add 2023/day4 solution

Author: libhide

src/main/kotlin/advent2023/Day04.kt

@@ -0,0 +1,73 @@
+package advent2023
+
+import kotlin.math.pow
+
+private const val CARD = "Card"
+private const val COLON_SEPARATOR = ":"
+
+class Day04(private val input: List<String>) {
+
+    fun solvePart1(): Int {
+        return input.sumOf { card ->
+            val matchCount = findMatchCountForCard(card)
+            2.0.pow(matchCount - 1).toInt()
+        }
+    }
+
+    fun solvePart2(): Int {
+        val cardToTotalCount = mutableMapOf<Int, Int>()
+
+        val cardToMatchCount = input.map { card ->
+            val cardNumber = card.substringAfter(CARD)
+                .substringBefore(COLON_SEPARATOR)
+                .trim()
+                .toInt()
+                .also {
+                    // init `cardToTotalCount` with each card occurring once
+                    cardToTotalCount[it] = 1
+                }
+            cardNumber to findMatchCountForCard(card)
+        }
+
+        cardToMatchCount.forEach { (cardNumber, matchCount) ->
+            repeat(matchCount) {
+                cardToTotalCount[cardNumber + it + 1] =
+                    checkNotNull(cardToTotalCount[cardNumber + it + 1]) + checkNotNull(cardToTotalCount[cardNumber])
+            }
+        }
+
+        return cardToTotalCount.map { it.value }.sum()
+    }
+
+    private fun findMatchCountForCard(cardInfo: String): Int {
+        val numbers = cardInfo.split(":").last().split("|")
+
+        val winningNumbers = numbers
+            .first()
+            .split(" ")
+            .filter { it.isNotEmpty() }
+            .map { it.trim() }
+            .map { it.toInt() }
+        val cardsInHand = numbers
+            .last()
+            .split(" ")
+            .filter { it.isNotEmpty() }
+            .map { it.trim() }
+            .map { it.toInt() }
+
+        var count = 0
+        cardsInHand.forEach { number ->
+            if (number in winningNumbers) {
+                count += 1
+            }
+        }
+        return count
+    }
+}
+
+fun main() {
+    val input = Resources.resourceAsListOfString("advent2023/day04.txt")
+    val day04 = Day04(input = input)
+    println("Part 1 Solution: ${day04.solvePart1()}")
+    println("Part 2 Solution: ${day04.solvePart2()}")
+}