% compress.m                created by:  Dion Monstavicius
%
% this function compresses an input image into
% a run length encoded bitstream.  The scheme is as follows:
% all non zero entries are encoded as is.
% whenever a zero is encountered, 0 is placed into the bitstream
% FOLLOWED by the number of zeros to encode that follow that zero.
% eg:  0 0 0 4 5 0 0 0 0 0 2 0 4
% would be compressed to:
% 0 3 4 5 0 5 2 0 1 4
% output = the bit stream
% entropy = the entropy of the bit stream

function [output, entropy] = compress(input)

[A B] = size(input);

input = reshape(input, 1, A*B);

j = 1;
zerocounter = 0;

for i = 1:length(input),
 if input(i) ~= 0,
  if zerocounter ~= 0,
   output(j) = 0;
   output(j+1) = zerocounter;
   j = j+2;
   zerocounter = 0;
  end
  output(j) = input(i);
  j = j+1;
 else
  zerocounter = zerocounter + 1;
 end
end

if zerocounter ~= 0

 output(j) = 0;
 output(j+1) = zerocounter;

end

limit = max(output);

p = hist(output, limit);
p = p/sum(p);

H = 0;
for i = 1:length(p);
 if p(i) ~= 0,
  H = H + p(i)*log2(1/p(i));
 end
end

entropy = H;