diff --git a/bin/day2401.ml b/bin/day2401.ml
index 754c762..a0ed08f 100644
--- a/bin/day2401.ml
+++ b/bin/day2401.ml
@@ -1,11 +1,7 @@
-(** [nums_from_string s] takes a string of space separated integers and gives
-    back a list of the integers. *)
-let nums_from_string s = List.map int_of_string (Str.split (Str.regexp " +") s)
-
 (** [pair_nums_from_string s] takes a string of two numbers separated by 
     whitespace and returns the pair of the numbers *)
 let pair_nums_from_string s =
-  match nums_from_string s with
+  match Aoc.nums_from_string s with
   | [ h; h' ] -> (h, h')
   | _ -> raise (Invalid_argument "pair_nums_from_string")
 
@@ -19,9 +15,6 @@ let rev_split lst =
   in
   impl [] [] lst
 
-(** [distance a b] returns the absolute difference between [a] and [b]. *)
-let distance a b = abs (a - b)
-
 (** [count lst n] counts the number of times [n] appears as an element in [lst].
     *)
 let count lst n =
@@ -33,12 +26,10 @@ let accumulate = List.fold_left ( + ) 0
 (** [lists_from_file fname] Read two lists of integers from [fname] and return
     as a pair. *)
 let lists_from_file fname =
-  In_channel.with_open_text fname In_channel.input_lines
-  |> List.map pair_nums_from_string
-  |> rev_split
+  Aoc.strings_from_file fname |> List.map pair_nums_from_string |> rev_split
 
 let day2401a a b =
-  List.map2 distance (List.sort Int.compare a) (List.sort Int.compare b)
+  List.map2 Aoc.distance1 (List.sort Int.compare a) (List.sort Int.compare b)
   |> accumulate
 
 let day2401b a b = List.map (count b) a |> List.map2 ( * ) a |> accumulate
diff --git a/bin/day2402.ml b/bin/day2402.ml
new file mode 100644
index 0000000..5e70a74
--- /dev/null
+++ b/bin/day2402.ml
@@ -0,0 +1,36 @@
+(** [is_monotonic pred lst] returns true if [pred] returns true when tested on
+    all consecutive elements of [lst]. *)
+let rec is_monotonic pred = function
+  | [] | _ :: [] -> true
+  | h :: h' :: t -> pred h h' && is_monotonic pred (h' :: t)
+
+let is_safe lst =
+  (is_monotonic ( < ) lst || is_monotonic ( > ) lst)
+  && is_monotonic (fun a b -> Aoc.distance1 a b <= 3) lst
+
+let is_safe_dampened lst =
+  let rec impl acc = function
+    | [] -> is_safe acc
+    | h :: t -> is_safe (acc @ t) || impl (acc @ [ h ]) t
+  in
+  impl [] lst
+
+let nums_from_file fname =
+  Aoc.strings_from_file fname |> List.map Aoc.nums_from_string
+
+let day2402a lsts = List.filter is_safe lsts |> List.length
+let day2402b lsts = List.filter is_safe_dampened lsts |> List.length
+
+let _ =
+  try
+    match Sys.argv with
+    | [| _; fname |] ->
+        let lines = nums_from_file fname in
+        Printf.printf "Part 1 = %d\n" (day2402a lines);
+        Printf.printf "Part 2 = %d\n" (day2402b lines)
+    | _ ->
+        Printf.printf "Usage: day2402 <fname>\n";
+        exit 1
+  with e ->
+    Printf.printf "An error occured: %s\n" (Printexc.to_string e);
+    exit 1
diff --git a/bin/dune b/bin/dune
index f5aa6c5..a3f35b9 100644
--- a/bin/dune
+++ b/bin/dune
@@ -1,4 +1,4 @@
 (executables
- (public_names day2401)
- (names day2401)
- (libraries str))
+ (public_names day2401 day2402)
+ (names day2401 day2402)
+ (libraries str aoc))
diff --git a/lib/aoc.ml b/lib/aoc.ml
new file mode 100644
index 0000000..cb7dff6
--- /dev/null
+++ b/lib/aoc.ml
@@ -0,0 +1,5 @@
+let nums_from_string s = List.map int_of_string (Str.split (Str.regexp " +") s)
+let distance1 a b = abs (a - b)
+
+let strings_from_file fname =
+  In_channel.with_open_text fname In_channel.input_lines
diff --git a/lib/aoc.mli b/lib/aoc.mli
new file mode 100644
index 0000000..b875046
--- /dev/null
+++ b/lib/aoc.mli
@@ -0,0 +1,10 @@
+val nums_from_string : string -> int list
+(** [nums_from_string s] takes a string of space separated integers and gives
+    back a list of the integers. *)
+
+val distance1 : int -> int -> int
+(** [distance1 a b] returns the absolute difference between [a] and [b]. *)
+
+val strings_from_file : string -> string list
+(** [strings_from_file fname] returns a list of strings from the file 
+    [fname].  Each string represents a line from the file. *)
diff --git a/lib/dune b/lib/dune
new file mode 100644
index 0000000..9982687
--- /dev/null
+++ b/lib/dune
@@ -0,0 +1,3 @@
+(library
+ (name aoc)
+ (libraries str))