49 lines
1.2 KiB
Kotlin
49 lines
1.2 KiB
Kotlin
package be.vandewalleh.aoc.days
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import be.vandewalleh.aoc.utils.input.Day
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import be.vandewalleh.aoc.utils.input.Lines
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import kotlin.math.abs
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private data class Bus(val index: Int, val id: Long)
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@Day(13)
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class Day13(@Lines val input: List<String>) {
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fun part1(): Int {
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val id = input[0].toInt()
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val (busId, min) = input[1]
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.splitToSequence(",")
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.filterNot { it == "x" }
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.map { it.toInt() }
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.map { bus -> bus to id / bus }
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.map { (bus, div) -> bus to bus * (div + 1) }
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.minByOrNull { it.second }!!
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return busId * abs(min - id)
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}
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private tailrec fun gcd(a: Long, b: Long): Long = if (b == 0L) a else gcd(b, a % b)
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private fun lcm(a: Long, b: Long): Long = a / gcd(a, b) * b
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fun part2(): Long {
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val buses = input[1]
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.splitToSequence(",")
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.mapIndexedNotNull { index, bus ->
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if (bus == "x") null
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else Bus(index, bus.toLong())
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}
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.toList()
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var step = 1L
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var t = 0L
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for ((i, id) in buses) {
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while ((t + i) % id != 0L) t += step
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step = lcm(step, id)
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}
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return t
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}
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}
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