code: function(file, cb) {
    var error,
        hash   = createHash('sha1'),
        stream = fs.ReadStream(file);
  
    stream.on('data', function(chunk) {
      hash.update(chunk);
    });
  
    stream.on('error', function(e) {
      if (!error) cb(e);
      error = e;
    })
  
    stream.on('end', function() {
      if (!error) cb(null, hash.digest('hex'));
    });
  }
docs: returns sha1 checksum for file