  A latest coding scheme named polarization-adjusted convolutional (PAC) codes
is shown to approach the dispersion bound for the code (128,64) under list
decoding. However, to achieve the near-bound performance, the list size of list
decoding needs to be excessively large, which leads to insufferable latency. In
this paper, to improve the speed of list decoding, fast list decoders for PAC
codes are proposed. We define four types of constituent nodes and provide fast
list decoding algorithms for each of them. Simulation results present that fast
list decoding with three types of constituent nodes can yield exactly the same
error-correction performance as list decoding, and reduce more than 50% time
steps for the code (128,64). Moreover, fast list decoding with four types of
constituent nodes can further reduce decoding latency with negligible
performance degradation.
