% cfncompress.m                created by:  Dion Monstavicius
%                              adapted by:  Chad Netzer
%
% 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, the total number of zeros to encode
% is placed in the bitstream as a negative number.
% eg:  0 0 0 4 5 0 0 0 0 0 2 0 4
% would be compressed to:
% -3 4 5 -5 2 -1 4
% output = the bit stream
% entropy = the entropy of the bit stream

function [output, entropy] = cfncompress(input)

[A B] = size(input);

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

j = 1;
zerocounter = 0;
zero_out = [];
non_zero_out = [];

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

if zerocounter ~= 0

 output(j) = -zerocounter;

end

p1 = histc(zero_out, 0:max(zero_out));
p2 = histc(non_zero_out, 0:max(non_zero_out));
p_sum = sum(p1) + sum(p2);
p1 = p1/p_sum;
p2 = p2/p_sum;
p = [ p1 p2];

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

entropy = H;