From 59cc25c5659e316a23b2d284971165f824498505 Mon Sep 17 00:00:00 2001
From: Shanti Chellaram <shanti@shanti.im>
Date: Thu, 22 Dec 2022 16:15:34 +0900
Subject: [PATCH] Day 21-2

---
 day21.erl | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/day21.erl b/day21.erl
index 9941fc8..de5de14 100644
--- a/day21.erl
+++ b/day21.erl
@@ -3,7 +3,13 @@
 -export([solve/1]).
 
 solve(InputData) ->
-    {eval(<<"root">>, parse_input(InputData)), none}.
+    Monkeys = parse_input(InputData),
+    {eval(<<"root">>, Monkeys), undo(human_path(<<"root">>, p2fix(Monkeys), []))}.
+
+% turn the root into a big equal sign
+
+p2fix(#{<<"root">> := {_Op, Left, Right}} = Monkeys) ->
+    Monkeys#{<<"root">> := {$-, Left, Right}}.
 
 parse_input(Input) -> parse_input(Input, #{}).
 
@@ -30,6 +36,30 @@ eval(Name, Monkeys) ->
             end
     end.
 
+undo(HumanPath) -> lists:foldl(fun do_undo/2, 0, HumanPath).
+
+do_undo({_Side, $+, Value}, Target) -> Target - Value;
+do_undo({_Side, $*, Value}, Target) -> Target div Value;
+do_undo({left, $/, Value}, Target) -> Target * Value;
+do_undo({right, $/, Value}, Target) -> Value div Target;
+do_undo({left, $-, Value}, Target) -> Target + Value;
+do_undo({right, $-, Value}, Target) -> Value - Target.
+
+
+human_path(<<"humn">>, _State, Path) -> lists:reverse(Path);
+human_path(Name, Monkeys, Path) ->
+    #{Name := Node} = Monkeys,
+    case Node of
+        Value when is_integer(Value) ->
+            not_found;
+        {Op, Left, Right} ->
+            case human_path(Left, Monkeys, [{left, Op, eval(Right, Monkeys)} | Path]) of
+                not_found -> human_path(Right, Monkeys, [{right, Op, eval(Left, Monkeys)} | Path]);
+                Found -> Found
+            end
+    end.
+
+
 %% Parse Integer standard code
 parse_int(<<$-, Rest/binary>>) ->
     {AbsoluteValue, Binary} = parse_int0(Rest, 0),