ungbr.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_UNGBR_HH
#define TLAPACK_UNGBR_HH
 
#include "tlapack/base/utils.hpp"
#include "tlapack/lapack/ungq.hpp"
 
namespace tlapack {
 
struct UngbrOpts {
    size_t nb = 32;  
};
 
template <class T, TLAPACK_SMATRIX matrix_t, TLAPACK_SVECTOR vector_t>
constexpr WorkInfo ungbr_q_worksize(const size_type<matrix_t> k,
                                    matrix_t& A,
                                    const vector_t& tau,
                                    const UngbrOpts& opts = {})
{
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const idx_t m = nrows(A);
 
    if (m >= k) {
        return ungq_worksize<T>(FORWARD, COLUMNWISE_STORAGE, A, tau,
                                UngqOpts{opts.nb});
    }
    else {
        auto&& A2 = slice(A, range{0, m - 1}, range{0, m - 1});
        auto&& tau2 = slice(tau, range{0, m - 1});
        return ungq_worksize<T>(FORWARD, COLUMNWISE_STORAGE, A2, tau2,
                                UngqOpts{opts.nb});
    }
}
 
template <class T, TLAPACK_SMATRIX matrix_t, TLAPACK_SVECTOR vector_t>
constexpr WorkInfo ungbr_p_worksize(const size_type<matrix_t> k,
                                    matrix_t& A,
                                    const vector_t& tau,
                                    const UngbrOpts& opts = {})
{
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const idx_t n = ncols(A);
 
    if (n <= k) {
        auto&& A2 = slice(A, range{0, n - 1}, range{0, n - 1});
        auto&& tau2 = slice(tau, range{0, n - 1});
        return ungq_worksize<T>(FORWARD, ROWWISE_STORAGE, A2, tau2,
                                UngqOpts{opts.nb});
    }
    else {
        return ungq_worksize<T>(FORWARD, ROWWISE_STORAGE, A, tau,
                                UngqOpts{opts.nb});
    }
}
 
template <TLAPACK_SMATRIX matrix_t,
          TLAPACK_SVECTOR vector_t,
          TLAPACK_WORKSPACE work_t>
int ungbr_q_work(const size_type<matrix_t> k,
                 matrix_t& A,
                 const vector_t& tau,
                 work_t& work,
                 const UngbrOpts& opts = {})
{
    using T = type_t<matrix_t>;
    using real_t = real_type<T>;
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const real_t zero(0);
    const real_t one(1);
    const idx_t m = nrows(A);
 
    if (m >= k) {
        // If m >= k, assume m >= n >= k
        ungq_work(FORWARD, COLUMNWISE_STORAGE, A, tau, work, UngqOpts{opts.nb});
    }
    else {
        // Shift the vectors which define the elementary reflectors one
        // column to the right, and set the first row and column of Q
        // to those of the unit matrix
        for (idx_t j = m - 1; j > 0; --j) {
            A(0, j) = zero;
            for (idx_t i = j + 1; i < m; ++i)
                A(i, j) = A(i, j - 1);
        }
        A(0, 0) = one;
        for (idx_t i = 1; i < m; ++i)
            A(i, 0) = zero;
        if (m > 1) {
            // Form Q(1:m,1:m)
            auto A2 = slice(A, range{1, m}, range{1, m});
            auto tau2 = slice(tau, range{0, m - 1});
            ungq_work(FORWARD, COLUMNWISE_STORAGE, A2, tau2, work,
                      UngqOpts{opts.nb});
        }
    }
 
    return 0;
}
 
template <TLAPACK_SMATRIX matrix_t, TLAPACK_SVECTOR vector_t>
int ungbr_q(const size_type<matrix_t> k,
            matrix_t& A,
            const vector_t& tau,
            const UngbrOpts& opts = {})
{
    using work_t = matrix_type<matrix_t, vector_t>;
    using T = type_t<work_t>;
 
    // Functor
    Create<work_t> new_matrix;
 
    // Allocates workspace
    WorkInfo workinfo = ungbr_q_worksize<T>(k, A, tau, opts);
    std::vector<T> work_;
    auto work = new_matrix(work_, workinfo.m, workinfo.n);
 
    return ungbr_q_work(k, A, tau, work, opts);
}
 
template <TLAPACK_SMATRIX matrix_t,
          TLAPACK_SVECTOR vector_t,
          TLAPACK_WORKSPACE work_t>
int ungbr_p_work(const size_type<matrix_t> k,
                 matrix_t& A,
                 const vector_t& tau,
                 work_t& work,
                 const UngbrOpts& opts = {})
{
    using T = type_t<matrix_t>;
    using real_t = real_type<T>;
    using idx_t = size_type<matrix_t>;
    using range = pair<idx_t, idx_t>;
 
    // constants
    const real_t zero(0);
    const real_t one(1);
    const idx_t n = ncols(A);
 
    //
    // Form P**H, determined by a call to gebrd to reduce a k-by-n
    // matrix
    //
    if (k < n) {
        // If m >= k, assume m >= n >= k
        ungq_work(FORWARD, ROWWISE_STORAGE, A, tau, work, UngqOpts{opts.nb});
    }
    else {
        // Shift the vectors which define the elementary reflectors one
        // row downward, and set the first row and column of P**H to
        // those of the unit matrix
        A(0, 0) = one;
        for (idx_t i = 1; i < n; ++i) {
            A(i, 0) = zero;
        }
        for (idx_t j = 1; j < n; ++j) {
            for (idx_t i = j - 1; i > 0; --i) {
                A(i, j) = A(i - 1, j);
            }
            A(0, j) = zero;
        }
        if (n > 1) {
            auto A2 = slice(A, range{1, n}, range{1, n});
            auto tau2 = slice(tau, range{0, n - 1});
            ungq_work(FORWARD, ROWWISE_STORAGE, A2, tau2, work,
                      UngqOpts{opts.nb});
        }
    }
 
    return 0;
}
 
template <TLAPACK_SMATRIX matrix_t, TLAPACK_SVECTOR vector_t>
int ungbr_p(const size_type<matrix_t> k,
            matrix_t& A,
            const vector_t& tau,
            const UngbrOpts& opts = {})
{
    using work_t = matrix_type<matrix_t, vector_t>;
    using T = type_t<work_t>;
 
    // Functor
    Create<work_t> new_matrix;
 
    // Allocates workspace
    WorkInfo workinfo = ungbr_p_worksize<T>(k, A, tau, opts);
    std::vector<T> work_;
    auto work = new_matrix(work_, workinfo.m, workinfo.n);
 
    return ungbr_p_work(k, A, tau, work, opts);
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_UNGBR_HH