gehd2.hpp

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//
// Copyright (c) 2025, University of Colorado Denver. All rights reserved.
//
// This file is part of <T>LAPACK.
// <T>LAPACK is free software: you can redistribute it and/or modify it under
// the terms of the BSD 3-Clause license. See the accompanying LICENSE file.
 
#ifndef TLAPACK_GEHD2_HH
#define TLAPACK_GEHD2_HH
 
#include "tlapack/base/utils.hpp"
#include "tlapack/lapack/larf.hpp"
#include "tlapack/lapack/larfg.hpp"
 
namespace tlapack {
 
template <class T, TLAPACK_SMATRIX matrix_t, TLAPACK_VECTOR vector_t>
constexpr WorkInfo gehd2_worksize(size_type<matrix_t> ilo,
                                  size_type<matrix_t> ihi,
                                  const matrix_t& A,
                                  const vector_t& tau)
{
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const idx_t n = ncols(A);
 
    WorkInfo workinfo;
    if (ilo + 1 < ihi && n > 0) {
        auto&& v = slice(A, range{ilo + 1, ihi}, ilo);
 
        auto&& C0 = slice(A, range{0, ihi}, range{ilo + 1, ihi});
        workinfo = larf_worksize<T>(RIGHT_SIDE, FORWARD, COLUMNWISE_STORAGE, v,
                                    tau[0], C0);
 
        auto&& C1 = slice(A, range{ilo + 1, ihi}, range{ilo + 1, n});
        workinfo.minMax(larf_worksize<T>(LEFT_SIDE, FORWARD, COLUMNWISE_STORAGE,
                                         v, tau[0], C1));
    }
 
    return workinfo;
}
 
template <TLAPACK_SMATRIX matrix_t,
          TLAPACK_VECTOR vector_t,
          TLAPACK_WORKSPACE work_t>
int gehd2_work(size_type<matrix_t> ilo,
               size_type<matrix_t> ihi,
               matrix_t& A,
               vector_t& tau,
               work_t& work)
{
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const idx_t n = ncols(A);
 
    // check arguments
    tlapack_check_false(ncols(A) != nrows(A));
    tlapack_check_false((idx_t)size(tau) < n - 1);
 
    // quick return
    if (n <= 0) return 0;
 
    for (idx_t i = ilo; i < ihi - 1; ++i) {
        // Define v := A[i+1:ihi,i]
        auto v = slice(A, range{i + 1, ihi}, i);
 
        // Generate the (i+1)-th elementary Householder reflection on v
        larfg(FORWARD, COLUMNWISE_STORAGE, v, tau[i]);
 
        // Apply Householder reflection from the right to A[0:ihi,i+1:ihi]
        auto C0 = slice(A, range{0, ihi}, range{i + 1, ihi});
        larf_work(RIGHT_SIDE, FORWARD, COLUMNWISE_STORAGE, v, tau[i], C0, work);
 
        // Apply Householder reflection from the left to A[i+1:ihi,i+1:n-1]
        auto C1 = slice(A, range{i + 1, ihi}, range{i + 1, n});
        larf_work(LEFT_SIDE, FORWARD, COLUMNWISE_STORAGE, v, conj(tau[i]), C1,
                  work);
    }
 
    return 0;
}
 
template <TLAPACK_SMATRIX matrix_t, TLAPACK_VECTOR vector_t>
int gehd2(size_type<matrix_t> ilo,
          size_type<matrix_t> ihi,
          matrix_t& A,
          vector_t& tau)
{
    using idx_t = size_type<matrix_t>;
    using T = type_t<matrix_t>;
 
    // functor
    Create<matrix_t> new_matrix;
 
    // constants
    const idx_t n = ncols(A);
 
    // check arguments
    tlapack_check_false(ncols(A) != nrows(A));
    tlapack_check_false((idx_t)size(tau) < n - 1);
 
    // quick return
    if (n <= 0) return 0;
 
    // Allocates workspace
    WorkInfo workinfo = gehd2_worksize<T>(ilo, ihi, A, tau);
    std::vector<T> work_;
    auto work = new_matrix(work_, workinfo.m, workinfo.n);
 
    return gehd2_work(ilo, ihi, A, tau, work);
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_GEHD2_HH