optimize day 5 solution

elixir/livebook/2024/day5.livemd

@@ -15,9 +15,11 @@ input = """
 rules = for rule <- rules |> String.split("\n"),
     [a, b] = rule |> String.split("|"),
     a = String.to_integer(a),
-    b = String.to_integer(b)
+    b = String.to_integer(b),
+    reduce: %{}
   do
-  {a, b}
+    acc -> 
+      Map.update(acc, a, MapSet.new([b]), fn c -> MapSet.put(c, b) end)
 end
 ```
 
@@ -33,11 +35,9 @@ updates = for update <- updates |> String.trim() |> String.split("\n"),
 
 ```elixir
 rule_sorter = fn a, b ->
-  a_precedes_b = Enum.any?(rules, &(&1 == {a, b}))
-  b_precedes_a = Enum.any?(rules, &(&1 == {b, a}))
+  b_precedes_a = Map.get(rules, b, MapSet.new()) |> MapSet.member?(a)
   cond do 
     b_precedes_a -> false
-    a_precedes_b -> true
     true -> true
   end
 end
@@ -53,6 +53,4 @@ for update <- updates,
     mid_idx = floor(len / 2)
     acc + Enum.at(sorted, mid_idx)
 end
-
-# 10495 too high
 ```