From 2742fabb808fcb6eb202e1029d69a5e62b1c606c Mon Sep 17 00:00:00 2001
From: Shanti Chellaram <shanti@shanti.im>
Date: Mon, 19 Dec 2022 11:06:50 +0900
Subject: [PATCH] Day 13-1

---
 day13.erl | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 day13.erl

diff --git a/day13.erl b/day13.erl
new file mode 100644
index 0000000..20589f6
--- /dev/null
+++ b/day13.erl
@@ -0,0 +1,44 @@
+-module(day13).
+
+-export([solve/1]).
+
+-compile(export_all).
+
+solve(Input) ->
+    PacketPairs = binary:split(Input, <<"\n\n">>, [global, trim_all]),
+    Part1 = part1(PacketPairs),
+    {Part1, none}.
+
+part1(PacketPairs) -> part1(PacketPairs, {1, 0}).
+
+part1([], {_Index, Score}) -> Score;
+part1([PacketPair|Rest], {Index, Score}) ->
+    [LeftStr, RightStr] = binary:split(PacketPair, <<$\n>>, [trim_all]),
+    Left = parse_packet(LeftStr),
+    Right = parse_packet(RightStr),
+    NewScore = case compare(Left, Right) of
+                   true -> Score + Index;
+                   false -> Score
+               end,
+    part1(Rest, {Index+1, NewScore}).
+
+compare([], []) -> undecided;
+compare([], [_|_Rest]) -> true;
+compare([_|_Rest], []) -> false;
+compare([Left | LeftRest], [Right | RightRest]) when is_list(Left) and is_list(Right) -> 
+    case compare(Left, Right) of
+        undecided -> compare(LeftRest, RightRest);
+        Result -> Result
+    end;
+compare([Left | _] = Lefts, [Right | RightRest]) when is_list(Left) and is_integer(Right) ->
+    compare(Lefts, [[Right] | RightRest]);
+compare([Left | LeftRest], [Right | _] = Rights) when is_integer(Left) and is_list(Right) ->
+    compare([[Left] | LeftRest], Rights);
+compare([Value | Left], [Value | Right]) when is_integer(Value) -> compare(Left, Right);
+compare([Left|_], [Right|_]) when is_integer(Left) and is_integer(Right) -> Left < Right;
+compare(Left, Right) when is_integer(Left) and is_integer(Right) -> Left < Right.
+
+parse_packet(Packet) ->
+    {ok, Tokens, _EndLine} = erl_scan:string(unicode:characters_to_list([Packet,$.])),
+    {ok, Parsed} = erl_parse:parse_term(Tokens),
+    Parsed.