day6.md

Day 6: Custom Customs

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Part 1

Problem

Given a list of input strings separated by blank lines, determine the unique characters in that grouping of the input. The answer is the sum of the size of each group.

Solution

Since I previously solved the issue of separating input by blank lines, I opted to re-use my fold solution from part 4, this time doing so as an extension function.

This left the simple matter of converting each group into a set (which de-duplicates), finding the size of each set, and summing the total:

    File("src/main/resources/day6_input.txt").readLines()
            .foldToBlankLineSeparated()
            .map { group -> group.filter { it.isLetter() } }
            .map { it.toSet().size }
            .sum()
            .apply { println(this) }

Part 2

Problem

Same as above, but now instead of unique characters in the input we want to know which characters are the same between lines in the same group.

Solution

The operation we want here is an intersection, which returns the elements which are shared between two sets. For read-ability I opted to create an extension function to handle the intersection of a list of strings.

fun List<String>.intersectAll(): Set<Char> =
        this.map { it.toSet() }.reduce { acc, element -> acc intersect element }

Using this, the mapping and aggregate functions can be updated as such:

    File("src/main/resources/day6_input.txt").readLines()
            .foldToBlankLineSeparated("\n")
            .map { group -> group.split("\n").intersectAll() }
            .map { it.toSet().size }
            .sum()
            .apply { println(this) }