Solution 2018-05 (Alchemical Reduction)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2018/day05/AlchemicalReduction.kt

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+package de.ronny_h.aoc.year2018.day05
+
+import de.ronny_h.aoc.AdventOfCode
+
+fun main() = AlchemicalReduction().run(9900, 4992)
+
+class AlchemicalReduction : AdventOfCode<Int>(2018, 5) {
+    override fun part1(input: List<String>): Int = input.first().react().length
+
+    override fun part2(input: List<String>): Int {
+        val polymer = input.first()
+        return "abcdefghijklmnopqrstuvwxyz".minOf {
+            polymer.react(it).length
+        }
+    }
+}
+
+fun String.react(toRemove: Char = ' '): String {
+    val toRemoveUpperCase = toRemove.uppercaseChar()
+    val toSkip = List(length) { i ->
+        get(i) == toRemove || get(i) == toRemoveUpperCase
+    }.toMutableList()
+    var somethingNewIsSkipped: Boolean
+
+    do {
+        somethingNewIsSkipped = false
+        var index = 0
+        while (index < lastIndex) {
+            if (toSkip[index]) {
+                index++
+                continue
+            }
+            val nextIndex = nextNonSkippedIndex(index, toSkip)
+            if (nextIndex > lastIndex) {
+                break
+            }
+            if (get(index) == get(nextIndex).oppositeCase()) {
+                toSkip[index] = true
+                toSkip[nextIndex] = true
+                index = nextIndex + 1
+                somethingNewIsSkipped = true
+            } else {
+                index++
+            }
+        }
+    } while (somethingNewIsSkipped)
+    return filterIndexed { i, _ -> !toSkip[i] }
+}
+
+private fun String.nextNonSkippedIndex(index: Int, toSkip: List<Boolean>): Int {
+    var nextIndex = index + 1
+    while (nextIndex < length && toSkip[nextIndex]) {
+        nextIndex++
+    }
+    return nextIndex
+}
+
+private fun Char.oppositeCase() = if (isLowerCase()) uppercaseChar() else lowercaseChar()

src/test/kotlin/de/ronny_h/aoc/year2018/day05/AlchemicalReductionTest.kt

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+package de.ronny_h.aoc.year2018.day05
+
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.data.forAll
+import io.kotest.data.row
+import io.kotest.matchers.shouldBe
+
+class AlchemicalReductionTest : StringSpec({
+
+    "simple reactions" {
+        forAll(
+            row("aA", ""),
+            row("abBA", ""),
+            row("abBAc", "c"),
+            row("cabBA", "c"),
+            row("abAB", "abAB"),
+            row("aabAAB", "aabAAB"),
+            row("dabAcCaCBAcCcaDA", "dabCBAcaDA"),
+        ) { chain, result ->
+            chain.react() shouldBe result
+        }
+    }
+
+    "part 1: the number of units after all possible reactions" {
+        val input = listOf("dabAcCaCBAcCcaDA")
+        AlchemicalReduction().part1(input) shouldBe 10
+    }
+
+    "part 2: the number of units after all possible reactions with the most problem-causing unit removed" {
+        val input = listOf("dabAcCaCBAcCcaDA")
+        AlchemicalReduction().part2(input) shouldBe 4
+    }
+})