2023 day 17, part 1

README.md

@@ -5,7 +5,7 @@
 My Elixir solutions for [Advent of Code](https://adventofcode.com/) (all years).
 
 <!-- stars start -->
-<p><a href="./lib/y2023/"><img src="https://img.shields.io/static/v1?label=2023&message=32%20stars&style=for-the-badge&color=yellow" alt="32 stars" /></a><br />
+<p><a href="./lib/y2023/"><img src="https://img.shields.io/static/v1?label=2023&message=33%20stars&style=for-the-badge&color=yellow" alt="33 stars" /></a><br />
 <a href="./lib/y2022/"><img src="https://img.shields.io/static/v1?label=2022&message=%E2%AD%90%EF%B8%8F%2050%20stars%20%E2%AD%90%EF%B8%8F&style=for-the-badge&color=brightgreen" alt="50 stars" /></a><br />
 <a href="./lib/y2021/"><img src="https://img.shields.io/static/v1?label=2021&message=46%20stars&style=for-the-badge&color=green" alt="46 stars" /></a><br />
 <a href="./lib/y2020/"><img src="https://img.shields.io/static/v1?label=2020&message=39%20stars&style=for-the-badge&color=yellow" alt="39 stars" /></a><br />

lib/advent/grid.ex

@@ -66,6 +66,6 @@ defmodule Advent.Grid do
   end
 
   defp colour(char) do
-    IO.ANSI.red() <> char <> IO.ANSI.reset()
+    IO.ANSI.color(4, 0, 5) <> "#{char}" <> IO.ANSI.reset()
   end
 end

lib/y2023/README.md

@@ -2,7 +2,7 @@
 
 My Elixir solutions for [Advent of Code 2023](https://adventofcode.com/2023).
 
-<!-- stars 2023 start --><img src="https://img.shields.io/static/v1?label=2023&message=32%20stars&style=for-the-badge&color=yellow" alt="32 stars" /><!-- stars 2023 end -->
+<!-- stars 2023 start --><img src="https://img.shields.io/static/v1?label=2023&message=33%20stars&style=for-the-badge&color=yellow" alt="33 stars" /><!-- stars 2023 end -->
 
 ## Benchmarks
 
@@ -47,4 +47,5 @@ day 15, part 1        1.97 K        0.51 ms    ±58.77%        0.47 ms        1.
 day 15, part 2        0.39 K        2.57 ms     ±9.13%        2.56 ms        3.05 ms
 day 16, part 1         97.00       10.31 ms    ±14.20%       10.08 ms       13.65 ms
 day 16, part 2          2.07      483.12 ms     ±2.51%      482.96 ms      503.06 ms
+day 17, part 1          1.42      704.79 ms     ±2.71%      713.38 ms      719.67 ms
 ```

lib/y2023/day17.ex

@@ -0,0 +1,102 @@
+defmodule Y2023.Day17 do
+  use Advent.Day, no: 17
+
+  alias Advent.Grid
+
+  def part1(grid) do
+    find_best_path(grid, {1, 1})
+  end
+
+  # @doc """
+  # iex> Day17.part2("update or delete me")
+  # "update or delete me"
+  # """
+  # def part2(input) do
+  #   input
+  # end
+
+  defp find_best_path(grid, {row, col}) do
+    PriorityQueue.new()
+    |> add_to_queue([[{row, col, nil, 0}]])
+    |> search(grid, Grid.size(grid), MapSet.new())
+  end
+
+  defp add_to_queue(queue, states) do
+    Enum.reduce(states, queue, fn state, queue ->
+      PriorityQueue.push(queue, state, elem(hd(state), 3))
+    end)
+  end
+
+  defp search(queue, grid, destination, cache) do
+    do_search(PriorityQueue.pop(queue), grid, destination, cache)
+  end
+
+  defp do_search({:empty, _queue}, _grid, _destination, _cache) do
+    raise("No path found")
+  end
+
+  defp do_search(
+         {{:value, [{row, col, _direction, count} | _rest]}, _queue},
+         _grid,
+         {row, col},
+         _cache
+       ) do
+    # Winner winner chicken dinner.
+    count
+  end
+
+  defp do_search({{:value, path}, queue}, grid, destination, cache) do
+    if MapSet.member?(cache, cache_key(path)) do
+      # Been here before at a lower heat loss value, ignore
+      search(queue, grid, destination, cache)
+    else
+      cache = MapSet.put(cache, cache_key(path))
+
+      queue
+      |> add_to_queue(valid_moves(path, grid))
+      |> search(grid, destination, cache)
+    end
+  end
+
+  defp valid_moves([{row, col, direction, value} | _rest] = path, grid) do
+    [{row - 1, col, :up}, {row + 1, col, :down}, {row, col - 1, :left}, {row, col + 1, :right}]
+    |> Enum.filter(&in_grid?(grid, &1))
+    |> Enum.reject(&backwards?(direction, &1))
+    |> Enum.reject(&too_far_straight?(path, &1))
+    |> Enum.map(fn {row, col, dir} ->
+      [{row, col, dir, value + Map.fetch!(grid, {row, col})} | path]
+    end)
+  end
+
+  defp cache_key([{row, col, direction, _} | _rest] = path) do
+    # The cache key has to include the last three directions
+    # Something going |  may give a different result to --
+    #                 |-                                 |
+    # even with the same current value, due to the straight run length
+    # limitation
+    next_cache = Enum.take_while(path, fn {_, _, dir, _} -> dir == direction end) |> length()
+    {{row, col, direction}, next_cache}
+  end
+
+  defp in_grid?(grid, {row, col, _dir}), do: Map.has_key?(grid, {row, col})
+
+  defp backwards?(from, {_, _, to}) do
+    {from, to} in [{:left, :right}, {:right, :left}, {:up, :down}, {:down, :up}]
+  end
+
+  defp too_far_straight?([{_, _, dir, _}, {_, _, dir, _}, {_, _, dir, _} | _rest], {_, _, dir}) do
+    true
+  end
+
+  defp too_far_straight?(_, _), do: false
+
+  def parse_input(input) do
+    input
+    |> Grid.new()
+    |> Enum.map(fn {a, b} -> {a, String.to_integer(b)} end)
+    |> Enum.into(%{})
+  end
+
+  def part1_verify, do: input() |> parse_input() |> part1()
+  # def part2_verify, do: input() |> parse_input() |> part2()
+end

test/y2023/day17_test.exs

@@ -0,0 +1,29 @@
+defmodule Y2023.Day17Test do
+  use ExUnit.Case, async: true
+  alias Y2023.Day17
+  doctest Day17
+
+  test "verification, part 1", do: assert(Day17.part1_verify() == 886)
+  # test "verification, part 2", do: assert(Day17.part2_verify() == "update or delete me")
+
+  @sample_input """
+  2413432311323
+  3215453535623
+  3255245654254
+  3446585845452
+  4546657867536
+  1438598798454
+  4457876987766
+  3637877979653
+  4654967986887
+  4564679986453
+  1224686865563
+  2546548887735
+  4322674655533
+  """
+
+  test "part 1" do
+    actual = Day17.parse_input(@sample_input) |> Day17.part1()
+    assert 102 == actual
+  end
+end