day(14): add solution

Signed-off-by: Matej Focko <mfocko@redhat.com>

src/year2021/day14/Day14.kt

@@ -0,0 +1,89 @@
+package year2021.day14
+
+import readInput
+
+typealias PolymerPair = String
+typealias PolymerRules = Map<PolymerPair, Char>
+typealias PolymerTemplate = Map<PolymerPair, Long>
+
+data class Polymer(
+    val rules: PolymerRules,
+    val template: MutableMap<Char, Long>,
+    val pairsInTemplate: MutableMap<PolymerPair, Long>
+) {
+    constructor(rules: PolymerRules, template: String) : this(rules, template.associate {
+        it to template.count { it2 -> it == it2 }.toLong()
+    }.toMutableMap(), template.toPolymerTemplate().toMutableMap())
+
+    fun step() {
+        val addedPairs = mutableMapOf<PolymerPair, Long>()
+
+        for ((pair, count) in pairsInTemplate) {
+            if (!rules.contains(pair) || count == 0.toLong()) {
+                continue
+            }
+
+            val addedElement = rules[pair]!!
+            val leftPair = pair[0].toString() + addedElement
+            val rightPair = addedElement + pair[1].toString()
+
+            addedPairs[leftPair] = (addedPairs[leftPair] ?: 0) + count
+            addedPairs[rightPair] = (addedPairs[rightPair] ?: 0) + count
+            addedPairs[pair] = (addedPairs[pair] ?: 0) - count
+
+            // update frequency counter
+            template[addedElement] = (template[addedElement] ?: 0) + count
+        }
+
+        for ((pair, change) in addedPairs) {
+            pairsInTemplate[pair] = (pairsInTemplate[pair] ?: 0) + change
+        }
+    }
+
+    val elements: Map<Char, Long>
+        get() = template
+}
+
+fun List<String>.toPolymerRules(): PolymerRules = this.associate {
+    val (polymerPair, insertedElement) = it.split(" -> ")
+    polymerPair to insertedElement[0]
+}
+
+fun String.toPolymerTemplate(): PolymerTemplate =
+    this.windowed(2)
+        .fold(mapOf()) { template, it ->
+            template + (it to template.getOrDefault(it, 0) + 1)
+        }
+
+fun part1(input: List<String>): Long {
+    val polymer = Polymer(input.drop(2).toPolymerRules(), input.first())
+
+    repeat(10) {
+        polymer.step()
+    }
+
+    val elements = polymer.elements
+    return elements.maxOf { it.value } - elements.minOf { it.value }
+}
+
+fun part2(input: List<String>): Long {
+    val polymer = Polymer(input.drop(2).toPolymerRules(), input.first())
+
+    repeat(40) {
+        polymer.step()
+    }
+
+    val elements = polymer.elements
+    return elements.maxOf { it.value } - elements.minOf { it.value }
+}
+
+fun main() {
+    val sample = readInput(14, "sample")
+    val input = readInput(14, "input")
+
+    check(part1(sample) == 1588.toLong())
+    println(part1(input))
+
+    check(part2(sample) == 2188189693529)
+    println(part2(input))
+}

src/year2021/day14/sample.txt

@@ -0,0 +1,18 @@
+NNCB
+
+CH -> B
+HH -> N
+CB -> H
+NH -> C
+HB -> C
+HC -> B
+HN -> C
+NN -> C
+BH -> H
+NC -> B
+NB -> B
+BN -> B
+BB -> N
+BC -> B
+CC -> N
+CN -> C