Day 1 part 1 solution

2025/day1.ex

@@ -1,5 +1,3 @@
-ops = ["L50", "R50", "R1"]
-
 defmodule Dial do
   # Initial conditions
   def rotate(operations), do: rotate(50, operations, 0)
@@ -9,18 +7,38 @@ defmodule Dial do
 
   # Compute the new dial position
   def rotate(index, [head | tail], zeroes) do
-    index = case head do
+    new_index = case head do
-      "R" <> number -> index + String.to_integer(number)
+      "R" <> number -> rotate_right(index, String.to_integer(number))
-      "L" <> number -> index - String.to_integer(number)
+      "L" <> number -> rotate_left(index, String.to_integer(number))
     end
 
-    case index do
+    case new_index do
-      0 -> rotate(index, tail, zeroes+1)
+      0 -> rotate(new_index, tail, zeroes+1)
-      _ -> rotate(index, tail, zeroes)
+      _ -> rotate(new_index, tail, zeroes)
+    end
+  end
+
+  defp rotate_right(index, number) do
+    diff = index + rem(number, 100)
+    case diff do
+      diff when diff <= 99 -> diff
+      diff when diff > 99 -> diff - 100
+    end
+  end
+
+  defp rotate_left(index, number) do
+    diff = index - rem(number, 100)
+    case diff do
+      diff when diff >= 0 -> diff
+      diff when diff < 0 -> diff + 100
     end
   end
 end
 
+ops = File.stream!("./day1.txt")
+  |> Stream.map(&String.trim_trailing/1)
+  |> Enum.to_list()
+
 dial = Dial.rotate(ops)
 
 IO.puts dial