Mux1H

s3   := Mux1H(Reg(next=UIntToOH(Reg(next=s1(log2Up(n) - 1, 0)))), s2)

The inner Reg selects LSBs of the s1. How many bits are selected is determined by n. For example, if n is 8, then selected LSBs are (2,0), i.e. the three LSBs.

Then, UIntToOH logic converts the three selected bits to the one-hot type.

The outer Reg stores thus obtained value.

The Mux1H syntax is as follows:

Mux1H(sel:Bits/Iterable[Bool], in:Iterable[Data]): Data

meaning that the value stored in the outer Reg represents the select input of the Mux, whereas the s2 represents the information input.

Finally, based on the value of the select input, the s3 takes the appropriate value from s2.

Further information: https://github.com/freechipsproject/chisel3/wiki/Muxes-and-Input-Selection#mux1h