2023 Day 3 Part 1

2023/day3.exs

@@ -0,0 +1,90 @@
+#!/usr/bin/env elixir
+defmodule Day3 do
+  def part1(_input) do
+    # for each number, check around its edges to see if there's a symbol
+    fn {{x, y}, number, length}, acc ->
+      top = for x <- (x - 1)..(x + length), do: {x, y - 1}
+      bottom = for x <- (x - 1)..(x + length), do: {x, y + 1}
+      box = [{x - 1, y}, {x + length, y}] ++ top ++ bottom
+      if part_number?(box), do: [number | acc], else: acc
+    end
+    |> :ets.foldl(_acc = [], :numbers)
+    |> Enum.sum()
+  end
+
+  def part_number?([]), do: false
+
+  def part_number?([coord | rest]) do
+    case :ets.lookup(:symbols, coord) do
+      [_symbol] -> true
+      [] -> part_number?(rest)
+    end
+  end
+
+  def part2(_input) do
+    :ok
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      :ets.new(:numbers, [:named_table])
+      :ets.new(:symbols, [:named_table])
+      parse_schematic(input, _x = 0, _y = 0)
+    else
+      _ -> :error
+    end
+  end
+
+  def parse_schematic("", _x, _y), do: :ok
+  def parse_schematic("." <> rest, x, y), do: parse_schematic(rest, x + 1, y)
+  def parse_schematic("\n" <> rest, _x, y), do: parse_schematic(rest, 0, y + 1)
+
+  def parse_schematic(<<digit::utf8>> <> rest, x, y) when digit in ?0..?9 do
+    {number, length, rest} = take_digits(<<digit::utf8>> <> rest, "", 0)
+    :ets.insert(:numbers, {{x, y}, number, length})
+    parse_schematic(rest, x + length, y)
+  end
+
+  def parse_schematic(<<symbol::utf8>> <> rest, x, y) do
+    :ets.insert(:symbols, {{x, y}, <<symbol::utf8>>})
+    parse_schematic(rest, x + 1, y)
+  end
+
+  def take_digits(<<digit::utf8>> <> rest, digits, count) when digit in ?0..?9,
+    do: take_digits(rest, digits <> <<digit::utf8>>, count + 1)
+
+  def take_digits(rest, digits, count), do: {String.to_integer(digits), count, rest}
+
+  #######################
+  # 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 day3.exs input_filename")
+  end
+end
+
+# Day3.run()