A <I>hash</I> is a function that maps arbitrarily-long character sequences to small, fixed-size strings.
add_substring b pos len adds len characters from byte array b, starting at character number pos, to the running hash computation.
add_string str adds all characters of string str to the running hash computation.
add_byte b adds the character having code b to the running hash computation. b must be between 0 and 255 inclusive.
Terminate the hash computation and return the hash value for the input data provided via the add_* methods. The hash value is a string of length hash_size characters. After calling result, the hash can no longer accept additional data. Hence, do not call any of the add_* methods after result.