pe011.ml

open Core.Std

let grid = [|
    [| 08; 02; 22; 97; 38; 15; 00; 40; 00; 75; 04; 05; 07; 78; 52; 12; 50; 77; 91; 08 |];
    [| 49; 49; 99; 40; 17; 81; 18; 57; 60; 87; 17; 40; 98; 43; 69; 48; 04; 56; 62; 00 |];
    [| 81; 49; 31; 73; 55; 79; 14; 29; 93; 71; 40; 67; 53; 88; 30; 03; 49; 13; 36; 65 |];
    [| 52; 70; 95; 23; 04; 60; 11; 42; 69; 24; 68; 56; 01; 32; 56; 71; 37; 02; 36; 91 |];
    [| 22; 31; 16; 71; 51; 67; 63; 89; 41; 92; 36; 54; 22; 40; 40; 28; 66; 33; 13; 80 |];
    [| 24; 47; 32; 60; 99; 03; 45; 02; 44; 75; 33; 53; 78; 36; 84; 20; 35; 17; 12; 50 |];
    [| 32; 98; 81; 28; 64; 23; 67; 10; 26; 38; 40; 67; 59; 54; 70; 66; 18; 38; 64; 70 |];
    [| 67; 26; 20; 68; 02; 62; 12; 20; 95; 63; 94; 39; 63; 08; 40; 91; 66; 49; 94; 21 |];
    [| 24; 55; 58; 05; 66; 73; 99; 26; 97; 17; 78; 78; 96; 83; 14; 88; 34; 89; 63; 72 |];
    [| 21; 36; 23; 09; 75; 00; 76; 44; 20; 45; 35; 14; 00; 61; 33; 97; 34; 31; 33; 95 |];
    [| 78; 17; 53; 28; 22; 75; 31; 67; 15; 94; 03; 80; 04; 62; 16; 14; 09; 53; 56; 92 |];
    [| 16; 39; 05; 42; 96; 35; 31; 47; 55; 58; 88; 24; 00; 17; 54; 24; 36; 29; 85; 57 |];
    [| 86; 56; 00; 48; 35; 71; 89; 07; 05; 44; 44; 37; 44; 60; 21; 58; 51; 54; 17; 58 |];
    [| 19; 80; 81; 68; 05; 94; 47; 69; 28; 73; 92; 13; 86; 52; 17; 77; 04; 89; 55; 40 |];
    [| 04; 52; 08; 83; 97; 35; 99; 16; 07; 97; 57; 32; 16; 26; 26; 79; 33; 27; 98; 66 |];
    [| 88; 36; 68; 87; 57; 62; 20; 72; 03; 46; 33; 67; 46; 55; 12; 32; 63; 93; 53; 69 |];
    [| 04; 42; 16; 73; 38; 25; 39; 11; 24; 94; 72; 18; 08; 46; 29; 32; 40; 62; 76; 36 |];
    [| 20; 69; 36; 41; 72; 30; 23; 88; 34; 62; 99; 69; 82; 67; 59; 85; 74; 04; 36; 16 |];
    [| 20; 73; 35; 29; 78; 31; 90; 01; 74; 31; 49; 71; 48; 86; 81; 16; 23; 57; 05; 54 |];
    [| 01; 70; 54; 71; 83; 51; 54; 69; 16; 92; 33; 48; 61; 43; 52; 01; 89; 19; 67; 48 |]
|]

let directions = [ (0, 1); (1, 0); (1, 1); (1, -1) ]

let get_max_product_adjacent how_many =
    Array.foldi grid ~init:0 ~f:(fun rowi acc row ->
        Array.foldi row ~init:acc ~f:(fun coli acc _ ->
            List.fold directions ~init:acc ~f:(fun acc (dx, dy) ->
                let rec iter x y prod = function
                    | 0 -> max acc prod
                    | steps ->
                        if x >= 0 && y >= 0 && x < 20 && y < 20 then
                            iter (x + dx) (y + dy)
                                 (prod * (Array.get (Array.get grid x) y))
                                 (steps - 1)
                        else
                            acc
                in
                iter rowi coli 1 how_many)))

let () = get_max_product_adjacent 4 |> Printf.printf "%d\n"