# Advent of Code 2022 - Day 9
## Day 9: Rope Bridge
[Description](https://adventofcode.com/2022/day/9)
## Utils
```elixir
defmodule Load do
def file(path) do
File.read!(__DIR__ <> path)
|> String.split("\n", trim: true)
end
def string(value) do
value |> String.split("\n", trim: true)
end
end
```
## Input
```elixir
input = Load.file("/data/day09.txt")
```
## Part 1
Simulate your complete hypothetical series of motions. How many positions does the tail of the rope visit at least once?
```elixir
defmodule Part1 do
def head_positions(input) do
input
|> move_head([{0, 0}])
|> Enum.reverse()
end
def move_head([], head_positions) do
head_positions
end
def move_head([instruction | input], head_positions) do
[dir, steps] = instruction |> String.split(" ", trim: true)
head = hd(head_positions)
direction =
case dir do
"R" -> {1, 0}
"L" -> {-1, 0}
"U" -> {0, 1}
"D" -> {0, -1}
end
steps = steps |> String.to_integer()
new_head_positions =
1..steps
|> Enum.map(fn step ->
{elem(head, 0) + step * elem(direction, 0), elem(head, 1) + step * elem(direction, 1)}
end)
|> Enum.reverse()
move_head(input, new_head_positions ++ head_positions)
end
def follow([], tail_positions) do
tail_positions
end
def follow([head_position | head_positions], [tail_position | _] = tail_positions) do
vd = vertical_distance(head_position, tail_position)
hd = horizontal_distance(head_position, tail_position)
if vd > 1 or hd > 1 do
offset = {
horizontal_offset(head_position, tail_position),
vertical_offset(head_position, tail_position)
}
normalized_offset = normalize(offset)
new_tail_position = {
elem(tail_position, 0) + elem(normalized_offset, 0),
elem(tail_position, 1) + elem(normalized_offset, 1)
}
follow(head_positions, [new_tail_position | tail_positions])
else
follow(head_positions, tail_positions)
end
end
defp vertical_offset(pos1, pos2) do
elem(pos1, 1) - elem(pos2, 1)
end
defp vertical_distance(pos1, pos2) do
abs(vertical_offset(pos1, pos2))
end
defp horizontal_offset(pos1, pos2) do
elem(pos1, 0) - elem(pos2, 0)
end
defp horizontal_distance(pos1, pos2) do
abs(horizontal_offset(pos1, pos2))
end
defp normalize(pos) do
x =
case elem(pos, 0) do
0 -> 0
n -> (n / abs(n)) |> round()
end
y =
case elem(pos, 1) do
0 -> 0
n -> (n / abs(n)) |> round()
end
{x, y}
end
end
input
|> Part1.head_positions()
|> Part1.follow([{0, 0}])
|> Enum.uniq()
|> Enum.count()
```
### Result
`5883`
<!-- livebook:{"break_markdown":true} -->
### Part 2
Simulate your complete series of motions on a larger rope with ten knots. How many positions does the tail of the rope visit at least once?
```elixir
# Derp
input
|> Part1.head_positions()
|> Part1.follow([{0, 0}])
|> Enum.reverse()
|> Part1.follow([{0, 0}])
|> Enum.reverse()
|> Part1.follow([{0, 0}])
|> Enum.reverse()
|> Part1.follow([{0, 0}])
|> Enum.reverse()
|> Part1.follow([{0, 0}])
|> Enum.reverse()
|> Part1.follow([{0, 0}])
|> Enum.reverse()
|> Part1.follow([{0, 0}])
|> Enum.reverse()
|> Part1.follow([{0, 0}])
|> Enum.reverse()
|> Part1.follow([{0, 0}])
|> Enum.uniq()
|> Enum.count()
```
### Result
`2367`
