git/c++/CMake
• Test for warnings coming from include/htool/*
• Coverage added
• Methods in ddm interface to get local numbering
• VirtualLowRankGenerator and VirtualAdmissibilityCondition added for better modularity

• Remove unnecessary arguments in HMatrix and cluster interfaces
• MultiHMatrix deprecated for the moment (everything related to this in htool/multi)

• Missing inlines added
• Fix bug when using htool via petsc with mkl

• Python interface with ctypes deprecated, see new python interface with pybind11.
• Performance tests removed, see dedicated repository.
• GUI deprecated.

• Interface for clustering via VirtualCluster, implementation with Cluster whose template parameter is the type of clustering (supported now: PCARegularClustering, PCAGeometricClustering, BoundingBox1RegularClustering and BoundingBoxGeometricClustering).
• Interface for hmatrix objects via VirtualHMatrix, implementation with HMatrix whose template parameter is the type of compressor (supported now: SVD, fullACA, partialACA, sympartialACA, all derived from abstract class LowRankMatrix).
• Interface for generator via VirtualGenerator. The user needs to define an object deriving from VirtualGenerator to populate a hmatrix.
• Block class with different admissible condition used in Block (only RjasanowSteinbach at the moment).
• Interface for a sub-block via IMatrix, from which both SubMatrix and LowRankMatrix inherit. They respectively correspond to dense and compressed sub-blocks.
• DDM solvers via HPDDM, and coarse space building.
• Test suite and CI with GitHub action.