On locally decodable source coding

With the boom of big data, traditional source coding techniques face the common obstacle to decode only a small portion of information efficiently. In this paper, we aim to resolve this difficulty by introducing a specific type of source coding scheme called locally decodable source coding (LDSC). Rigorously, LDSC is capable of recovering an arbitrary bit of the unencoded message from its encoded version, by only feeding a small number of the encoded message to the decoder, and we call the decoder t-local if only t encoded symbols are required.We consider both almost lossless (block error) and lossy (bit error) cases for LDSC. First, we show that using linear encoder and a decoder with bounded locality, the reliable compress rate can not be less than one. More importantly, we show that even with a general encoder and 2-local decoders (t = 2), the rate of LDSC is still one. On the contrary, the achievability bounds for almost lossless and lossy compressions with excess distortion suggest that optimal compression rate is achievable when O(log n) encoded symbols is queried by the decoder with block-length n. We also show that, rate distortion is achievable when the number of queries is scaled over n with a bound on the rate in finite-length regime. Although the achievability bounds are simply based on the concatenation of code blocks, they outperform the existing bounds in succinct data structures literature.