lu_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_LU_MULT_HH
#define TLAPACK_LU_MULT_HH
 
#include "tlapack/base/utils.hpp"
#include "tlapack/blas/trmm.hpp"
 
namespace tlapack {
 
struct LuMultOpts {
    size_t nx = 1;
};
 
template <TLAPACK_SMATRIX matrix_t>
void lu_mult(matrix_t& A, const LuMultOpts& opts = {})
{
    using idx_t = size_type<matrix_t>;
    using T = type_t<matrix_t>;
    using range = pair<idx_t, idx_t>;
    using real_t = real_type<T>;
 
    const idx_t m = nrows(A);
    const idx_t n = ncols(A);
    tlapack_check(m == n);
    tlapack_check(opts.nx >= 1);
 
    // quick return
    if (n == 0) return;
 
    if (n <= (idx_t)opts.nx) {  // Matrix is small, do not use recursion
        for (idx_t i2 = n; i2 > 0; --i2) {
            idx_t i = i2 - 1;
            for (idx_t j2 = n; j2 > 0; --j2) {
                idx_t j = j2 - 1;
                T sum(0);
                for (idx_t k = 0; k <= min(i, j); ++k) {
                    if (i == k)
                        sum += A(k, j);
                    else
                        sum += A(i, k) * A(k, j);
                }
                A(i, j) = sum;
            }
        }
        return;
    }
 
    const idx_t n0 = n / 2;
 
    /*
        Matrix A is splitted into 4 submatrices:
        A = [ A00 A01 ]
            [ A10 A11 ]
        and, hereafter,
            L00 is the strict lower triangular part of A00, with unitary main
       diagonal. L11 is the strict lower triangular part of A11, with unitary
       main diagonal. U00 is the upper triangular part of A00. U11 is the upper
       triangular part of A11.
    */
    auto A00 = slice(A, range(0, n0), range(0, n0));
    auto A01 = slice(A, range(0, n0), range(n0, n));
    auto A10 = slice(A, range(n0, n), range(0, n0));
    auto A11 = slice(A, range(n0, n), range(n0, n));
 
    lu_mult(A11, opts);
 
    // A11 = A10*A01 + L11*U11
    gemm(NO_TRANS, NO_TRANS, T(1), A10, A01, T(1), A11);
 
    // A01 = L00*A01
    trmm(LEFT_SIDE, LOWER_TRIANGLE, NO_TRANS, UNIT_DIAG, real_t(1), A00, A01);
 
    // A10 = A10*U00
    trmm(RIGHT_SIDE, UPPER_TRIANGLE, NO_TRANS, NON_UNIT_DIAG, real_t(1), A00,
         A10);
 
    // A00 = L00*U00
    lu_mult(A00, opts);
 
    return;
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_LU_MULT_HH