Day 4 solution

src/main/kotlin/net/rafaeltoledo/kotlin/advent/Day04.kt

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+package net.rafaeltoledo.kotlin.advent
+
+import kotlin.math.pow
+
+class Day04 {
+
+  fun invoke(input: List<String>): Int {
+    val cards = input.map { it.toScratchCard() }
+
+    return cards
+      .map { card -> card.numbers.filter { card.winningNumbers.contains(it) } }
+      .sumOf { it.size.calculateCardValue() }
+  }
+
+  fun invoke2(input: List<String>): Int {
+    val cards = input.map { it.toScratchCard() }
+
+    val cardCounter = cards.associate { it.identifier to 1 }.toMutableMap()
+
+    cards
+      .forEachIndexed { index, card ->
+        val id = index + 1
+        val multiplier = cardCounter[id] ?: 1
+        val extras = card.numbers.filter { card.winningNumbers.contains(it) }.count()
+
+        for (i in 0 until extras) {
+          val targetId = id + (i + 1)
+          cardCounter[targetId]?.let { current ->
+            cardCounter[targetId] = current + multiplier
+          }
+        }
+      }
+
+    return cardCounter.toList().sumOf { it.second }
+  }
+}
+
+private fun Int.calculateCardValue(): Int {
+  return 2.0.pow((this - 1).toDouble()).toInt()
+}
+
+private fun String.toScratchCard(): ScratchCard {
+  val parts = split(":")
+
+  val identifier = parts.first().replace("Card", "").trim().toInt()
+
+  val content = parts.last().split("|")
+  val winningNumbers =
+    content.first().split(" ").filter { it.isNotEmpty() }.map { it.trim().toInt() }
+  val numbers = content.last().split(" ").filter { it.isNotEmpty() }.map { it.trim().toInt() }
+
+  return ScratchCard(identifier, numbers, winningNumbers)
+}
+
+data class ScratchCard(
+  val identifier: Int,
+  val numbers: List<Int>,
+  val winningNumbers: List<Int>,
+)

src/test/kotlin/net/rafaeltoledo/kotlin/advent/Day04Test.kt

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+package net.rafaeltoledo.kotlin.advent
+
+import com.google.common.truth.Truth
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+
+class Day04Test : BaseTest() {
+
+  override val day: String = "04"
+
+  @Test
+  fun `Sample data`() {
+    val input = readInput("sample")
+
+    val output = Day04().invoke(input)
+    assertThat(output).isEqualTo(13)
+  }
+
+  @Test
+  fun `Sample data increment`() {
+    val input = readInput("sample")
+
+    val output = Day04().invoke2(input)
+    assertThat(output).isEqualTo(30)
+  }
+
+  @Test
+  fun `Real data`() {
+    val input = readInput()
+
+    val output = Day04().invoke(input)
+    assertThat(output).isEqualTo(26914)
+  }
+
+  @Test
+  fun `Real data increment`() {
+    val input = readInput()
+
+    val output = Day04().invoke2(input)
+    assertThat(output).isEqualTo(13080971)
+  }
+}

src/test/resources/04.sample.txt

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+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11