lauum_recursive.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_LAUUM_RECURSIVETLAPACK_HH
#define TLAPACK_LAUUM_RECURSIVETLAPACK_HH
 
#include "tlapack/base/utils.hpp"
#include "tlapack/blas/trmm.hpp"
 
namespace tlapack {
 
template <TLAPACK_SMATRIX matrix_t>
int lauum_recursive(const Uplo& uplo, matrix_t& C)
 
{
    tlapack_check(nrows(C) == ncols(C));
 
    using T = type_t<matrix_t>;
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
    using real_t = real_type<T>;
 
    const idx_t n = nrows(C);
 
    // check arguments
    tlapack_check_false(uplo != Uplo::Lower && uplo != Uplo::Upper);
    tlapack_check_false(nrows(C) != ncols(C));
 
    // Quick return
    if (n <= 0) return 0;
 
    idx_t n0 = n / 2;
 
    // 1-by-1 case for recursion
    if (n == 1) {
        real_t rC00 = real(C(0, 0));
        real_t iC00 = imag(C(0, 0));
        C(0, 0) = rC00 * rC00 + iC00 * iC00;
    }
    else {
        if (uplo == Uplo::Lower) {
            // Upper computes U * U_hermitian
            auto C00 = slice(C, range(0, n0), range(0, n0));
            auto C10 = slice(C, range(n0, n), range(0, n0));
            auto C11 = slice(C, range(n0, n), range(n0, n));
 
            lauum_recursive(uplo, C00);
            herk(LOWER_TRIANGLE, CONJ_TRANS, real_t(1), C10, real_t(1), C00);
            trmm(LEFT_SIDE, uplo, CONJ_TRANS, NON_UNIT_DIAG, real_t(1), C11,
                 C10);
            lauum_recursive(uplo, C11);
        }
        else {
            // Lower computes  L_hermitian * L
            auto C00 = slice(C, range(0, n0), range(0, n0));
            auto C01 = slice(C, range(0, n0), range(n0, n));
            auto C11 = slice(C, range(n0, n), range(n0, n));
 
            lauum_recursive(uplo, C00);
            herk(UPPER_TRIANGLE, NO_TRANS, real_t(1), C01, real_t(1), C00);
            trmm(RIGHT_SIDE, uplo, CONJ_TRANS, NON_UNIT_DIAG, real_t(1), C11,
                 C01);
            lauum_recursive(UPPER_TRIANGLE, C11);
        }
    }
 
    return 0;
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_LAUUM_RECURSIVETLAPACK_HH