Part 1

Task

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?

Solution

• Implement a function that checks if IntRange is fully contained inside another IntRange.

• Break each line into 2 IntRanges.

• Count all lines that contain 1 range that is fully contained inside the other.
  

fun IntRange.containedIn(other: IntRange) =
    this.first >= other.first && this.last <= other.last

fun toIntRange(range: String): IntRange {
    val (start, end) = range.split("-")
    return start.toInt()..end.toInt()
}

fun solvePart1(): Int =
    input.filter { it.isNotBlank() }
        .map {
            val (firstElf, secondElf) = it.split(",")
            toIntRange(firstElf) to toIntRange(secondElf)
        }
        .count { it.first.containedIn(it.second) || it.second.containedIn(it.first) }

That's it, and now for our test cases:

@Test
fun `Part 1 - Example`() {
    val day04 = Day04(exampleInput)
    assertEquals(2, day04.solvePart1())
}

@Test
fun `Part 1 - Real Input`() {
    val day04 = Day04(resourceAsList("2022/day04.txt"))
    assertEquals(556, day04.solvePart1())
}

Part 2

Task

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?

Solution

There is only 1 change that we need to make in this task. We need to check if any range includes at least 1 area from the other.

fun IntRange.anyContainedIn(other: IntRange) =
    this.any { other.contains(it) }

fun solvePart2(): Int =
    input.filter { it.isNotBlank() }
        .map {
            val (firstElf, secondElf) = it.split(",")
            toIntRange(firstElf) to toIntRange(secondElf)
        }
        .count { it.first.anyContainedIn(it.second) || it.second.anyContainedIn(it.first) }

That's it, the code is done! let's wrap it out with our test cases for part 2:

@Test
fun `Part 2 - Example`() {
    val day04 = Day04(exampleInput)
    assertEquals(4, day04.solvePart2())
}

@Test
fun `Part 2 - Real Input`() {
    val day04 = Day04(resourceAsList("2022/day04.txt"))
    assertEquals(876, day04.solvePart2())
}

All of the code that was presented in the article, along with the utility classes that are used are available in my GitHub account.

See you on the next challenge!