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I am using the coding puzzels of https://adventofcode.com for improving my F# skills.

Problem:

Day 1 Part 2:

Starting with frequency 0, a list of numbers should be added successive generating new frequencies. Searched is the first frequency which occurs twice. If the end of the list has been reached without success; the list should be processed again.

Example Input: [+1, -2, +3, +1]

  • Current frequency 0, change of +1; resulting frequency 1.
  • Current frequency 1, change of -2; resulting frequency -1. C* urrent frequency -1, change of +3; resulting frequency 2.
  • Current frequency 2, change of +1; resulting frequency 3. (At this point, we continue from the start of the list.)
  • Current frequency 3, change of +1; resulting frequency 4.
  • Current frequency 4, change of -2; resulting frequency 2, which has already been seen.

In this example, the first frequency reached twice is 2.

Solution

let input = [1;-2;3;1]

type State =
    | FinalState of int
    | IntermediateState of int * int Set

type State with
    static member Zero = IntermediateState(0, Set.empty.Add(0)) 

let foldIntermediateState (freqHistory: int Set, freq : int) : State =
    let isFinal = freqHistory |> Set.contains freq
    if isFinal then
        FinalState (freq)
    else
        IntermediateState (freq, freqHistory.Add(freq))

let foldState state value =
    match state with
        | FinalState _ -> state
        | IntermediateState (lastFreq, freqHistory) -> foldIntermediateState (freqHistory, lastFreq + value)

let rec processListRec state lst =
    let result = lst |> Seq.fold foldState state
    match result with
        | FinalState result -> printfn "The result is: %i" result
        | IntermediateState _ -> lst |> processListRec result

input |> Seq.map int |> processListRec State.Zero

Questions

In fact, I am proud to have found a solution without mutable states that looks (at least for me) functional! :D. So any feeback that helps improving the solution or even alternative function approches for solving that problem are welcome :).

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Caveat : I don't know F#.

I liked your solution, the use of algebraic data types, pattern matching and use of list processing via fold and higher order functions are good.

However I'm trying to apply a functional style in python (which I'm learning) and I think there is a way to do this that use simple list data structures, with (list) comprehension or the equivalent

Here is my attempt in python to present an alternative approach

import functools
import itertools

fchanges = []
with open('frequency_input') as f:
    for line in f:
            fchanges.append(int(line))

frequencies = [sum(fchanges[:index]) for index in range(1, len(fchanges)+1)]
skew = frequencies[-1]

print('Skew is ' + str(skew))
accumulator = itertools.accumulate(itertools.cycle(fchanges))
prefix = itertools.takewhile(lambda x: (x + skew) not in frequencies, accumulator)

# takewhile consumes the value I'm looking for to check the predicate, hence this hack
# don't need the list, memory optimization - just need the length
plen = functools.reduce(lambda x, y: x+1, prefix, 0)
accumulator = itertools.accumulate(itertools.cycle(fchanges))
print('found first repetition ' + str(next(itertools.islice(accumulator, plen, None)) + skew))

Happy to receive feedback on my attempt too for any observers.

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