Day13

days/src/main/kotlin/Day13.kt

@@ -0,0 +1,55 @@
+package be.vandewalleh.aoc.days
+
+import be.vandewalleh.aoc.utils.input.Day
+import be.vandewalleh.aoc.utils.input.Input
+import be.vandewalleh.aoc.utils.input.Lines
+import be.vandewalleh.aoc.utils.input.createDay
+import kotlin.math.abs
+
+data class Bus(val index: Int, val id: Long)
+
+@Day(13)
+class Day13(@Lines val input: Input<List<String>>) {
+
+    fun part1(): Int {
+        val id = input.value[0].toInt()
+
+        val (busId, min) = input.value[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.value[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
+    }
+}
+
+fun main() = with(createDay<Day13>()) {
+    println(part1())
+    println(part2())
+}

days/src/main/resources/day13.txt

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+1000511
+29,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,409,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,17,13,19,x,x,x,23,x,x,x,x,x,x,x,353,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

days/src/test/kotlin/Day13Test.kt

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+package be.vandewalleh.aoc.days
+
+import be.vandewalleh.aoc.utils.input.Input
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.CsvSource
+
+class Day13Test {
+
+    @CsvSource(
+        value = [
+            "           7,13 | 77",
+            "     17,x,13,19 | 3417",
+            "     67,7,59,61 | 754018",
+            "   67,x,7,59,61 | 779210",
+            "   67,7,x,59,61 | 1261476",
+            "1789,37,47,1889 | 1202161486",
+        ],
+        delimiter = '|'
+    )
+    @ParameterizedTest
+    fun examples(buses: String, answer: Long) {
+        val input = Input(listOf("", buses))
+        assertThat(Day13(input).part2()).isEqualTo(answer)
+    }
+
+}