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.

翻译：AIFV（几乎瞬时固定到可变长度）编码是一种无噪声源编码，通过允许使用具有两个码表的时变编码器以及最多2比特的解码延迟，能够实现比霍夫曼编码更短的平均码字长度。首先，我们考虑一类通用的无噪声源编码，称为$k$-比特延迟可解码编码，其中允许有限数量的码表，且对于$k \geq 0$，解码延迟最多为$k$比特。随后，我们证明AIFV编码在$2$-比特延迟可解码编码类中能够达到最优的平均码字长度。