function y = log( x )

%   Disciplined convex programming information:
%       LOG(X) is concave and nondecreasing in X. When used in CVX
%       expressions, X must be concave.
%
%   Disciplined geometric programming information:
%       LOG(X) is typically not used in geometric programs. Technically it
%       possible to do so in certain advanced cases, because monomials and
%       posynomials are treated by CVX as log-affine and log-convex
%       constructs, respectively. However, such usage is undocumented and
%       will not be officially supported.

global cvx___
error(nargchk(1,1,nargin)); %#ok
cvx_expert_check( 'log', x );

%
% Determine the expression types
%

persistent remap
if isempty( remap ),
    remap_0 = cvx_remap( 'nonpositive' );
    remap_1 = cvx_remap( 'positive' );
    remap_2 = cvx_remap( 'real-affine', 'concave' ) & ~remap_1 & ~remap_0;
    remap_3 = cvx_remap( 'monomial' );
    remap_4 = cvx_remap( 'posynomial' );
    remap   = remap_1 + 2 * remap_2 + 3 * remap_3 + 4 * remap_4;
end
v = remap( cvx_classify( x ) );

%
% Process each type of expression one piece at a time
%

vu = sort( v(:) );
vu = vu([true;diff(vu)~=0]);
nv = length( vu );
if nv ~= 1,
    y = cvx( size( x ), [] );
end
for k = 1 : nv,

    %
    % Select the category of expression to compute
    %

    vk = vu( k );
    if nv == 1,
        xt = x;
    else
        t = v == vk;
        xt = cvx_subsref( x, t );
    end

    %
    % Perform the computations
    %

    switch vk,
        case 0,
            % Invalid
            error( 'Disciplined convex programming error:\n    Illegal operation: log( {%s} ).', cvx_class( xt, true, true, true ) );
        case 1,
            % Positive constant
            yt = cvx( log( cvx_constant( xt ) ) );
        case 2,
            % Affine, convex (invalid)
            sx = xt.size_; %#ok
            yt = [];
            cvx_begin
                hypograph variable yt( sx ) 
                exp( yt ) <= xt;            %#ok
            cvx_end
        case 3,
            % Monomial
            nb = prod( xt.size_ );
            [ rx, cx, vx ] = find( xt.basis_ );
            logs = cvx___.logarithm( rx, 1 );
            tt = vx ~= 1; nt = sum( tt );
            bx = sparse( [ ones( nt, 1 ) ; logs ], [ cx( tt ) ; cx ], [ log( vx( tt ) ) ; ones( nb, 1 ) ], full( max( logs ) ), size( xt.basis_, 2 ) );
            yt = cvx( xt.size_, bx );
        case 4,
            % Posynomial
            sx = xt.size_;
            xt = xt.basis_;
            rc = full( sum( xt ~= 0, 1 ) );
            ru = sort( rc(:) );
            ru = ru([true;diff(ru)~=0]);
            nu = length( ru );
            if nu ~= 1,
                yt = cvx( sx, [] );
            end
            for kk = 1 : nu,
                rk = ru( kk );
                if nu == 1,
                    xtt = xt;
                else
                    tt  = rc == rk;
                    xtt = xt( :, tt );
                end
                [ rx, cx, vx ] = find( xtt ); %#ok
                rx = rx( : ); vx = vx( : );
                nq = length( vx );
                vx = log( vx );
                tz = rx ~= 1;
                rx = cvx___.logarithm( rx( tz ), 1 );
                vx = vx + cvx( nq, sparse( rx, find( tz ), 1, full( max( rx ) ), nq ) );
                vx = reshape( vx, rk, nq / rk );
                vx = log_sum_exp( vx );
                if nu == 1,
                    yt = reshape( vx, sx );
                else
                    yt = cvx_subsasgn( yt, tt, vx );
                end
            end
        otherwise,
            error( 'Shouldn''t be here.' );
    end

    %
    % Store the results
    %

    if nv == 1,
        y = yt;
    else
        y = cvx_subsasgn( y, t, yt );
    end

end

% Copyright 2005-2014 CVX Research, Inc.
% See the file LICENSE.txt for full copyright information.
% The command 'cvx_where' will show where this file is located.
