Add a generic main function.

lib/aoc.ml

@@ -3,3 +3,20 @@ let distance1 a b = abs (a - b)
 
 let strings_from_file fname =
   In_channel.with_open_text fname In_channel.input_lines
+
+let main prep parts =
+  try
+    match Sys.argv with
+    | [| _; fname |] ->
+        let lines = prep fname in
+        let do_part i (fmt, fn) =
+          Printf.printf "Part %d = %s\n" i (fmt (fn lines))
+        in
+        List.iteri do_part parts;
+        exit 0
+    | _ ->
+        Printf.printf "Usage: %s <fname>\n" Sys.executable_name;
+        exit 2
+  with e ->
+    Printf.printf "An error occured: %s\n" (Printexc.to_string e);
+    exit 1