function rewritedat(datfile, inffile, hdrfile, I)

% rewritedat(datfile, inffile, hdrfile, I)
% Rewrite training files with a subset of the examples.
% Used to shrink the cache.

fhdr = fopen(hdrfile, 'rb');
header = fread(fhdr, 3, 'int32');
labelsize = header(2);
fclose(fhdr);

oldfile = [datfile '_tmp'];
unix(['mv ' datfile ' ' oldfile]);
fin = fopen(oldfile, 'rb');
fout = fopen(datfile, 'wb');

% sort indexes so we never have to seek before the current position
I = sort(I);

pos = 1;
for i = 1:length(I)
  cnt = I(i)-pos;
  while cnt > 0
    % + 2 to include the num non-zero blocks and example length
    info = fread(fin, labelsize+2, 'int32');
    dim = info(end);
    fseek(fin, dim*4, 0);
    cnt = cnt - 1;
  end
  y = fread(fin, labelsize+2, 'int32');
  dim = y(end);
  x = fread(fin, dim, 'single');
  fwrite(fout, y, 'int32');
  fwrite(fout, x, 'single');
  pos = I(i)+1;
end

fclose(fin);
fclose(fout);

% remove the old cache file
unix(['rm ' oldfile]);

% keep the info file in sync with the data file
oldfile = [inffile '_tmp'];
unix(['cp ' inffile ' ' oldfile]);
[labels, scores, unique] = readinfo(inffile);
labels = labels(I);
scores = scores(I);
unique = unique(I);
fid = fopen(inffile, 'w');
for i = 1:length(I)
  fprintf(fid, '%d\t%f\t%d\n', labels(i), scores(i), unique(i));
end
fclose(fid);