day 3

day3.ml

@@ -0,0 +1,50 @@
+let read_file_rev name =
+    let ic = open_in name in
+    let try_read () =
+        try Some (input_line ic)
+        with End_of_file -> None in
+    let rec loop acc =
+        match try_read () with
+        | Some s -> loop (s::acc)
+        | None -> close_in ic; acc in
+    List.rev (loop [])
+
+
+let stepped_on_a_tree line pos =
+    let width = String.length line in
+    String.get line (pos mod width) = '#'
+
+
+let rec skiii count pos_x pos_y right down = function
+    | [] -> count
+    | h :: t -> if pos_y mod down <> 0 then skiii count pos_x (pos_y + 1) right down t
+                else skiii (count + (if (stepped_on_a_tree h pos_x) then 1 else 0))
+                     (pos_x + right)
+                     (pos_y + 1)
+                     right down t
+
+let part1 l =
+    skiii 0 0 0 3 1 l
+
+let part2 l =
+    (skiii 0 0 0 1 1 l) *
+    (skiii 0 0 0 3 1 l) *
+    (skiii 0 0 0 5 1 l) *
+    (skiii 0 0 0 7 1 l) *
+    (skiii 0 0 0 1 2 l)
+
+
+let do_count filename = function
+    | 1 -> read_file_rev filename |> part1
+    | 2 -> read_file_rev filename |> part2
+
+
+let () =
+    assert ((do_count "day3.test" 1) = 7)
+let () =
+    print_endline (string_of_int (do_count "day3.input" 1))
+
+let () =
+    assert ((do_count "day3.test" 2) = 336)
+let () =
+    print_endline (string_of_int (do_count "day3.input" 2))