gelqt.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_GELQT_HH
#define TLAPACK_GELQT_HH
 
#include "tlapack/base/utils.hpp"
#include "tlapack/lapack/gelq2.hpp"
#include "tlapack/lapack/larfb.hpp"
#include "tlapack/lapack/larft.hpp"
 
namespace tlapack {
 
template <class T, TLAPACK_SMATRIX matrix_t>
constexpr WorkInfo gelqt_worksize(const matrix_t& A, const matrix_t& TT)
{
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const idx_t m = nrows(A);
    const idx_t n = ncols(A);
    const idx_t k = min(m, n);
    const idx_t nb = min((idx_t)ncols(TT), k);
 
    auto&& TT1 = slice(TT, range(0, nb), range(0, nb));
    auto&& A11 = rows(A, range(0, nb));
    auto&& tauw1 = diag(TT1);
 
    return gelq2_worksize<T>(A11, tauw1);
}
 
template <TLAPACK_SMATRIX matrix_t, TLAPACK_WORKSPACE work_t>
int gelqt_work(matrix_t& A, matrix_t& TT, work_t& work)
{
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const idx_t m = nrows(A);
    const idx_t n = ncols(A);
    const idx_t k = min(m, n);
    const idx_t nb = ncols(TT);
 
    // check arguments
    tlapack_check_false(nrows(TT) < m || ncols(TT) < nb);
 
    for (idx_t j = 0; j < k; j += nb) {
        // Use blocked code initially
        idx_t ib = min(nb, k - j);
 
        // Compute the LQ factorization of the current block A(j:j+ib-1,j:n)
        auto TT1 = slice(TT, range(j, j + ib), range(0, ib));
        auto A11 = slice(A, range(j, j + ib), range(j, n));
        auto tauw1 = diag(TT1);
 
        gelq2_work(A11, tauw1, work);
 
        // Form the triangular factor of the block reflector H = H(j) H(j+1)
        // . . . H(j+ib-1)
        larft(FORWARD, ROWWISE_STORAGE, A11, tauw1, TT1);
 
        if (j + ib < k) {
            // Apply H to A(j+ib:m,j:n) from the right
            auto A12 = slice(A, range(j + ib, m), range(j, n));
            auto work1 = slice(TT, range(j + ib, m), range(0, ib));
            larfb_work(RIGHT_SIDE, NO_TRANS, FORWARD, ROWWISE_STORAGE, A11, TT1,
                       A12, work1);
        }
    }
 
    return 0;
}
 
template <TLAPACK_SMATRIX matrix_t>
int gelqt(matrix_t& A, matrix_t& TT)
{
    using T = type_t<matrix_t>;
 
    // functors
    Create<matrix_t> new_matrix;
 
    // Allocates workspace
    WorkInfo workinfo = gelqt_worksize<T>(A, TT);
    std::vector<T> work_;
    auto work = new_matrix(work_, workinfo.m, workinfo.n);
 
    return gelqt_work(A, TT, work);
}
}  // namespace tlapack
#endif  // TLAPACK_GELQT_HH