2021 day 7

2021/day7.exs

@@ -0,0 +1,81 @@
+defmodule Day7 do
+  def part1(input) do
+    input
+    |> Enum.frequencies()
+    |> Enum.into(Map.new(0..Enum.max(input), &{&1, 0}))
+    |> calculate_costs()
+    |> Enum.min()
+  end
+
+  defp calculate_costs(crabs) do
+    for here <- Map.keys(crabs) do
+      for {there, crabs_there} <- Map.delete(crabs, here), reduce: 0 do
+        cost -> cost + abs(here - there) * crabs_there
+      end
+    end
+  end
+
+  def part2(input) do
+    input
+    |> Enum.frequencies()
+    |> Enum.into(Map.new(0..Enum.max(input), &{&1, 0}))
+    |> calculate_sum_costs()
+    |> Enum.min()
+  end
+
+  defp calculate_sum_costs(crabs) do
+    for here <- Map.keys(crabs) do
+      for {there, crabs_there} <- Map.delete(crabs, here), reduce: 0 do
+        cost ->
+          distance = abs(here - there)
+          # Mmm maths. Can get the sum of 1..n by doing (n * n+1)/2. Guess you have to know it.
+          sum = div(distance * (distance + 1), 2)
+          cost + sum * crabs_there
+      end
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> String.split(",")
+      |> Enum.map(&String.to_integer/1)
+    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("#{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 day7.exs input_filename")
+  end
+end
+
+Day7.run()