Day 9: Mirage Maintenance

[andreiz]

https://adventofcode.com/2023/day/9

Input: https://adventofcode.com/2023/day/9/input

[andreiz]

First thing we need to do is calculate arrays of successive differences until all the numbers in the array are the same (or, equivalently, we reach an array where the first element is 0).

I asked ChatGPT to write me a function that determines whether all the elements in the array are the same and it gave me this:

func allElementsAreEqual<T: Equatable>(_ array: [T]) -> Bool {
    guard let firstElement = array.first else { return true } // Empty array is considered to have all equal elements
    return array.allSatisfy { $0 == firstElement }
}

So now I know how to templatize a function.

[andreiz]

Then we just iterate over the input array of numbers, computing differences and appending the resulting array to the result, until allElementsAreEqual gives us the stopping condition.

func calculateDifferences(_ input: [Int]) -> [[Int]] {
    var result = [input]
    var currentSequence = input

    while true {
        let newSequence = zip(currentSequence, currentSequence.dropFirst()).map { $1 - $0 }
        result.append(newSequence)
        if allElementsAreEqual(newSequence) {
            break
        }
        currentSequence = newSequence
    }
    return result
}

[andreiz]

Once we have this, part 1 is trivial. Taking extended arrays from the puzzle text:

10  13  16  21  30  45  *68*
   3   3   5   9  15  *23*
     0   2   4   6  *8*
       2   2   2  *2*

We can see that 8 is equal to 2+6, then 23 is equal to 2+6+15, then 68 is 2+6+15+45. So we just need to sum the last values of the array.

[andreiz]

Part 2 is almost as trivial.

*5*  10  13  16  21  30  45
  *5*   3   3   5   9  15
   *-2*   0   2   4   6
      *2*   2   2   2

Starting with 2, we calculate the previous line as 0 - 2 = -2, then the one before is 3 - (-2) = 5, then 10 - 5 = 5.

[andreiz]

Translated into code:

var sumLast = 0
var sumFirst = 0
for line in input {
    let differences = calculateDifferences(line)
    let lastValue = differences.reduce(0) { current, sequence in
        current + (sequence.last ?? 0)
    }
    let firstValue = differences.reversed().reduce(0) { current, sequence in
        -current + (sequence.first ?? 0)
    }
    print("lastValue: \(lastValue)")
    print("firstValue: \(firstValue)")
    sumLast += lastValue
    sumFirst += firstValue
}

[andreiz]

This passes with the test and actual inputs.