Day 4: Camp Cleanup

Mix.install([
  {:benchee, "~> 1.1"},
  {:benchee_html, "~> 1.0"}
])

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Story

Space needs to be cleared before the last supplies can be unloaded from the ships, and so several Elves have been assigned the job of cleaning up sections of the camp. Every section has a unique ID number, and each Elf is assigned a range of section IDs.

However, as some of the Elves compare their section assignments with each other, they’ve noticed that many of the assignments overlap. To try to quickly find overlaps and reduce duplicated effort, the Elves pair up and make a big list of the section assignments for each pair (your puzzle input).

For example, consider the following list of section assignment pairs:

2-4,6-8
2-3,4-5
5-7,7-9
2-8,3-7
6-6,4-6
2-6,4-8

For the first few pairs, this list means:

• Within the first pair of Elves, the first Elf was assigned sections 2-4 (sections 2, 3, and 4), while the second Elf was assigned sections 6-8 (sections 6, 7, 8).
• The Elves in the second pair were each assigned two sections.
• The Elves in the third pair were each assigned three sections: one got sections 5, 6, and 7, while the other also got 7, plus 8 and 9.

This example list uses single-digit section IDs to make it easier to draw; your actual list might contain larger numbers. Visually, these pairs of section assignments look like this:

.234.....  2-4
.....678.  6-8

.23......  2-3
...45....  4-5

....567..  5-7
......789  7-9

.2345678.  2-8
..34567..  3-7

.....6...  6-6
...456...  4-6

.23456...  2-6
...45678.  4-8

Some of the pairs have noticed that one of their assignments fully contains the other. For example, 2-8 fully contains 3-7, and 6-6 is fully contained by 4-6. In pairs where one assignment fully contains the other, one Elf in the pair would be exclusively cleaning sections their partner will already be cleaning, so these seem like the most in need of reconsideration. In this example, there are 2 such pairs.

In how many assignment pairs does one range fully contain the other?

defmodule Advent1 do
  def solve do
    File.stream!("#{File.cwd!()}/input/4.txt")
    |> Stream.map(&String.trim/1)
    |> Stream.map(fn x -> String.split(x, ",") end)
    |> Stream.map(&split/1)
    |> Stream.map(&in_range/1)
    |> Enum.sum()
  end

  def split([left, right]) do
    (String.split(left, "-") ++ String.split(right, "-"))
    |> Stream.map(&String.to_integer/1)
    |> Enum.to_list()
  end

  def in_range([a, b, x, y]) when a <= x and b >= y, do: 1
  def in_range([a, b, x, y]) when x <= a and y >= b, do: 1
  def in_range(_), do: 0
end

Advent1.solve()

560

Part Two

It seems like there is still quite a bit of duplicate work planned. Instead, the Elves would like to know the number of pairs that overlap at all.

In the above example, the first two pairs (2-4,6-8 and 2-3,4-5) don’t overlap, while the remaining four pairs (5-7,7-9, 2-8,3-7, 6-6,4-6, and 2-6,4-8) do overlap:

• 5-7,7-9 overlaps in a single section, 7.
• 2-8,3-7 overlaps all of the sections 3 through 7.
• 6-6,4-6 overlaps in a single section, 6.
• 2-6,4-8 overlaps in sections 4, 5, and 6.

So, in this example, the number of overlapping assignment pairs is 4.

In how many assignment pairs do the ranges overlap?

defmodule Advent2 do
  def solve() do
    File.stream!("#{File.cwd!()}/input/4.txt")
    |> Stream.map(&String.trim/1)
    |> Stream.map(fn x -> String.split(x, ",") end)
    |> Stream.map(&split/1)
    |> Stream.map(&overlap?/1)
    |> Stream.filter(fn x -> x end)
    |> Enum.to_list()
    |> length()
  end

  def split([left, right]) do
    (String.split(left, "-") ++ String.split(right, "-"))
    |> Stream.map(&String.to_integer/1)
    |> Enum.to_list()
  end

  def overlap?([a, b, x, y]) do
    between?(a, x, b) or
      between?(a, y, b) or
      between?(x, a, y) or
      between?(x, b, y)
  end

  def between?(l, m, r) do
    l <= m and m <= r
  end

  def bool_to_int(true), do: 1
  def bool_to_int(_), do: 0
end

Advent2.solve()

839

Benchmark

Benchee.run(%{
  "Part 1" => &Advent1.solve/0,
  "Part 2" => &Advent2.solve/0
})

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Operating System: Linux
CPU Information: Intel(R) Core(TM) i7-5500U CPU @ 2.40GHz
Number of Available Cores: 4
Available memory: 7.69 GB
Elixir 1.14.0
Erlang 25.1.2

Benchmark suite executing with the following configuration:
warmup: 2 s
time: 5 s
memory time: 0 ns
reduction time: 0 ns
parallel: 1
inputs: none specified
Estimated total run time: 14 s

Benchmarking Part 1 ...
Benchmarking Part 2 ...

Name             ips        average  deviation         median         99th %
Part 1        284.59        3.51 ms    ±23.62%        3.16 ms        6.47 ms
Part 2        275.95        3.62 ms    ±25.04%        3.23 ms        6.60 ms

Comparison: 
Part 1        284.59
Part 2        275.95 - 1.03x slower +0.110 ms

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