day1.md

Day 1: Report Repair

Part 1

Problem

Find two numbers from the list of input data that add up to 2020. Return the product of these numbers.

Solution

I simply use a stream to iterate over the values, comparing each to the remaining values until I find one which satisfies the condition.

val firstResult: Int? = entries.firstOrNull {
        entries.minusElement(it).contains(total - it)
}

Part 2

Problem

Find three numbers that add up to 2020 and return the product.

Solution

Though I could have simplified this to always look for exactly 3 numbers, I opted to generalize it to run for any number of values desired. This solution replaces my part 1 code completely, as the same code works in both cases. In theory, I could run this for 4 values that sum to 2020, though no solution exists in that case.

It works by iteratively doing a breadth-wise expansion of options, filtering out impossibilities as it goes. There are likely faster ways to traverse the tree, but this works well enough for the given dataset.

fun expandExpenseValueCandidatesSelections(total: Int): List<ExpenseValueCandidateTracker> {
    return this.remainingCandidates
        .filter { it >= selectedCandidates.maxOrNull()?:0 }
        .filter { selectedCandidates.sum() + it <= total }
        .run {
            this.map {
                ExpenseValueCandidateTracker(selectedCandidates+it, this.minus(it))
            }
        }
}