Day 2: Gift Shop

Megathread guidelines

  • Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
  • You can send code in code blocks by using three backticks, the code, and then three backticks or use something such as https://topaz.github.io/paste/ if you prefer sending it through a URL

FAQ

  • eco_game@discuss.tchncs.de
    2·
    4 days ago

    Kotlin

    got up early for this one, sadly it took me over 30 minutes to realize, that my code at the time was also considering a single digit valid .-.

    also still pretty scuffed, but hey it works:

    Solution 
    class Day02 : Puzzle {

    val ids = mutableSetOf<String>()

    override fun readFile() {
        val input = readInputFromFile("src/main/resources/a2025/day02.txt")
        ids.addAll(
            input.replace("\n", "")
                .split(",")
                .map { it.split("-") }
                .map(this::buildList)
                .flatten()
        )
    }

    private fun buildList(rangeList: List<String>): List<String> {
        val start = rangeList[0].toLong()
        val end = rangeList[1].toLong()
        val ids = mutableListOf<String>()
        for (i in start..end) {
            ids.add(i.toString())
        }
        return ids
    }

    override fun solvePartOne(): String {
        return ids.filter(this::idNotValid)
            .sumOf(String::toLong).toString()
    }

    override fun solvePartTwo(): String {
        return ids.filter { idNotValid(it, true) }
            .sumOf(String::toLong).toString()
    }

    private fun idNotValid(id: String, multipleSplits: Boolean = false): Boolean {
        val length = id.length

        // try all splits
        var split = 2
        while (split <= length) {
            if (length % split == 0) {
                val splits = mutableListOf<String>()
                var beg = 0
                var end = length / split
                val step = end
                for (i in 0..<split) {
                    splits.add(id.substring(beg, end))
                    beg += step
                    end += step
                }
                if (splits.all { it == splits[0] }) return true
            }
            if (multipleSplits) {
                split++
            } else {
                break
            }
        }
        return false
    }
}

    full code on Codeberg

    • chunkystyles@sopuli.xyzEnglish
      2·
      3 days ago

      My part 2 Kotlin solution:

      val factors = intArrayOf(2, 3, 5, 7)

fun main() {
    var total = 0L
    val input = getInput(2)
    val ranges = parseInput1(input)
    ranges.forEach {
        val start = it.first.toLong()
        val end = it.second.toLong()
        for (id in start..end) {
            val idString = id.toString()
            if (isIdInvalid2(idString)) {
                total += id
            }
        }
    }
    println(total)
}

fun parseInput1(input: String): List<Pair<String, String>> {
    return input.split(",")
        .filter { it.isNotBlank() }
        .map {
            val secondSplit = it.split("-")
            secondSplit.first().trim() to secondSplit.last().trim()
        }
}

fun isIdInvalid2(id: String): Boolean {
    for (factor in factors) {
        if (id.length % factor == 0) {
            val gap = id.length / factor
            var areAllMatching = true
            for (i in 0..<gap) {
                val end = (factor - 1) * gap + i
                if (!areCharactersTheSame(id, gap, i, end)) {
                    areAllMatching = false
                    break
                }
            }
            if (areAllMatching) {
                return true
            }
        }
    }
    return false
}

fun areCharactersTheSame(string: String, gap: Int, start: Int, end: Int): Boolean {
    val character = string[start]
    for (i in start + gap..end step gap) {
        if (character != string[i]) {
            return false
        }
    }
    return true
}

      I didn’t look closely enough at the input to know how big an entire list of IDs would be huge or not. But I assumed it was. So instead I just did ranges as Pairs.

      I also only considered the prime factors up to 7, because there weren’t any IDs that needed any higher.

      Edit: I also worried that a brute force solution might be slow, but being day 2, I might have been a little too worried about that. The main algorithm ran in 68ms.