1 files changed, 51 insertions, 0 deletions
diff --git a/aoc-2021-kotlin/src/Day03.kt b/aoc-2021-kotlin/src/Day03.kt
new file mode 100644
index 0000000..1418a2e
--- /dev/null
+++ b/aoc-2021-kotlin/src/Day03.kt
@@ -0,0 +1,51 @@
+object Day03 {
+    private fun addLists(l1: List<Int>, l2: List<Int>) = l1.zip(l2).map { it.first + it.second }
+
+    private fun valueOfBits(bitList: List<Int>) = bitList.reduce { acc, bit -> acc * 2 + bit }
+
+    private fun invertBits(bitList: List<Int>) = bitList.map { bit -> 1 - bit }
+
+    private fun mostCommonBits(input: List<List<Int>>): List<Int> =
+        input
+            .reduce(::addLists)
+            .map { bit -> if (bit.toDouble() / input.size >= 0.5) 1 else 0 }
+
+    private fun selectByBitCriteria(
+        input: List<List<Int>>,
+        comparisonCriteria: (currentList: List<List<Int>>) -> List<Int>
+    ): List<Int>? {
+        var list = input
+        var index = 0
+
+        while (list.size > 1 && index < list[0].size) {
+            list = list.filter { e -> e[index] == comparisonCriteria(list)[index] }
+            index += 1
+        }
+
+        return list.getOrNull(0)
+    }
+
+    fun part1(input: List<List<Int>>): Int =
+        mostCommonBits(input)
+            .let { gammaBits ->
+                val epsilonBits = invertBits(gammaBits)
+                return valueOfBits(gammaBits) * valueOfBits(epsilonBits)
+            }
+
+    fun part2(input: List<List<Int>>): Int {
+        val oxygenRating = selectByBitCriteria(input) { mostCommonBits(it) }!!
+        val scrubberRating = selectByBitCriteria(input) { invertBits(mostCommonBits(it)) }!!
+
+        return valueOfBits(oxygenRating) * valueOfBits(scrubberRating)
+    }
+}
+
+fun main() {
+    val testInput = readInputAsBitLists("Day03_test")
+    check(Day03.part1(testInput) == 198)
+    check(Day03.part2(testInput) == 230)
+
+    val input = readInputAsBitLists("Day03")
+    println(Day03.part1(input))
+    println(Day03.part2(input))
+}