function [A0, A0star, AR, B] = get_companion_matrix(auxiliary_model_name, auxiliary_model_type)

% Gets the companion VAR representation of a PAC auxiliary model.
% Depending on the nature of this auxiliary model the output is
% saved in oo_.{var,trend_component}.(auxiliary_model_name).CompanionMatrix
%
% INPUTS
% - auxiliary_model_name     [string]    the name of the auxiliary model
% - auxiliary_model_type     [string]    the type of the auxiliary model
%                                        ('var' or 'trend_component')
%
% OUTPUTS
% - None

% Copyright © 2018-2022 Dynare Team
%
% This file is part of Dynare.
%
% Dynare 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 3 of the License, or
% (at your option) any later version.
%
% Dynare 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 Dynare.  If not, see <https://www.gnu.org/licenses/>.

global oo_ M_

if nargin < 2
    if isfield(M_, 'var') && isfield(M_.var, auxiliary_model_name)
        auxiliary_model_type = 'var';
    elseif isfield(M_, 'trend_component') && isfield(M_.trend_component, auxiliary_model_name)
        auxiliary_model_type = 'trend_component';
    else
        error('Unknown type of auxiliary model.')
    end
end

if strcmp(auxiliary_model_type, 'var')
    [AR, ~, Constant] = feval(sprintf('%s.varmatrices', M_.fname), auxiliary_model_name, M_.params, M_.var.(auxiliary_model_name).structural);
    isconstant = any(abs(Constant)>0);
    M_.var.(auxiliary_model_name).isconstant = isconstant; % FIXME Could be done by preprocessor instead…
elseif strcmp(auxiliary_model_type, 'trend_component')
    [AR, A0, A0star] = feval(sprintf('%s.trend_component_ar_a0', M_.fname), auxiliary_model_name, M_.params);
else
    error('Unknown type of auxiliary model.')
end

% Get the number of lags
p = size(AR, 3);

% Get the number of variables
n = length(M_.(auxiliary_model_type).(auxiliary_model_name).lhs);

switch auxiliary_model_type
  case 'var'
    oo_.var.(auxiliary_model_name).CompanionMatrix = zeros(n*p+isconstant);
    oo_.var.(auxiliary_model_name).CompanionMatrix(isconstant+(1:n),isconstant+(1:n)) = AR(:,:,1);
    for i = 2:p
        oo_.var.(auxiliary_model_name).CompanionMatrix(isconstant+(1:n),isconstant+(i-1)*n+(1:n)) = AR(:,:,i);
        oo_.var.(auxiliary_model_name).CompanionMatrix(isconstant+(i-1)*n+(1:n),isconstant+(i-2)*n+(1:n)) = eye(n);
    end
    if isconstant
        oo_.var.(auxiliary_model_name).CompanionMatrix(1,1) = 1;
        for i=1:n
            oo_.var.(auxiliary_model_name).CompanionMatrix(1+i,1) = Constant(i);
        end
    end
    M_.var.(auxiliary_model_name).list_of_variables_in_companion_var = M_.endo_names(M_.var.(auxiliary_model_name).lhs);
    if nargout
        A0 = [];
        A0star = [];
        B  = [];
    end
  case 'trend_component'
    % Get number of trends.
    q = sum(M_.trend_component.(auxiliary_model_name).targets);
    % Get the number of equations with error correction.
    m  = n - q;
    % Get the indices of trend and EC equations in the auxiliary model.
    target_eqnums_in_auxiliary_model = M_.trend_component.(auxiliary_model_name).target_eqn;
    ecm_eqnums_in_auxiliary_model = find(~M_.trend_component.(auxiliary_model_name).targets);

    % REMARK It is assumed that the non trend equations are the error correction
    %        equations. We assume that the model can be cast in the following form:
    %
    %        Δ Xₜ₋₁ = A₀ (Xₜ₋₁ - C₀Zₜ₋₁) + Σᵢ₌₁ᵖ Aᵢ Δ Xₜ₋ᵢ + ϵₜ
    %
    %        Zₜ = Zₜ₋₁ + ηₜ
    %
    %        where Xₜ is a n×1 vector and Zₜ is an m×1 vector, A₀ is a
    %        n×n matrix, C₀ a n×m matrix, and Aᵢ (i=1,…,p) are n×n
    %        matrices. Matrix C₀ can be factorized as C₀ = (A₀)⁻¹×Λ,
    %        where Λ is a n×m matrix.
    %
    %        We rewrite the model in levels (we integrate the first set
    %        of equations) and rewrite the model as a VAR(1) model. Let
    %        Yₜ = [Xₜ; Zₜ] be the vertical concatenation of vectors
    %        Xₜ (variables with EC) and Zₜ (trends). We have
    %
    %        Yₜ = Σᵢ₌₁ᵖ⁺¹ Bᵢ Yₜ₋ᵢ + [εₜ; ηₜ]
    %
    %        with
    %
    %               B₁ = [I+A₀+A₁, -Λ; 0, I]
    %
    %               Bᵢ = [Aᵢ-Aᵢ₋₁, 0; 0, 0]   for i = 2,…, p
    %        and
    %               Bₚ₊₁ = -[Aₚ, 0; 0, 0]
    %
    %        where the dimensions of I and 0 matrices can easily be
    %        deduced from the number of EC and trend equations.

    % Check that the lhs of candidate ecm equations are at least first differences.
    for i = 1:m
        if ~get_difference_order(M_.trend_component.(auxiliary_model_name).lhs(ecm_eqnums_in_auxiliary_model(i)))
            error([auxiliary_model_name ' is not a trend component model. The LHS variables should be in differences'])
        end
    end
    % Get the EC matrix (the EC term is assumend to be in t-1).
    %
    % TODO: Check that the EC term is the difference between the
    %       endogenous variable and the trend variable.
    %
    % Build B matrices (VAR in levels)
    B = zeros(m+q, m+q, p+1);
    B(ecm_eqnums_in_auxiliary_model, ecm_eqnums_in_auxiliary_model, 1) = eye(m) + A0 + AR(:,:,1);
    B(ecm_eqnums_in_auxiliary_model, target_eqnums_in_auxiliary_model) = -A0star;
    B(target_eqnums_in_auxiliary_model, target_eqnums_in_auxiliary_model) = eye(q);
    for i = 2:p
        B(ecm_eqnums_in_auxiliary_model,ecm_eqnums_in_auxiliary_model,i) = AR(:,:,i) - AR(:,:,i-1);
    end
    B(ecm_eqnums_in_auxiliary_model,ecm_eqnums_in_auxiliary_model,p+1) = -AR(:,:,p);
    % Write Companion matrix
    oo_.trend_component.(auxiliary_model_name).CompanionMatrix = zeros(size(B, 1)*size(B, 3));
    for i = 1:p
        oo_.trend_component.(auxiliary_model_name).CompanionMatrix(1:n, (i-1)*n+(1:n)) = B(:,:,i);
        oo_.trend_component.(auxiliary_model_name).CompanionMatrix(i*n+(1:n),(i-1)*n+(1:n)) = eye(n);
    end
    oo_.trend_component.(auxiliary_model_name).CompanionMatrix(1:n, p*n+(1:n)) = B(:,:,p+1);
    M_.trend_component.(auxiliary_model_name).list_of_variables_in_companion_var = M_.endo_names(M_.trend_component.(auxiliary_model_name).lhs);
    variables_rewritten_in_levels = M_.trend_component.(auxiliary_model_name).list_of_variables_in_companion_var(ecm_eqnums_in_auxiliary_model);
    for i=1:m
        id = get_aux_variable_id(variables_rewritten_in_levels{i});
        if id
            auxinfo = M_.aux_vars(id);
            if auxinfo.type==8
                M_.trend_component.(auxiliary_model_name).list_of_variables_in_companion_var(ecm_eqnums_in_auxiliary_model(i)) = ...
                    {M_.endo_names{auxinfo.orig_index}};
            else
                error('This is a bug. Please contact the Dynare Team.')
            end
        else
            error('This is a bug. Please contact the Dynare Team.')
        end
    end
end