hetd2.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_HETD2_HH
#define TLAPACK_HETD2_HH
 
#include "tlapack/base/utils.hpp"
#include "tlapack/blas/axpy.hpp"
#include "tlapack/blas/dot.hpp"
#include "tlapack/blas/hemv.hpp"
#include "tlapack/blas/her2.hpp"
#include "tlapack/lapack/larfg.hpp"
 
namespace tlapack {
 
template <TLAPACK_SMATRIX A_t, TLAPACK_SVECTOR tau_t, class uplo_t>
int hetd2(uplo_t uplo, A_t& A, tau_t& tau)
{
    using T = type_t<A_t>;
    using real_t = real_type<T>;
    using idx_t = size_type<A_t>;
    using pair = pair<idx_t, idx_t>;
 
    // constants
    const idx_t n = ncols(A);
    const real_t one(1);
    const real_t zero(0);
    const real_t half(0.5);
 
    // check arguments
    tlapack_check_false(uplo != Uplo::Lower && uplo != Uplo::Upper);
    tlapack_check(nrows(A) == ncols(A));
    tlapack_check((idx_t)size(tau) >= n - 1);
 
    // quick return
    if (n <= 0) return 0;
 
    if (uplo == Uplo::Upper) {
        //
        // Reduce upper triangle of A
        //
 
        // Only access real part of the main diagonal
        A(n - 1, n - 1) = real(A(n - 1, n - 1));
 
        for (idx_t i = n - 2; i != idx_t(-1); --i) {
            // Define v := A[0:i,i+1]
            auto v = slice(A, pair{0, i + 1}, i + 1);
 
            // Generate elementary reflector H(i) = I - tau * v * v**T
            // to annihilate A(0:i-1,i+1)
            T taui;
            larfg(BACKWARD, COLUMNWISE_STORAGE, v, taui);
 
            if (taui != zero) {
                // Apply H(i) from both sides to A(0:i, 0:i)
                auto C = slice(A, pair{0, i + 1}, pair{0, i + 1});
                auto w = slice(tau, pair{0, i + 1});
 
                // Store the offdiagonal element
                auto beta = A(i, i + 1);
                A(i, i + 1) = one;
 
                // Compute x:= taui * C * v storing x in w
                hemv(UPPER_TRIANGLE, taui, C, v, w);
 
                // Compute w := w - (1/2) * tau * (w**H * v) * v
                axpy(-half * taui * dot(w, v), v, w);
 
                // Apply the transformation as a rank-2 update:
                //    C := C - v * w**H - w * v**H
                her2(UPPER_TRIANGLE, -one, v, w, C);
 
                // Reload the offdiagonal element
                A(i, i + 1) = beta;
            }
            else {
                // Only access real part of the main diagonal
                A(i, i) = real(A(i, i));
            }
 
            tau[i] = taui;
        }
    }
    else {
        //
        // Reduce lower triangle of A
        //
 
        // Only access real part of the main diagonal
        A(0, 0) = real(A(0, 0));
 
        for (idx_t i = 0; i < n - 1; ++i) {
            // Define v := A[i+1:n,i]
            auto v = slice(A, pair{i + 1, n}, i);
 
            // Generate elementary reflector H(i) = I - tau * v * v**T
            // to annihilate A(i+2:n,i)
            T taui;
            larfg(FORWARD, COLUMNWISE_STORAGE, v, taui);
 
            if (taui != zero) {
                // Apply H(i) from both sides to A(i+1:n, i+1:n)
                auto C = slice(A, pair{i + 1, n}, pair{i + 1, n});
                auto w = slice(tau, pair{i, n - 1});
 
                // Store the offdiagonal element
                auto beta = A(i + 1, i);
                A(i + 1, i) = one;
 
                // Compute x:= taui * C * v storing x in w
                hemv(LOWER_TRIANGLE, taui, C, v, w);
 
                // Compute w := w - (1/2) * tau * (w**H * v) * v
                axpy(-half * taui * dot(w, v), v, w);
 
                // Apply the transformation as a rank-2 update:
                //    C := C - v * w**H - w * v**H
                her2(LOWER_TRIANGLE, -one, v, w, C);
 
                // Reload the offdiagonal element
                A(i + 1, i) = beta;
            }
            else {
                // Only access real part of the main diagonal
                A(i + 1, i + 1) = real(A(i + 1, i + 1));
            }
 
            tau[i] = taui;
        }
    }
 
    return 0;
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_HETD2_HH