ul_mult.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_ul_mult_HH
#define TLAPACK_ul_mult_HH
 
#include "tlapack/base/utils.hpp"
#include "tlapack/blas/gemm.hpp"
#include "tlapack/blas/trmm.hpp"
 
namespace tlapack {
 
template <TLAPACK_SMATRIX matrix_t>
int ul_mult(matrix_t& A)
{
    using idx_t = size_type<matrix_t>;
    using T = type_t<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // check arguments
    tlapack_check(nrows(A) == ncols(A));
 
    // constant
    const idx_t n = ncols(A);
 
    // if L and U are 1-by-1, then L is 1 and we simply UL=A(0,0)
    if (n == 1) return 0;
 
    // if n>1
    idx_t n0 = n / 2;
 
    // break A into four parts
    auto A00 = tlapack::slice(A, range(0, n0), range(0, n0));
    auto A10 = tlapack::slice(A, range(n0, n), range(0, n0));
    auto A01 = tlapack::slice(A, range(0, n0), range(n0, n));
    auto A11 = tlapack::slice(A, range(n0, n), range(n0, n));
 
    // calculate top left corner
    ul_mult(A00);
    tlapack::gemm(NO_TRANS, NO_TRANS, T(1), A01, A10, T(1), A00);
 
    // calculate bottom left corner
    tlapack::trmm(LEFT_SIDE, UPPER_TRIANGLE, NO_TRANS, NON_UNIT_DIAG, T(1), A11,
                  A10);
 
    // calculate top right
    tlapack::trmm(RIGHT_SIDE, LOWER_TRIANGLE, NO_TRANS, UNIT_DIAG, T(1), A11,
                  A01);
 
    // calculate bottom right
    ul_mult(A11);
 
    return 0;
 
}  // ul_mult
 
}  // namespace tlapack
 
#endif  // TLAPACK_ul_mult_HH