From 787610c66b0208ac0d21138bb639f052c9f9aeef Mon Sep 17 00:00:00 2001
From: Dustin Swan <dustin@dustinswan.com>
Date: Wed, 16 Dec 2020 15:43:45 -0500
Subject: [PATCH] day15

---
 day15.ml  | 85 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 day15.txt |  3 ++
 2 files changed, 88 insertions(+)
 create mode 100644 day15.ml
 create mode 100644 day15.txt

diff --git a/day15.ml b/day15.ml
new file mode 100644
index 0000000..a4a0838
--- /dev/null
+++ b/day15.ml
@@ -0,0 +1,85 @@
+(* Common util. Should probably put this in a module or something.. *)
+let get_one_line file =
+  try Some (input_line file) with
+    End_of_file -> None
+
+let get_lines file =
+  let rec input lines =
+    match get_one_line file with
+      Some line -> input (line :: lines)
+    | None -> List.rev lines
+  in input []
+
+let read_file file =
+  let channel = open_in(file) in
+  get_lines channel
+
+let explode s = (* convert a string to a list of chars *)
+  let rec exp i l =
+   if i < 0 then l else exp (i - 1) (s.[i] :: l) in
+  exp (String.length s - 1) [];;
+
+let implode l = String.of_seq (List.to_seq l)
+
+let (<<) f g x = f(g(x))
+let (>>) f g x = g(f(x))
+let flip f x y = f y x
+
+(* --- *)
+
+let rec find_idx x = function (* find index of x *)
+  | [] -> raise (Failure "Not Found")
+  | h :: t -> if x = h then 0 else 1 + find_idx x t
+
+let rec last = function
+  | [] -> raise (Failure "Empty List")
+  | x :: [] -> x
+  | x :: xs -> last xs
+
+let hash = Hashtbl.create 1000
+
+let rec go idx last num =
+  (* Printf.printf "\nidx: %i. last num: %i\n" idx num; *)
+
+  if idx = last - 1
+  then num
+  else
+  let next =
+    try
+      let idx' = Hashtbl.find hash num in
+      let new_num = idx - idx' in
+      new_num
+    with Not_found _ -> 0
+  in
+  let _ = Hashtbl.replace hash num idx in
+  go (idx+1) last next
+
+let () =
+  let nums = read_file "day15.txt"
+    |> flip List.nth 0 (* grab the first line *)
+    |> String.split_on_char ','
+    |> List.map int_of_string
+  in
+
+  List.iter (Printf.printf "%i ") nums;
+  print_newline ();
+
+  (* pluck out the last number to start things off *)
+  let rev = List.rev nums in
+  let x :: xs = rev in
+  let nums' = List.rev xs in
+
+  let starting_idx = List.length xs in
+
+  (* Add the initial list to our hash *)
+  List.iteri (fun i x -> Hashtbl.add hash x i;) nums';
+
+  let final = go starting_idx 2020 x in
+  Printf.printf "after 2020: %i\n" final;
+
+  Hashtbl.clear hash;
+
+  List.iteri (fun i x -> Hashtbl.add hash x i;) nums';
+
+  let final' = go starting_idx 30000000 x in
+  Printf.printf "after 30000000: %i\n" final';
diff --git a/day15.txt b/day15.txt
new file mode 100644
index 0000000..28af522
--- /dev/null
+++ b/day15.txt
@@ -0,0 +1,3 @@
+2,0,6,12,1,3
+3,2,1
+0,3,6