Day10AdapterArray.kt

package adventofcode.year2020

import adventofcode.Puzzle
import adventofcode.PuzzleInput
import adventofcode.common.product

class Day10AdapterArray(customInput: PuzzleInput? = null) : Puzzle(customInput) {
    private val sortedInput by lazy { input.lines().map(String::toInt).sorted() }

    override fun partOne() =
        (sortedInput + listOf(sortedInput.last() + 3))
            .foldIndexed(emptyMap<Int, Int>()) { index, acc, elem ->
                val previous = sortedInput.getOrElse(index - 1) { 0 }
                acc + mapOf(elem - previous to (acc[elem - previous] ?: 0) + 1)
            }
            .values
            .product()

    override fun partTwo(): Long {
        val k = longArrayOf(1, 0, 0, 0)

        (sortedInput + listOf(sortedInput.last() + 3))
            .fold(0) { a, b ->
                val d = b - a
                k.copyInto(k, d, 0, k.size - d)
                k.fill(0, 0, d)
                k[0] += k.sum()
                b
            }

        return k[0]
    }
}