Day 12 Parts 1 and 2

README.md

@@ -2,7 +2,7 @@
 
 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="975" alt="image" src="https://user-images.githubusercontent.com/498229/101872701-0a2b3080-3bc9-11eb-97da-884b52168ecc.png">
+<img width="975" alt="image" src="https://user-images.githubusercontent.com/498229/101977622-1a5b1280-3c92-11eb-8dc5-73c0998623e0.png">
 
 ## Strategy
 

day12/README

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+Day 12 Notes
+
++--------+
+| Part 1 |
++--------+
+
+$ elixir day12part1.exs
+923
+
+Thoughts:
+
+This was pretty fun :-)
+
+I spent ages fixing stupid typos and didn't realise I'd forgotten to take account for the fact that
+a rotation command could contain more than 90 degrees. I submitted so many wrong answers I had to
+wait 3 minutes to submit the correct  one... oops.
+
+Decided to store the direction as an {x, y} vector, and handle rotations as just flipping the
+values as required. Use recursion to do it repetedly for rotations of more than 90 degrees.
+
+NSEW operations are just simply adjusting the coordinates as required.
+
+For F, apply the vector to the boat's current position.
+
++--------+
+| Part 2 |
++--------+
+
+$ elixir day12part2.exs
+24769
+
+Thoughts:
+
+Interesting. The fact that the waypoint moves when the boat moves makes this part very simple.
+
+Thankfully I stored the rotation as a vector in part 1, which is basically the waypoint
+fixed at a distance of 1. So for this part, can just adjust the NSEW commands to move the
+waypoint/vector instead of the boat.
+
+The rest of the commands are the same.
+
++------------------+
+| Overall Thoughts |
++------------------+
+
+I liked this one. Figuring out how to do the rotation efficiently probably took the most time,
+but I'm pretty happy with the results 😊

day12/day12part1.exs

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+defmodule Day12Part1 do
+  def run do
+    File.read!("input")
+    |> String.split("\n", trim: true)
+    |> Enum.map(fn <<action::binary-1>> <> value -> {action, String.to_integer(value)} end)
+    |> Enum.reduce({0, 0, {1, 0}}, &process/2)
+    |> manhattan_distance()
+    |> IO.puts()
+  end
+
+  def process({"N", value}, {x, y, direction}), do: {x, y + value, direction}
+  def process({"S", value}, {x, y, direction}), do: {x, y - value, direction}
+  def process({"E", value}, {x, y, direction}), do: {x + value, y, direction}
+  def process({"W", value}, {x, y, direction}), do: {x - value, y, direction}
+  def process({"L", value}, {x, y, direction}), do: {x, y, turn_left(direction, value)}
+  def process({"R", value}, {x, y, direction}), do: {x, y, turn_right(direction, value)}
+  def process({"F", value}, {x, y, {vx, vy}}), do: {x + vx * value, y + vy * value, {vx, vy}}
+
+  def turn_left(direction, 0), do: direction
+  def turn_left({vx, vy}, degrees), do: turn_left({-vy, vx}, degrees - 90)
+
+  def turn_right(direction, 0), do: direction
+  def turn_right({vx, vy}, degrees), do: turn_right({vy, -vx}, degrees - 90)
+
+  def manhattan_distance({x, y, _}), do: abs(x) + abs(y)
+end
+
+Day12Part1.run()

day12/day12part2.exs

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+defmodule Day12Part2 do
+  def run do
+    File.read!("input")
+    |> String.split("\n", trim: true)
+    |> Enum.map(fn <<action::binary-1>> <> value -> {action, String.to_integer(value)} end)
+    |> Enum.reduce({{0, 0}, {10, 1}}, &process/2)
+    |> manhattan_distance()
+    |> IO.puts()
+  end
+
+  def process({"N", value}, {boat, {vx, vy}}), do: {boat, {vx, vy + value}}
+  def process({"S", value}, {boat, {vx, vy}}), do: {boat, {vx, vy - value}}
+  def process({"E", value}, {boat, {vx, vy}}), do: {boat, {vx + value, vy}}
+  def process({"W", value}, {boat, {vx, vy}}), do: {boat, {vx - value, vy}}
+  def process({"L", value}, {boat, waypoint}), do: {boat, rotate_left(waypoint, value)}
+  def process({"R", value}, {boat, waypoint}), do: {boat, rotate_right(waypoint, value)}
+  def process({"F", value}, {{x, y}, {vx, vy}}), do: {{x + vx * value, y + vy * value}, {vx, vy}}
+
+  def rotate_left(waypoint, 0), do: waypoint
+  def rotate_left({vx, vy}, degrees), do: rotate_left({-vy, vx}, degrees - 90)
+
+  def rotate_right(waypoint, 0), do: waypoint
+  def rotate_right({vx, vy}, degrees), do: rotate_right({vy, -vx}, degrees - 90)
+
+  def manhattan_distance({{x, y}, _}), do: abs(x) + abs(y)
+end
+
+Day12Part2.run()

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