Day 6 Part 2

2023/day6.exs

@@ -0,0 +1,69 @@
+#!/usr/bin/env elixir
+defmodule Day6 do
+  def part1(input) do
+    input
+    |> Enum.map(fn line -> Enum.map(line, &String.to_integer/1) end)
+    |> Enum.zip()
+    |> Enum.map(fn {race, record} -> count_winning(race, record) end)
+    |> Enum.product()
+  end
+
+  def count_winning(race, record) do
+    scores = for n <- 1..div(race, 2), do: n * (race - n)
+    [_middle | rev_no_middle] = reversed = Enum.reverse(scores)
+    scores = scores ++ if rem(race, 2) == 1, do: reversed, else: rev_no_middle
+    Enum.count(scores, fn score -> score > record end)
+  end
+
+  def part2(input) do
+    [race, record] = Enum.map(input, fn line -> line |> Enum.join() |> String.to_integer() end)
+    count_winning(race, record)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n", trim: true)
+      |> Enum.map(fn line ->
+        line
+        |> String.split()
+        |> Enum.drop(1)
+      end)
+    else
+      _ -> :error
+    end
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def run do
+    case input() do
+      :error -> print_usage()
+      input -> run_parts_with_timer(input)
+    end
+  end
+
+  defp run_parts_with_timer(input) do
+    run_with_timer(1, fn -> part1(input) end)
+    run_with_timer(2, fn -> part2(input) end)
+  end
+
+  defp run_with_timer(part, fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
+    IO.puts("#{inspect(result)}\n")
+  end
+
+  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
+  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
+  defp format_time(μsec), do: "#{μsec / 1_000_000}s"
+
+  defp print_usage do
+    IO.puts("Usage: elixir day6.exs input_filename")
+  end
+end
+
+# Day6.run()