solved day 3 !

src/day3.ml

@@ -0,0 +1,58 @@
+open Containers
+module CharSet = Set.Make (Char)
+
+let set_of_string string = CharSet.of_list (String.to_list string)
+
+let priority char =
+  match char with
+  | 'a' .. 'z' -> 1 + Char.compare char 'a'
+  | 'A' .. 'Z' -> 27 + Char.compare char 'A'
+  | _ -> failwith @@ Printf.sprintf "'%c' is not a valid item" char
+;;
+
+
+let parse_line line =
+  let half_len = String.length line / 2 in
+  let first_half = String.sub line 0 half_len in
+  let second_half = String.sub line half_len half_len in
+  set_of_string first_half, set_of_string second_half
+;;
+
+let part1 lines =
+  List.fold_left
+    (fun acc line ->
+      let first, second = parse_line line in
+      let item =
+        match CharSet.elements @@ CharSet.inter first second with
+        | [ item ] -> item
+        | _ -> failwith "sad"
+      in
+      acc + priority item)
+    0
+    lines
+;;
+
+(*
+x thank you https://doc.sherlocode.com/ for helping me find Containers and its
+x List.chunks and List.reduce functions
+x*)
+let part2 lines =
+  let groups = List.chunks 3 (List.map set_of_string lines) in
+  List.fold_left
+    (fun acc group ->
+      let item =
+        match CharSet.elements @@ List.reduce_exn CharSet.inter group with
+        | [ item ] -> item
+        | _ -> failwith "sad"
+      in
+      acc + priority item)
+    0
+    groups
+;;
+
+let print () =
+  let lines = Utils.lines_of_input 3 in
+  Printf.(
+    printf "Day 3.1: %d\n" @@ part1 lines;
+    printf "Day 3.2: %d\n" @@ part2 lines)
+;;