AUDIO ENCODER, AUDIO DECODER,METHOD FOR ENCODING AND AUDIO INFORMATION, METHOD FOR DECODING AN AUDIO INFORMATION AND COMPUTER PROGRAM USING AN OPTIMIZED HASH TABLE

An audio decoder (200; 800) for providing a decoded audio information (212; 812) on the basis of an encoded audio information (210; 810) comprises an arithmetic decoder (230; 820) for providing a plurality of decoded spectral values (232; 822) on the basis of an arithmetically encoded representation (222; 821) of the spectral values, and a frequency- domain-to-time-domain converter (260; 830) for providing a time-domain audio representation (262; 812) using the decoded spectral values (232; 822), in order to obtain the decoded audio information (212; 812). The arithmetic decoder (230; 820) is configured to select a mapping rule (297; cum_freq[]) describing a mapping of a code value (value) representing a spectral value, or a most significant bit-plane of a spectral value, in an encoded form, onto a symbol code (symbol) representing a spectral value, or a most significant bit- plane of a spectral value, in a decoded form, in dependence on a context state (s) described by a numeric current context value (c). The arithmetic decoder (230; 820) is configured to determine the numeric current context value (c) in dependence on a plurality of previously decoded spectral values. The arithmetic decoder is configured to evaluate a hash table (ari_hash_m[]), entries of which define both significant state values amongst the numeric context values and boundaries of intervals of numeric context values, in order to select the mapping rule, wherein the hash table ari_hash_m is defined as given in Figs. 22(1), 22(2), 22(3) and 22(4). The arithmetic decoder is configured to evaluate the hash table, to determine whether the numeric current context value (c) is identical to a table context value described by an entry of the hash table (ari_hash_m[]) or to determine an interval described by entries of the hash table (ari_hash_m[]) within which the numeric current context value (c) lies, and to derive a mapping rule index value describing a selected mapping rule in dependence on a result of the evaluation.