private string ReadKeyIfIncluded(Bitstream stream, List<string> keyHistory)
{
var has_key = stream.ReadBool();
if (!has_key)
{
return null;
}
var is_substring = stream.ReadBool();
string key;
if (!is_substring)
{
key = stream.ReadString();
}
else
{
var fromIndex = (int) stream.ReadBits(5);
var fromLength = (int) stream.ReadBits(5);
key = keyHistory[fromIndex].Substring(0, fromLength);
key += stream.ReadString();
}
if (keyHistory.Count == KEY_HISTORY_SIZE)
{
keyHistory.RemoveAt(0);
}
keyHistory.Add(key);
return key;
}