  AIFV (almost instantaneous fixed-to-variable length) codes are noiseless
source codes that can attain a shorter average codeword length than Huffman
codes by allowing a time-variant encoder with two code tables and a decoding
delay of at most 2 bits. First, we consider a general class of noiseless source
codes, called k-bit delay decodable codes, in which one allows a finite number
of code tables and a decoding delay of at most k bits for k >= 0. Then we prove
that AIFV codes achieve the optimal average codeword length in the 2-bit delay
decodable codes class.
