Advent of Code 2023 9

advent2023-9.fs

let input = """0 3 6 9 12 15
1 3 6 10 15 21
10 13 16 21 30 45""" |> fun x -> x.Split("\n")

let parse (input:string[]) =
    input
    |> Array.map (fun x -> x.Split(" ") |> Array.map int)

let rec extrapolateSequence (sequence: int[]) =
    let diffs =
        sequence
        |> Array.pairwise
        |> Array.map (fun (a,b) -> b-a)
    let diff = Array.last diffs
    let allDiffsAreTheSame = diffs |> Array.forall ((=) diff)
    let headDiff, lastDiff =
        if allDiffsAreTheSame then
            diff, diff
        else
            extrapolateSequence diffs
    Array.head sequence - headDiff, Array.last sequence + lastDiff
        
let game = parse input
let part1 = Array.sumBy (extrapolateSequence >> snd) game // 114
let part2 = Array.sumBy (extrapolateSequence >> fst) game // 2