Day03.kt

fun main() {

    fun List<String>.getBits(col: Int): Pair<Char, Char> {
        val ones = count { it[col] == '1' }
        return if (ones >= size - ones) '1' to '0' else '0' to '1'
    }

    fun List<String>.filterByBit(getBit: (Pair<Char, Char>) -> Char): Int {
        var list = this
        var c = -1
        while (list.size > 1) {
            val bits = list.getBits(++c)
            list = list.filter { it[c] == getBit(bits) }
        }
        return list[0].toInt(2)
    }

    fun part1(input: List<String>): Int {
        val (mcb, lcb) = input[0].indices.map { input.getBits(it) }.unzip()
        return mcb.joinToString("").toInt(2) * lcb.joinToString("").toInt(2)
    }

    fun part2(input: List<String>): Int {
        return input.filterByBit { it.first } * input.filterByBit { it.second }
    }

    val testInput = readInput("Day03_test")
    check(part1(testInput) == 198)
    check(part2(testInput) == 230)

    val input = readInput("Day03")
    println(part1(input))
    println(part2(input))
}