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Day14OneTimePad.kt
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Day14OneTimePad.kt
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package adventofcode.year2016
import adventofcode.Puzzle
import adventofcode.PuzzleInput
import adventofcode.common.String.md5
class Day14OneTimePad(customInput: PuzzleInput? = null) : Puzzle(customInput) {
override val name = "One-Time Pad"
override fun partOne() = generateSequence(0, Int::inc)
.filter { index -> isKey(input, index) }
.take(64)
.last()
companion object {
private val cache = mutableMapOf<String, String>()
private fun String.hash() = cache.getOrPut(this) { md5() }
private fun String.findFirstTriplet() = this
.windowed(3)
.find { triplet -> triplet.toSet().count() == 1 }
?.first()
private fun isKey(salt: String, index: Int): Boolean {
val hash = "$salt$index".hash()
val triplet = hash.findFirstTriplet() ?: return false
return ((index + 1 until index + 1000).any { "$salt$it".hash().contains(triplet.toString().repeat(5)) })
}
}
}