Day 8: Matchsticks

Mix.install([
  {:kino, "~> 0.12.3"}
])

Input

input = Kino.Input.textarea("Please paste your puzzle input:")

Part 1

defmodule Part1 do
  @doc ~S"""
  Decodes string by interpreting all escape sequences
  of the inner string.

  ## Example
  ```elixir
  iex> Part1.decode(~S("\x27"))
  "'"
  ```
  """
  def decode(str) do
    str
    |> String.slice(1..-2)
    |> Macro.unescape_string()
  end
end

We split on newlines to gather all of the individual strings. When processing with Part1.decode/1 we use the built in Macro.unescape_string/1 to perform the neccessary string interpolation. We take the difference of each string and then sum the results to derive our answer.

input
|> Kino.Input.read()
|> String.split("\n")
|> Stream.map(&(String.length(&1) - String.length(Part1.decode(&1))))
|> Enum.sum()

Part 2

defmodule Part2 do
  @doc ~S"""
  Encodes a string into a textual representation where all 
  escape sequences and surrounding quotes are printed literally

  ## Example
  ```elixir
  iex> Part2.encode(~S("\x27"))
  ~S("\"\\x27\"")
  ```
  """
  def encode(str) do
    str
    |> String.replace("\\", "\\\\")
    |> String.replace("\"", "\\\"")
    |> then(&amp;("\"" <> &amp;1 <> "\""))
  end
end

For part 2 we need to convert the string the other way. We would like to print each backslash character, so any escape character with a backslash needs to be replaced to accommodate this and the new quotes attached. Once this is done it is the same process as part 1.

input
|> Kino.Input.read()
|> String.split("\n")
|> Enum.map(&(String.length(Part2.encode(&1)) - String.length(&1)))
|> Enum.sum()