function h = h2nats(p, q)
%H2NATS binary (cross-)entropy in nats for a Bernoulli distribution
%
% H2NATS is applied elementwise to probabilities p in [0,1]
%
% h2nats(p, q) = -p.*log(q) - (1-p).*log(1-q)
% If p is omitted, q = p.
% 
% However, unlike the above straight-forward implementation:
% - when 0*log(0) is evaluated, the limit of 0 is returned.
% - q probabilities outside [0,1] give errors rather than complex numbers
%
% See also: H2BITS

% Iain Murray, November 2007, September 2008

if nargin < 2
    z = (p == 0) | (p == 1);
    
    p1 = 1-p;
    h = -p.*reallog(p) - p1.*reallog(p1);
    
    h(z) = 0;
else
    assert(isequal(size(p), size(q)));

    h = zeros(size(p));
    m = (p ~= 0);
    h(m) = -p(m).*reallog(q(m));
    m = (p ~= 1);
    h(m) = h(m) - (1-p(m)).*reallog(1-q(m));
end