1 files changed, 91 insertions, 0 deletions
diff --git a/aoc-2021-kotlin/src/Day10.kt b/aoc-2021-kotlin/src/Day10.kt
new file mode 100644
index 0000000..d6d026c
--- /dev/null
+++ b/aoc-2021-kotlin/src/Day10.kt
@@ -0,0 +1,91 @@
+object Day10 {
+    private enum class ChunkDelimiter(val open: Char, val close: Char) {
+        Parentheses('(', ')'),
+        Brackets('[', ']'),
+        Braces('{', '}'),
+        Angled('<', '>');
+
+        companion object {
+            fun isOpening(character: Char): Boolean = values().any { it.open == character }
+            fun isClosing(character: Char): Boolean = values().any { it.close == character }
+            fun from(character: Char): ChunkDelimiter =
+                values().find { it.open == character || it.close == character }!!
+        }
+    }
+
+    private sealed class SyntaxError {
+        object None : SyntaxError()
+        class IncompleteLine(private val stack: ArrayDeque<ChunkDelimiter>) : SyntaxError() {
+            val score: Long
+                get() {
+                    fun delimiterValue(delimiter: ChunkDelimiter): Int = when (delimiter) {
+                        ChunkDelimiter.Parentheses -> 1
+                        ChunkDelimiter.Brackets -> 2
+                        ChunkDelimiter.Braces -> 3
+                        ChunkDelimiter.Angled -> 4
+                    }
+
+                    return stack.fold(0) { acc, elem -> acc * 5 + delimiterValue(elem) }
+                }
+        }
+
+        class CorruptedLine(private val invalidDelimiter: ChunkDelimiter) : SyntaxError() {
+            val score: Int
+                get() = when (invalidDelimiter) {
+                    ChunkDelimiter.Parentheses -> 3
+                    ChunkDelimiter.Brackets -> 57
+                    ChunkDelimiter.Braces -> 1197
+                    ChunkDelimiter.Angled -> 25137
+                }
+        }
+    }
+
+    private fun parse(line: String): SyntaxError {
+        val stack = ArrayDeque<ChunkDelimiter>()
+
+        for (char in line) {
+            when {
+                ChunkDelimiter.isOpening(char) -> stack.addFirst(ChunkDelimiter.from(char))
+                ChunkDelimiter.isClosing(char) -> {
+                    val closingDelimiter = ChunkDelimiter.from(char)
+                    if (stack.first() == closingDelimiter) {
+                        stack.removeFirst()
+                    } else {
+                        return SyntaxError.CorruptedLine(closingDelimiter)
+                    }
+                }
+            }
+        }
+
+        if (stack.isNotEmpty()) {
+            return SyntaxError.IncompleteLine(stack)
+        }
+
+        return SyntaxError.None
+    }
+
+    fun part1(input: List<String>): Int = input
+        .map { parse(it) }
+        .filterIsInstance<SyntaxError.CorruptedLine>()
+        .sumOf { it.score }
+
+    fun part2(input: List<String>): Long =
+        input
+            .asSequence()
+            .map { parse(it) }
+            .filterIsInstance<SyntaxError.IncompleteLine>()
+            .map { it.score }
+            .sorted()
+            .toList()
+            .let { it[it.size / 2] }
+}
+
+fun main() {
+    val testInput = readInputAsLines("Day10_test")
+    check(Day10.part1(testInput) == 26397)
+    check(Day10.part2(testInput) == 288957L)
+
+    val input = readInputAsLines("Day10")
+    println(Day10.part1(input))
+    println(Day10.part2(input))
+}