function f = plus(f, g)
%+   Addition of two TRIGTECH objects.
%   F + G adds F and G, where F and G may be TRIGTECH objects or scalars.
%
%   If F is an array-valued TRIGTECH, then F + C is supported if C is a row
%   vector of doubles with the same number of columns as F.
%
% See also MINUS, UPLUS.

% Copyright 2017 by The University of Oxford and The Chebfun Developers. 
% See http://www.chebfun.org/ for Chebfun information.

if ( isempty(f) || isempty(g) ) % TRIGTECH + [] = [].
    
    f = [];
    
elseif ( isa(g, 'double') ) % TRIGTECH + double.
    
    % Update values (use bsxfun() to handle the case in which g is a vector
    % and f is an array-valued TRIGTECH):
    f.values = bsxfun(@plus, f.values, g);
    
    % Update coeffs:
    if ( (size(g, 2) > 1) && (size(f.coeffs, 2) == 1) )
        % Perform singleton expansion of f:
        f.coeffs = repmat(f.coeffs, 1, size(g, 2));
    end
    N = size(f.coeffs,1);
    
    % Determine the index in the coefficient vector where the constant term
    % is stored. The way we have arranged the coefficients means it should
    % be in the middle of the array, but that depends on whether the number
    % of coefficients is even or odd.
    if mod(N,2) == 1
        const_index = (N+1)/2;
    else
        const_index = N/2+1;
    end
    f.coeffs(const_index,:) = f.coeffs(const_index,:) + g;
    
    % Update isReal:
    f.isReal = f.isReal & isreal(g);
    
elseif ( isa(f, 'double') ) % double + TRIGTECH.
    
    % Switch argument order and call TRIGTECH/PLUS again:
    f = plus(g, f);
    
elseif ( isa(f, 'trigtech') && isa(g, 'trigtech') )  % TRIGTECH + TRIGTECH.
    
    % We will simply add the values together then compute the coefficients
    % of the result. This is probably not the most efficient means of
    % determining the sum.
    nf = size(f.values, 1);
    ng = size(g.values, 1);
    if ( nf > ng )
        % Increase the length of g (via PROLONG):
        g = prolong(g, nf);
    elseif ( nf < ng )
        % Increase the length of f (via PROLONG):
        f = prolong(f, ng);
    end
    
    % Update values and coefficients:
    f.values = f.values + g.values;
    f.coeffs = f.vals2coeffs(f.values);

    % Update isReal:
    f.isReal = f.isReal & g.isReal;
    
    % Force the values to be real where f is real.
    f.values(:,f.isReal) = real(f.values(:,f.isReal));
    
    % Look for a zero output:
    if ( ~any(f.values(:)) || ~any(f.coeffs(:)) )
        % Create a zero TRIGTECH:
        ishappy = f.ishappy && g.ishappy;
        z = zeros(1, size(f.values, 2));
        data.vscale = z;
        f = f.make(z, data);
        f.ishappy = ishappy;
    else
        f.ishappy = f.ishappy && g.ishappy;
    end
    
else    % Don't know how to do the addition of the objects.
    
    error('CHEBFUN:TRIGTECH:plus:typeMismatch',['Incompatible operation between objects.\n', ...
    'Make sure functions are of the same type.']);
    
end

end
