unghr.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_UNGHR_HH
#define TLAPACK_UNGHR_HH
 
#include "tlapack/base/utils.hpp"
#include "tlapack/lapack/ungq.hpp"
 
namespace tlapack {
 
template <class T, TLAPACK_SMATRIX matrix_t, TLAPACK_SVECTOR vector_t>
constexpr WorkInfo unghr_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 nh = (ihi > ilo + 1) ? ihi - 1 - ilo : 0;
 
    if (nh > 0 && ilo + 1 < ihi) {
        auto&& A_s = slice(A, range{ilo + 1, ihi}, range{ilo + 1, ihi});
        auto&& tau_s = slice(tau, range{ilo, ihi - 1});
        return ungq_worksize<T>(FORWARD, COLUMNWISE_STORAGE, A_s, tau_s);
    }
    return WorkInfo(0);
}
 
template <TLAPACK_SMATRIX matrix_t,
          TLAPACK_SVECTOR vector_t,
          TLAPACK_WORKSPACE work_t>
int unghr_work(size_type<matrix_t> ilo,
               size_type<matrix_t> ihi,
               matrix_t& A,
               const vector_t& tau,
               work_t& work)
{
    using T = type_t<matrix_t>;
    using real_t = real_type<T>;
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const real_t zero(0);
    const real_t one(1);
    const idx_t m = nrows(A);
    const idx_t n = ncols(A);
    const idx_t nh = ihi > ilo + 1 ? ihi - 1 - ilo : 0;
 
    // check arguments
    tlapack_check_false((idx_t)size(tau) < min(m, n));
 
    // Shift the vectors which define the elementary reflectors one
    // column to the right, and set the first ilo and the last n-ihi
    // rows and columns to those of the unit matrix
 
    // This is currently optimised for column matrices, it may be interesting
    // to also write these loops for row matrices
    for (idx_t j = ihi - 1; j > ilo; --j) {
        for (idx_t i = 0; i < j; ++i) {
            A(i, j) = zero;
        }
        for (idx_t i = j + 1; i < ihi; ++i) {
            A(i, j) = A(i, j - 1);
        }
        for (idx_t i = ihi; i < n; ++i) {
            A(i, j) = zero;
        }
    }
    for (idx_t j = 0; j < ilo + 1; ++j) {
        for (idx_t i = 0; i < n; ++i) {
            A(i, j) = zero;
        }
        A(j, j) = one;
    }
    for (idx_t j = ihi; j < n; ++j) {
        for (idx_t i = 0; i < n; ++i) {
            A(i, j) = zero;
        }
        A(j, j) = one;
    }
 
    // Now that the vectors are shifted, we can call orgqr to generate the
    // matrix orgqr is not yet implemented, so we call org2r instead
    if (nh > 0) {
        auto A_s = slice(A, range{ilo + 1, ihi}, range{ilo + 1, ihi});
        auto tau_s = slice(tau, range{ilo, ihi - 1});
        return ungq_work(FORWARD, COLUMNWISE_STORAGE, A_s, tau_s, work);
    }
    else {
        return 0;
    }
}
 
template <TLAPACK_SMATRIX matrix_t, TLAPACK_SVECTOR vector_t>
int unghr(size_type<matrix_t> ilo,
          size_type<matrix_t> ihi,
          matrix_t& A,
          const vector_t& tau)
{
    using T = type_t<matrix_t>;
 
    // Functor
    Create<matrix_t> new_matrix;
 
    // Allocates workspace
    WorkInfo workinfo = unghr_worksize<T>(ilo, ihi, A, tau);
    std::vector<T> work_;
    auto work = new_matrix(work_, workinfo.m, workinfo.n);
 
    return unghr_work(ilo, ihi, A, tau, work);
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_UNGHR_HH