For decoding messages, a decoder exchanges single-bit messages for a data channel between a plurality of M parity nodes and a plurality of N symbol nodes. Each parity node has one or more adjacent symbol nodes with a plurality of edges between the parity node and each adjacent symbol node. An extrinsic decision and an extrinsic parity value are calculated based on a time-varying lookup table. The lookup table stores the locally maximum-likelihood extrinsic decision for a quantized number of data channel states as a function of adjacent extrinsic parity values.