2021 day 9 part 1

2021/day9.exs

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+defmodule Day9 do
+  defmodule Cave do
+    defstruct floor: %{}, max_x: 0, max_y: 0
+  end
+
+  def part1(cave) do
+    for x <- 0..cave.max_x, y <- 0..cave.max_y, lowest_neighbour?(cave, x, y), reduce: 0 do
+      risk_level -> risk_level + cave.floor[{x, y}] + 1
+    end
+  end
+
+  defp neighbours(cave, x, y) do
+    [{x - 1, y}, {x + 1, y}, {x, y - 1}, {x, y + 1}]
+    |> Enum.reject(fn {x, y} -> x < 0 or x > cave.max_x or y < 0 or y > cave.max_y end)
+  end
+
+  defp lowest_neighbour?(cave, x, y) do
+    here = cave.floor[{x, y}]
+    Enum.all?(neighbours(cave, x, y), fn {xn, yn} -> here < cave.floor[{xn, yn}] end)
+  end
+
+  def part2(_input) do
+    :ok
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      rows =
+        input
+        |> String.split("\n", trim: true)
+        |> Enum.map(&String.split(&1, "", trim: true))
+
+      floor =
+        for {row, y} <- Enum.with_index(rows), {height, x} <- Enum.with_index(row), into: %{} do
+          {{x, y}, String.to_integer(height)}
+        end
+
+      max_x = length(hd(rows)) - 1
+      max_y = length(rows) - 1
+
+      %Cave{floor: floor, max_x: max_x, max_y: max_y}
+    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 day9.exs input_filename")
+  end
+end
+
+# Day9.run()