package eu.janvdb.aoc2020.day13

import eu.janvdb.aocutil.kotlin.ChineseRemainderTheoremEquation
import eu.janvdb.aocutil.kotlin.ModuloMath
import java.math.BigInteger

//const val TIME_ = 939L
//const val BUSSES = "7,13,x,x,59,x,31,19"

const val TIME = 1000052L
const val BUSSES =
	"23,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,863,x,x,x,x,x,x,x,x,x,x,x,19,13,x,x,x,17,x,x,x,x,x,x,x,x,x,x,x,29,x,571,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,41"

fun main() {
	part1()
	part2()
}

private fun part1() {
	val busses = BUSSES.split(",").filter { it != "x" }.map(String::toInt)
	val earliestStartTimesPerBus = busses.map { Pair(it, (TIME + it - 1) / it * it) }
	val earliestBus = earliestStartTimesPerBus.minByOrNull { it.second }!!

	println(earliestBus.first * (earliestBus.second - TIME))
}

private fun part2() {
	val startTimes = BUSSES.split(",")
		.mapIndexed { index, value -> Pair(index, value) }
		.filter { it.second != "x" }
		.map { StartTime(it.second.toLong().toBigInteger(), it.first.toLong().toBigInteger()) }


	// Equations: (t + 0) mod 7 = 0, (t + 1) mod 13 = 0, (t + 4) mod 59 = 0, (t + 6) mod 31 = 0, (t + 7) mod 19 = 0
	// Or: t = 0 (mod 7), t = 12 (mod 13), t = 54 (mod 59), t = 25 (mod 31), t = 12 (mod 19)
	// Since the values are pairwise coprime, this can be solved with the Chinese Remainder Theorem.
	val equations = startTimes.map(StartTime::asChineseTheoremEquation)
	val solution = ModuloMath.solveChineseRemainderTheorem(equations)
	println(solution)
}

data class StartTime(val busNumber: BigInteger, val startTime: BigInteger) {
	fun asChineseTheoremEquation() : ChineseRemainderTheoremEquation {
		return ChineseRemainderTheoremEquation(ModuloMath.minusModN(busNumber, startTime, busNumber), busNumber)
	}
}
