// -*- mode: C++; c-indent-level: 4; c-basic-offset: 4; indent-tabs-mode: nil; -*-
//
// fastLm.cpp: Rcpp and GSL based implementation of lm
//
// Copyright (C)  2010 - 2024  Dirk Eddelbuettel and Romain Francois
//
// This file is part of RcppGSL.
//
// RcppGSL is free software: you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 2 of the License, or
// (at your option) any later version.
//
// RcppGSL is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with RcppGSL.  If not, see <http://www.gnu.org/licenses/>.

#include <RcppGSL.h>

#include <gsl/gsl_multifit.h>
#include <cmath>

// [[Rcpp::export(fastLm_impl)]]
Rcpp::List fastLm(const RcppGSL::Matrix &X, const RcppGSL::Vector &y) {

    int n = X.nrow(), k = X.ncol();
    double chisq;

    RcppGSL::Vector coef(k);                // to hold the coefficient vector 
    RcppGSL::Matrix cov(k,k);               // and the covariance matrix
    
    // the actual fit requires working memory we allocate and free
    gsl_multifit_linear_workspace *work = gsl_multifit_linear_alloc (n, k);
    gsl_multifit_linear (X, y, coef, cov, &chisq, work);
    gsl_multifit_linear_free (work);

    // assign diagonal to a vector, then take square roots to get std.error
    Rcpp::NumericVector std_err;
    std_err = gsl_matrix_diagonal(cov); 	// need two step decl. and assignment
    std_err = Rcpp::sqrt(std_err);         	// sqrt() is an Rcpp sugar function

    return Rcpp::List::create(Rcpp::Named("coefficients") = coef, 
                              Rcpp::Named("stderr")       = std_err,
                              Rcpp::Named("df.residual")  = n - k);
    
}

