Create Counter class

2021/src/main/kotlin/Counter.kt

@@ -0,0 +1,29 @@
+package be.vandewalleh.aoc.days
+
+class Counter<K> : Iterable<Map.Entry<K, Long>> {
+    private val map = HashMap<K, Long>()
+    operator fun get(key: K) = map[key] ?: 0L
+
+    operator fun set(key: K, count: Long) {
+        map[key] = count
+    }
+
+    operator fun set(key: K, count: Int) {
+        if (count == 0)
+            map.remove(key)
+        else
+            map[key] = count.toLong()
+    }
+
+    fun toMap() = map.toMap(HashMap())
+    override fun toString() = map.toString()
+
+    fun minValue() = map.minOf { it.value }
+    fun maxValue() = map.maxOf { it.value }
+
+    override fun iterator() = toMap().iterator()
+
+    companion object
+}
+
+operator fun <T> Counter.Companion.invoke(items: Iterable<T>) = Counter<T>().apply { items.forEach { this[it] += 1 } }

2021/src/main/kotlin/Day14.kt

@@ -12,25 +12,20 @@ class Day14 : BaseDay() {
         val pairs = input.lines.value.drop(2)
             .associate { it.split(" -> ").let { (a, b) -> a to (a[0] + b to b + a[1]) } }
 
-        fun <T, K> Grouping<T, K>.toCounter() = eachCount().mapValues { it.value.toLong() }.toMutableMap()
-        fun <K> MutableMap<K, Long>.inc(key: K, count: Long) = compute(key) { _, old -> (old ?: 0) + count }
-        fun <K> MutableMap<K, Long>.dec(key: K, count: Long) = compute(key) { _, old -> (old ?: 0) - count }
+        val polymer = Counter(template.windowed(2)) // {NN=1, NC=1, CB=1}
+        val counts = Counter(template.toCharArray().toList()) // {N=2, C=1, ...}
 
-        // {NN=1, NC=1, CB=1}
-        val polymer = template.windowed(2).groupingBy { it }.toCounter()
-
-        // {N=2, C=1, ...}
-        val counts = template.groupingBy { it }.toCounter()
         repeat(step) {
-            polymer.toMap().forEach { (pair, value) ->
+            polymer.forEach { (pair, value) ->
                 val (left, right) = pairs[pair]!!
-                polymer.dec(pair, value)
-                polymer.inc(left, value)
-                polymer.inc(right, value)
-                counts.inc(left[1], value)
+                polymer[pair] -= value
+                polymer[left] += value
+                polymer[right] += value
+                counts[left[1]] += value
             }
         }
-        return counts.maxOf { it.value } - counts.minOf { it.value }
+
+        return counts.maxValue() - counts.minValue()
     }
 
     override fun part1() = run(10)