I found a first optimization on reddit.

Instead of doing:

seq = {a,b,c,d} # last 4 price changes
Map.put(sequences, seq, price)

It is possible to do:

<<seq::integer-20>> = <<a::5, b::5, c::5, d::5>>
Map.put_new(sequences, seq, price)

That’s not a game changer but it definitely improve times.

This was a nice break after the last two days.

The combined runtime for both parts is 2.4 seconds.

https://github.com/bjorng/advent-of-code/blob/main/2024/day22/lib/day22.ex

(lud beat me to it while typing)

It says that the bid is placed as soon as the sequence occurs. So even if there’s a better price for a later secret (same seed, later iteration) with the same sequence, that’s too late and shouldn’t be used.