I've refactored my code for day 12 of advent of code by using monocle, a lens library in scala.
Is it possible to improve this code :
type Register = String
type Mem = Map[String, Int]
@Lenses
case class State(mem: Mem, pointer: Int)
def processInstruction(instructions: Seq[Instruction]): State => State = { s =>
(instructions(s.pointer) match {
case Inc(r) =>
State.pointer.modify( _ + 1) andThen (State.mem composeLens at(r)).modify(_.map(_ + 1))
case Dec(r) =>
State.pointer.modify( _ + 1) andThen (State.mem composeLens at(r)).modify(_.map(_ - 1))
case CpyInt(v, to) =>
State.pointer.modify( _ + 1) andThen (State.mem composeLens at(to)).set(Some(v))
case CpyReg(from, to) =>
State.pointer.modify( _ + 1) andThen (State.mem composeLens at(to)).set(Some(s.mem(from)))
case Jnz(r, v) => if (r != "1" && s.mem(r) == 0)
State.pointer.modify( _ + 1)
else
State.pointer.modify( _ + v )
}).apply(s)
}
And here is another try, separating the modification of each field
def processInstruction2(instructions: Seq[Instruction]): State => State = { s =>
val ptr = instructions(s.pointer) match {
case Jnz(r, v) if !(r != "1" && s.mem(r) == 0) => State.pointer.modify(_ + v)
case _ => State.pointer.modify(_ + 1)
}
val mem = instructions(s.pointer) match {
case Inc(r) => (State.mem composeLens at(r)).modify(_.map(_ + 1))
case Dec(r) => (State.mem composeLens at(r)).modify(_.map(_ - 1))
case CpyInt(v, to) => (State.mem composeLens at(to)).set(Some(v))
case CpyReg(from, to) => (State.mem composeLens at(to)).set(Some(s.mem(from)))
case _ => identity[State]
}
(ptr andThen mem)(s)
}
One more question : is there a way to use Map.withDefaultValue
with monocle ?
The full code is here : https://gist.github.com/YannMoisan/b8ba25afc041d88706545527d9ec1988
You might want to use the second approach, because it separates handling of two fields. However, the functions shouldn't be interpreted in sequence (
andThen
), but rather they should be combined asPartialFunction
s withorElse
.UPDATE (after the Yann Moisan comment)
The execution may not terminate in case of infinite loop in user's program. So instead of the
foldLeft
we need some recursive function that will extract the next instruction by pointer:(The last line of
processInstruction3
should be replaced with the above code)