day 13

build.gradle.kts

@@ -12,6 +12,7 @@ repositories {
 }
 
 dependencies {
+    implementation("com.github.h0tk3y.betterParse:better-parse:0.4.4")
     testImplementation(kotlin("test"))
 }
 

src/main/kotlin/me/gergo/Aoc13.kt

@@ -0,0 +1,83 @@
+package me.gergo
+
+import com.github.h0tk3y.betterParse.combinators.*
+import com.github.h0tk3y.betterParse.grammar.Grammar
+import com.github.h0tk3y.betterParse.grammar.parseToEnd
+import com.github.h0tk3y.betterParse.grammar.parser
+import com.github.h0tk3y.betterParse.lexer.literalToken
+import com.github.h0tk3y.betterParse.lexer.regexToken
+import com.github.h0tk3y.betterParse.parser.Parser
+import java.io.File
+
+fun main() {
+    val packetPairs = File("src/main/resources/input13.txt").readText().split("\n\n")
+        .map(::parsePacketPair)
+
+    // Part One
+    val result1 = packetPairs
+        .mapIndexed { i, p -> if (p.first < p.second) i + 1 else 0 }
+        .sum()
+    println("Sum of packet indices in right order: $result1")
+
+    // Part Two
+    val divider1 = Packet(listOf(listOf(2)))
+    val divider2 = Packet(listOf(listOf(6)))
+    val result2 = File("src/main/resources/input13.txt").readLines()
+        .asSequence()
+        .filter { it.isNotBlank() }
+        .map(::parsePacket)
+        .plus(divider1)
+        .plus(divider2)
+        .sorted()
+        .mapIndexed { i, p -> if (p == divider1 || p == divider2) i + 1 else 0 }
+        .filter { it != 0 }
+        .reduce(Int::times)
+    println("Decoder key: $result2")
+}
+
+private data class Packet(val value: List<Any>) : Comparable<Packet> {
+    override fun compareTo(other: Packet) = compareLists(value, other.value)
+}
+
+private fun compareLists(a: List<*>, b: List<*>): Int {
+    for (i in a.indices) {
+        if (i == b.size) return 1 // Right side ran out of items => left is "larger"
+        val cmp = compareValues(a[i], b[i])
+        if (cmp != 0) return cmp // One of the items was larger or smaller
+    }
+    if (a.size < b.size) return -1 // Left side ran out of items => right is "larger"
+    return 0 // Equally large and items were equal, too
+}
+
+private fun compareValues(a: Any?, b: Any?): Int {
+    return if (a is Int && b is Int) {
+        a.compareTo(b)
+    } else if (a is List<*> && b is List<*>) {
+        compareLists(a, b)
+    } else if (a is Int) {
+        compareLists(listOf(a), b as List<*>)
+    } else {
+        compareLists(a as List<*>, listOf(b))
+    }
+}
+
+private fun parsePacketPair(packets: String): Pair<Packet, Packet> {
+    val (p1, p2) = packets.split("\n")
+    return Pair(parsePacket(p1), parsePacket(p2))
+}
+
+private fun parsePacket(line: String) = packetGrammar.parseToEnd(line)
+
+private val packetGrammar = object : Grammar<Packet>() {
+    val comma by literalToken(",")
+    val openingBracket by literalToken("[")
+    val closingBracket by literalToken("]")
+    val integer by regexToken("\\d+")
+    val whiteSpace by regexToken("\\s+", true)
+
+    val integerParser = integer map { it.text.toInt() }
+    val listParser = (-openingBracket and separated(parser(this::value), comma, true) and -closingBracket)
+        .map { it.terms }
+    val value: Parser<Any> = integerParser or listParser
+    override val rootParser = listParser map ::Packet
+}