laset.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_LASET_HH
#define TLAPACK_LASET_HH
 
#include "tlapack/base/types.hpp"
 
namespace tlapack {
 
template <TLAPACK_UPLO uplo_t, TLAPACK_MATRIX matrix_t>
void laset(uplo_t uplo,
           const type_t<matrix_t>& alpha,
           const type_t<matrix_t>& beta,
           matrix_t& A)
{
    using idx_t = size_type<matrix_t>;
 
    // constants
    const idx_t m = nrows(A);
    const idx_t n = ncols(A);
 
    // check arguments
    tlapack_check_false(uplo != Uplo::Lower && uplo != Uplo::Upper &&
                        uplo != Uplo::General);
 
    if (uplo == Uplo::Upper) {
        // Set the strictly upper triangular or trapezoidal part of
        // the array to alpha.
        for (idx_t j = 1; j < n; ++j) {
            const idx_t M = min(m, j);
            for (idx_t i = 0; i < M; ++i)
                A(i, j) = alpha;
        }
    }
    else if (uplo == Uplo::Lower) {
        // Set the strictly lower triangular or trapezoidal part of
        // the array to alpha.
        const idx_t N = min(m, n);
        for (idx_t j = 0; j < N; ++j) {
            for (idx_t i = j + 1; i < m; ++i)
                A(i, j) = alpha;
        }
    }
    else {
        // Set all elements in A to alpha.
        for (idx_t j = 0; j < n; ++j)
            for (idx_t i = 0; i < m; ++i)
                A(i, j) = alpha;
    }
 
    // Set the first min(m,n) diagonal elements to beta.
    const idx_t N = min(m, n);
    for (idx_t i = 0; i < N; ++i)
        A(i, i) = beta;
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_LASET_HH