AdventOfCode 2020, December 9

AdventOfCode2020.12.09.fsx

    let input =
        File.ReadAllLines("Data/input09.txt")
        |> Seq.toArray 
        |> Array.map Int64.Parse

    // Reused from Day 1
    let rec findNwithSum sum n numbers acc_sum acc_numbers =
        if sum < 0L || n < 0 then
            None
        else if sum = 0L && n = 0 then
            Some acc_numbers
        else
            match numbers with
            | [] -> None
            | head :: tail ->
                match findNwithSum (sum-head) (n-1) tail (acc_sum+head) (head::acc_numbers) with
                | Some result -> Some result
                | None -> findNwithSum sum n tail acc_sum acc_numbers

    let rec findFirstNotValid size =
        input
        |> Array.mapi (fun i x -> 
            if i < size then 
                x, [x] 
            else
                match findNwithSum x 2 (List.ofArray input.[i-size..i-1]) 0L [] with
                | Some s -> x, s
                | None -> x, [])
        |> Array.filter (fun (i,v) -> List.isEmpty v)
        |> Array.head
        |> fst

    let notValid = findFirstNotValid 25

    let rec findContiguousSet sum setSize ndx =
        if ndx + setSize >= input.Length then
            Array.empty
        else
            let slice = input.[ndx..ndx+setSize-1]
            if slice |> Array.sum = sum then slice
            else findContiguousSet sum setSize (ndx+1)

    for setSize in [2..input.Length-1] do
        let result = findContiguousSet notValid setSize 0
        if result.Length > 0 then
            printfn "%A" <| (result |> Array.min) + (result |> Array.max)