Day 8.

src/aoc2023/Day08.kt

@@ -0,0 +1,144 @@
+package aoc2023
+
+import println
+import readInput
+
+/*
+--- Day 8: Haunted Wasteland ---
+You're still riding a camel across Desert Island when you spot a sandstorm quickly approaching. When you turn to warn the Elf, she disappears before your eyes! To be fair, she had just finished warning you about ghosts a few minutes ago.
+One of the camel's pouches is labeled "maps" - sure enough, it's full of documents (your puzzle input) about how to navigate the desert. At least, you're pretty sure that's what they are; one of the documents contains a list of left/right instructions, and the rest of the documents seem to describe some kind of network of labeled nodes.
+It seems like you're meant to use the left/right instructions to navigate the network. Perhaps if you have the camel follow the same instructions, you can escape the haunted wasteland!
+After examining the maps for a bit, two nodes stick out: AAA and ZZZ. You feel like AAA is where you are now, and you have to follow the left/right instructions until you reach ZZZ.
+This format defines each node of the network individually. For example:
+RL
+
+AAA = (BBB, CCC)
+BBB = (DDD, EEE)
+CCC = (ZZZ, GGG)
+DDD = (DDD, DDD)
+EEE = (EEE, EEE)
+GGG = (GGG, GGG)
+ZZZ = (ZZZ, ZZZ)
+
+Starting with AAA, you need to look up the next element based on the next left/right instruction in your input. In this example, start with AAA and go right (R) by choosing the right element of AAA, CCC. Then, L means to choose the left element of CCC, ZZZ. By following the left/right instructions, you reach ZZZ in 2 steps.
+Of course, you might not find ZZZ right away. If you run out of left/right instructions, repeat the whole sequence of instructions as necessary: RL really means RLRLRLRLRLRLRLRL... and so on. For example, here is a situation that takes 6 steps to reach ZZZ:
+LLR
+
+AAA = (BBB, BBB)
+BBB = (AAA, ZZZ)
+ZZZ = (ZZZ, ZZZ)
+
+Starting at AAA, follow the left/right instructions. How many steps are required to reach ZZZ?
+
+*/
+fun main() {
+
+    val inlineTestInput = """
+RL
+
+AAA = (BBB, CCC)
+BBB = (DDD, EEE)
+CCC = (ZZZ, GGG)
+DDD = (DDD, DDD)
+EEE = (EEE, EEE)
+GGG = (GGG, GGG)
+ZZZ = (ZZZ, ZZZ)
+"""
+    val inlineTestInput2 = """
+LR
+
+ABA = (ABB, XXX)
+ABB = (XXX, ABZ)
+ABZ = (ABB, XXX)
+BAA = (BAB, XXX)
+BAB = (BAC, BAC)
+BAC = (BAZ, BAZ)
+BAZ = (BAB, BAB)
+XXX = (XXX, XXX)
+"""
+
+    fun part1(input: List<String>): Int {
+        val cmd = input[0]
+        val map = HashMap<String, Pair<String, String>>()
+        for (i in input.drop(2)) {
+            "([A-Z][A-Z][A-Z]) = .([A-Z][A-Z][A-Z]), ([A-Z][A-Z][A-Z]).".toRegex().matchEntire(i)!!.destructured
+                .let { (id, left, right) -> map[id] = left to right }
+        }
+        var instPos = 0
+        var steps = 0
+        var loc = "AAA"
+        do {
+            loc = if (cmd[instPos] == 'L') map[loc]!!.first else map[loc]!!.second
+            instPos = (instPos + 1) % cmd.length
+            steps++
+        } while (loc != "ZZZ")
+        return steps
+    }
+
+    fun part2(input: List<String>): Long {
+        val cmd = input[0]
+        val map = HashMap<String, Pair<String, String>>()
+        for (i in input.drop(2)) {
+            "([A-Z][A-Z][A-Z]) = .([A-Z][A-Z][A-Z]), ([A-Z][A-Z][A-Z]).".toRegex().matchEntire(i)!!.destructured
+                .let { (id, left, right) -> map[id] = left to right }
+        }
+        val starts = map.keys.filter { it.endsWith("A") }
+        var ggts = HashSet<Int>()
+        for (start in starts) {
+            var instPos = 0
+            var steps = 0
+            var loc = start
+            do {
+                loc = if (cmd[instPos] == 'L') map[loc]!!.first else map[loc]!!.second
+                instPos = (instPos + 1) % cmd.length
+                steps++
+            } while (!loc.endsWith("Z"))
+            ggts.add(steps)
+        }
+        val primes = ArrayList<Int>()
+        val factors = HashSet<Int>()
+        var prime = 2
+        primes.add(2)
+        do {
+            val newGgts = HashSet<Int>()
+            for (f in ggts) {
+                if (prime * 2 - 1 > f) {
+                    factors.add(f)
+                } else {
+                    if (f % prime == 0) {
+                        newGgts.add(f / prime)
+                        factors.add(prime)
+                    } else {
+                        newGgts.add(f)
+                    }
+                }
+            }
+            ggts = newGgts
+            if (prime == 2) {
+                prime = 3
+            } else {
+                do {
+                    prime += 2
+                } while (primes.any { (prime % it) == 0 })
+            }
+            primes.add(prime)
+        } while (ggts.isNotEmpty())
+
+        return factors.map { it.toLong() }.reduce(Long::times)
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    val testInput2 = inlineTestInput2.trim().reader().readLines()
+    //val testInput = readInput("aoc2023/Day08_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput2)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 2)
+    check(testInputPart2Result == 6L)
+
+    val input = readInput("aoc2023/Day08")
+    part1(input).println()
+    part2(input).println()
+}