Discussion on: AoC Day 14: Chocolate Charts

[Jon Bristow]

Kotlin Solution

Part 1

Another tailrec solution for the books. I had some issue with how long this took. After a few frustrating minutes, I figured out that .takeLast() was not as efficient as the .get() or array access notation.

private fun answer1(input: Int): Any? {
    val elf1 = 0
    val elf2 = 1
    val recipes = mutableListOf(3, 7)
    return findRecipes(2, elf1, elf2, recipes, input)
}

private tailrec fun findRecipes(
    n: Int,
    elf1: Int,
    elf2: Int,
    recipes: MutableList<Int>,
    limit: Int
): String {
    if (n > limit + 10) {
        return recipes.drop(limit).take(10).joinToString("")
    }
    val re1 = recipes[elf1]
    val re2 = recipes[elf2]
    val newRecipes = when {
        re1 + re2 > 9 -> mutableListOf(1, (re1 + re2) % 10)
        else -> mutableListOf((re1 + re2) % 10)
    }
    val nextN = n + newRecipes.count()

    recipes.addAll(newRecipes)
    return findRecipes(
        nextN,
        (elf1 + re1 + 1) % nextN,
        (elf2 + re2 + 1) % nextN,
        recipes,
        limit
    )
}

Part 2

This one threw me, since the solution was so much farther out than I expected...

private fun answer2(input: Int): Any? {
  val elf1 = 0
  val elf2 = 1
  val recipes = mutableListOf(3, 7)
  return findRecipes2( 2, elf1, elf2, recipes, input.toString().map { it.toDigit() })
}
private tailrec fun findRecipes2(
    n: Int,
    elf1: Int,
    elf2: Int,
    recipes: MutableList<Int>,
    check: List<Int>
): Int {
    val ccount = check.count()
    if ((n > ccount) && (n - ccount until n).map { recipes[it] } == check) {
        return n - ccount
    } else if ((n > ccount) && (n - ccount until n).map { recipes[it - 1] } == check) {
        return n - ccount - 1
    }

    val re1 = recipes[elf1]
    val re2 = recipes[elf2]

    val newRecipes = when {
        re1 + re2 > 9 -> mutableListOf(1, (re1 + re2) % 10)
        else -> mutableListOf((re1 + re2) % 10)
    }
    val nextN = n + newRecipes.count()
    recipes.addAll(newRecipes)
    return findRecipes2(
        nextN,
        (elf1 + re1 + 1) % nextN,
        (elf2 + re2 + 1) % nextN,
        recipes,
        check
    )
}