solve day 15 part 1 !

src/day15.ml

@@ -44,22 +44,30 @@ let example_lines =
   ]
 ;;
 
-let%expect_test "Day 15 example" =
-  example_lines |> List.map parse_sensor |> List.iter @@ Format.printf "%a@ " pp_sensor;
-  [%expect
-    {|
-    { pos = (2, 18); beacon = (-2, 15); dist = 7 }
-    { pos = (9, 16); beacon = (10, 16); dist = 1 }
-    { pos = (13, 2); beacon = (15, 3); dist = 3 }
-    { pos = (12, 14); beacon = (10, 16); dist = 4 }
-    { pos = (10, 20); beacon = (10, 16); dist = 4 }
-    { pos = (14, 17); beacon = (10, 16); dist = 5 }
-    { pos = (8, 7); beacon = (2, 10); dist = 9 }
-    { pos = (2, 0); beacon = (2, 10); dist = 10 }
-    { pos = (0, 11); beacon = (2, 10); dist = 3 }
-    { pos = (20, 14); beacon = (25, 17); dist = 8 }
-    { pos = (17, 20); beacon = (21, 22); dist = 6 }
-    { pos = (16, 7); beacon = (15, 3); dist = 5 }
-    { pos = (14, 3); beacon = (15, 3); dist = 1 }
-    { pos = (20, 1); beacon = (15, 3); dist = 7 } |}]
+let bounds sensors =
+  let max_dist = sensors |> List.fold_left (fun acc s -> max acc s.dist) 0 in
+  sensors
+  |> List.flat_map (fun sensor -> [ sensor.pos; sensor.beacon ])
+  |> Vec2.bounds max_dist
+;;
+
+let%expect_test "Day 15.1" =
+  let sensors = Utils.lines_of_input 15 |> List.map parse_sensor in
+  let scan_row = 2000000 in
+  let Vec2.{ x = lx; _ }, Vec2.{ x = hx; _ } = bounds sensors in
+  let is_impossible pos =
+    sensors
+    |> List.map (fun s ->
+      let d = taxicab_dist pos s.pos in
+      if d <= s.dist && not Vec2.(pos = s.beacon) then 1 else 0)
+    |> List.reduce_exn ( + )
+    |> Int.sign
+  in
+  Vec2.fold_region
+    Vec2.{ x = lx; y = scan_row }
+    Vec2.{ x = hx; y = scan_row }
+    (fun acc pos -> acc + is_impossible pos)
+    0
+  |> Printf.printf "%i\n";
+  [%expect {| 5525990 |}]
 ;;

src/vec2.ml

@@ -27,3 +27,28 @@ let iter_region a b callback =
     done
   done
 ;;
+
+let fold_region a b callback init =
+  let acc = ref init in
+  iter_region a b (fun pos -> acc := callback !acc pos);
+  !acc
+;;
+
+let bounds padding points =
+  let lx, hx, ly, hy =
+    points
+    |> List.fold_left
+         (fun (lx, hx, ly, hy) { x; y } ->
+           let lx = min lx x in
+           let hx = max hx x in
+           let ly = min ly y in
+           let hy = max hy y in
+           lx, hx, ly, hy)
+         (Int.max_int, 0, Int.max_int, 0)
+  in
+  let lx = Int.(lx - padding) in
+  let hx = Int.(hx + padding) in
+  let ly = Int.(ly - padding) in
+  let hy = Int.(hy + padding) in
+  of_tuple (lx, ly), of_tuple (hx, hy)
+;;

src/vec2.mli

@@ -18,3 +18,5 @@ val ( - ) : t -> t -> t
 val ( = ) : t -> t -> bool
 val abs : t -> t
 val iter_region : t -> t -> (t -> unit) -> unit
+val fold_region : t -> t -> ('a -> t -> 'a) -> 'a -> 'a
+val bounds : int -> t list -> t * t