Day 6. Part 2 took 15 seconds to run... hm. But it works! Elixir is comfortable, but maybe not very exciting
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defmodule Day6 do
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def run do
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map =
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File.read!("data.txt")
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|> String.split("\n")
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|> Enum.drop(-1)
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|> Enum.map(&String.graphemes(&1))
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height = length(map)
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width = length(hd(map))
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# find initial location
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idx = Enum.concat(map) |> Enum.find_index(&(&1 != "#" and &1 != "."))
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location = {div(idx, height), rem(idx, width)}
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initialState = %{
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map: map,
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visited: MapSet.new(),
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location: location,
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bearing: 0,
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height: height,
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width: width
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}
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visited = walk(initialState, false)
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IO.puts("Part 1: #{MapSet.size(visited)}")
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# for each visited location, try putting an obstacle there
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num_positions =
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Enum.reduce(visited, 0, fn {row, col}, acc ->
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# generate new map with obstacle in the path
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new_row = List.replace_at(Enum.at(map, row), col, "#")
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new_map = List.replace_at(map, row, new_row)
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acc + walk(%{initialState | map: new_map}, true)
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end)
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IO.puts("Part 2: #{num_positions}")
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end
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defp walk(state, detect_loops?) do
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# if we're detecting loops, and this location & bearing is already in our map, get out
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if detect_loops? and MapSet.member?(state.visited, {state.location, state.bearing}) do
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1
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else
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# add current location to set
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visited =
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if detect_loops? do
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# if we're detecting loops, also record the bearing
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MapSet.put(state.visited, {state.location, state.bearing})
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else
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MapSet.put(state.visited, state.location)
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end
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{row, col} = state.location
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# grab next location
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{next_row, next_col} =
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case state.bearing do
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0 -> {row - 1, col}
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1 -> {row, col + 1}
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2 -> {row + 1, col}
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3 -> {row, col - 1}
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end
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in_bounds =
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next_row >= 0 and next_row < state.height and
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next_col >= 0 and next_col < state.width
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if !in_bounds do
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# if not in bounds, end recursion
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if detect_loops? do
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# if we're detecting loops, we didn't find one. return 0
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0
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else
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# otherwise return the list of visited locations
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visited
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end
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else
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cell = Enum.at(state.map, next_row) |> Enum.at(next_col)
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new_state =
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if cell == "#" do
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# keep location the same, turn 90° right
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new_bearing = Integer.mod(state.bearing + 1, 4)
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%{state | visited: visited, bearing: new_bearing}
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else
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%{state | visited: visited, location: {next_row, next_col}}
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end
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walk(new_state, detect_loops?)
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end
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end
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end
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end
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Day6.run()
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......#...
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