Add solution to day 4

src/Day04.kt

@@ -0,0 +1,55 @@
+import kotlin.math.pow
+
+fun main() {
+    fun part1(input: List<String>): Int {
+        var matchValue = 0
+        input.forEach { card ->
+            val winningNumbers = card.winningNumbers
+            val matches = card.myNumbers.sumOf { number ->
+                (if (winningNumbers.contains(number)) 1 else 0).toInt()
+            }
+            matchValue += when {
+                matches > 1 -> 2.0.pow(matches - 1).toInt()
+                else -> matches
+            }
+        }
+        return matchValue
+    }
+
+    fun part2(input: List<String>): Int {
+        val cardCounts = MutableList(input.size) { 1 }
+        val matches = MutableList(input.size) { 0 }
+        input.forEachIndexed { index, card ->
+            val winningNumbers = card.winningNumbers
+            val matchCount = card.myNumbers.sumOf { number ->
+                (if (winningNumbers.contains(number)) 1 else 0).toInt()
+            }
+            matches[index] = matchCount * cardCounts[index]
+            for (i in index + 1..index + matchCount) {
+                cardCounts[i] = cardCounts[i] + (1 * cardCounts[index])
+            }
+        }
+        return cardCounts.subList(0, input.size).sum()
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = readInput("Day04_test")
+    check(part1(testInput) == 13)
+
+    val input = readInput("Day04")
+    part1(input).println()
+    check(part2(testInput) == 30)
+    part2(input).println()
+}
+
+val String.winningNumbers: Set<Int>
+    get() = substringAfter(':')
+        .substringBefore('|')
+        .split(' ')
+        .mapNotNull { it.toIntOrNull() }
+        .toSet()
+
+val String.myNumbers: List<Int>
+    get() = substringAfter('|')
+        .split(' ')
+        .mapNotNull { it.toIntOrNull() }