package be.vandewalleh.aoc.days

import be.vandewalleh.aoc.utils.input.Day
import be.vandewalleh.aoc.utils.input.Lines
import kotlin.math.abs

private data class Bus(val index: Int, val id: Long)

@Day(13)
class Day13(@Lines val input: List<String>) {

    fun part1(): Int {
        val id = input[0].toInt()

        val (busId, min) = input[1]
            .splitToSequence(",")
            .filterNot { it == "x" }
            .map { it.toInt() }
            .map { bus -> bus to id / bus }
            .map { (bus, div) -> bus to bus * (div + 1) }
            .minByOrNull { it.second }!!

        return busId * abs(min - id)
    }

    private tailrec fun gcd(a: Long, b: Long): Long = if (b == 0L) a else gcd(b, a % b)
    private fun lcm(a: Long, b: Long): Long = a / gcd(a, b) * b

    fun part2(): Long {
        val buses = input[1]
            .splitToSequence(",")
            .mapIndexedNotNull { index, bus ->
                if (bus == "x") null
                else Bus(index, bus.toLong())
            }
            .toList()

        var step = 1L
        var t = 0L

        for ((i, id) in buses) {
            while ((t + i) % id != 0L) t += step
            step = lcm(step, id)
        }

        return t
    }
}