2 files changed, 83 insertions, 8 deletions

diff --git a/src/day06.exs b/src/day06.exs
index 09a1604..8e339ff 100644
--- a/src/day06.exs
+++ b/src/day06.exs
@@ -32,15 +32,13 @@ defmodule Day06 do
         {v, j} <- Enum.with_index(String.codepoints(line)) do
       if v == element do
         {i, j}
-      else
-        nil
       end
     end
     |> Enum.filter(& &1 != nil)
   end
 
-  def part1({i, j}, grid) do
-    path({i, j}, :up, grid)
+  def part1({guard, grid, _, _}) do
+    path(guard, :up, grid)
     |> Enum.map(fn {i, j, _} -> {i, j} end)
     |> Enum.uniq()
     |> Enum.count()
@@ -89,7 +87,7 @@ defmodule Day06 do
     end
   end
 
-  def part2(guard, grid, overt, ohori) do
+  def part2({guard, grid, overt, ohori}) do
     path(guard, :up, grid)
     |> Enum.map(fn {i, j, _} -> {i, j} end)
     |> Enum.uniq()
@@ -160,12 +158,12 @@ defmodule Day06 do
   end
 end
 
-{guard, grid, overt, ohori} = IO.read(:stdio, :eof) |> Day06.parse_data()
+data = IO.read(:stdio, :eof) |> Day06.parse_data()
 
-{time1 , ans1} = :timer.tc(fn -> Day06.part1(guard, grid) end)
+{time1 , ans1} = :timer.tc(fn -> Day06.part1(data) end)
 IO.puts("Time  : #{time1 / 1000000}")
 IO.puts("Answer: #{ans1}")
 
-{time2 , ans2} = :timer.tc(fn -> Day06.part2(guard, grid, overt, ohori) end)
+{time2 , ans2} = :timer.tc(fn -> Day06.part2(data) end)
 IO.puts("Time  : #{time2 / 1000000}")
 IO.puts("Answer: #{ans2}")
diff --git a/src/day07.exs b/src/day07.exs
new file mode 100644
index 0000000..1adeb54
--- /dev/null
+++ b/src/day07.exs
@@ -0,0 +1,77 @@
+defmodule Utils do
+  def parse_integer_list(string, split, trim \\ true) do
+    string
+    |> String.split(split, trim: trim)
+    |> Enum.map(& String.to_integer(&1))
+  end
+end
+
+defmodule Equation do
+  @derive Inspect
+  defstruct [:result, :terms]
+
+  def from_string(string) do
+    string
+    |> Utils.parse_integer_list([":", " "])
+    |> then(& %Equation{result: hd(&1), terms: tl(&1)})
+  end
+
+  def has_lr_solution(equation, ops) do
+    has_lr_solution(hd(equation.terms), tl(equation.terms), equation.result, ops)
+  end
+
+  defp has_lr_solution(acc, terms, result, ops) do
+    cond do
+      acc > result ->
+        false
+      terms == [] and acc == result ->
+        true
+      terms == [] ->
+        false
+      true ->
+        ops
+        |> Enum.any?(& has_lr_solution(apply_op(acc, hd(terms), &1), tl(terms), result, ops))
+    end
+  end
+
+  defp apply_op(l, r, op) do
+    case op do
+      :add ->
+        l + r
+      :mul ->
+        l * r
+      :concat ->
+        String.to_integer("#{l}#{r}")
+    end
+  end
+end
+
+defmodule Day07 do
+  def part1(data) do
+    data
+    |> String.split("\n", trim: true)
+    |> Enum.map(& Equation.from_string(&1))
+    |> Enum.filter(& Equation.has_lr_solution(&1, [:mul, :add]))
+    |> Enum.map(& &1.result)
+    |> Enum.sum()
+  end
+
+  def part2(data) do
+    data
+    |> String.split("\n", trim: true)
+    |> Enum.map(& Equation.from_string(&1))
+    |> Enum.filter(& Equation.has_lr_solution(&1, [:mul, :concat, :add]))
+    |> Enum.map(& &1.result)
+    |> Enum.sum()
+  end
+end
+
+data = IO.read(:stdio, :eof)
+
+{time1 , ans1} = :timer.tc(fn -> Day07.part1(data) end)
+IO.puts("Time  : #{time1 / 1000000}")
+IO.puts("Answer: #{ans1}")
+
+{time2 , ans2} = :timer.tc(fn -> Day07.part2(data) end)
+IO.puts("Time  : #{time2 / 1000000}")
+IO.puts("Answer: #{ans2}")