function h = hnats(pp, qq)
%HNATS (cross-)entropy for discrete distributions measured in nats
%
% h = hnats(pp, qq)
%   = -pp(:)'*log(qq(:)), but with zeros from pp removed.
%
% If one argument is given, encoding distribution qq is equal to the source
% distribution pp.
%
% See also, HBITS, H2NATS, H2BITS, KL_NATS, KL_BITS

% Iain Murray, September 2008

if nargin < 2
    qq = pp;
elseif ~isequal(size(pp), size(qq))
    error('Input probability arrays must have the same size.');
end

mask = (pp(:)~=0);
m = @(x) x(mask);
s = @(x) m(x(:));

h = -s(pp)'*reallog(s(qq));