2021 Day3

class BinaryDiagnostic(data: List<String>) {
    /* Part 1 */
    private val gamma = commonBit(data, '1', '0').toInt(2)
    private val epsilon = commonBit(data, '0', '1').toInt(2)
    val powerConsumption = gamma * epsilon

    private fun commonBit(data: List<String>, most: Char, least: Char) =
        countBits(data).fold("")
        { result, count -> result + (if (count > data.size / 2) most else least) }

    private fun countBits(data: List<String>) =
        data.fold(Array(data[0].length) { 0 }) { counts, binaryString ->
            (0 until binaryString.length).forEach {
                counts[it] += binaryString[it].toString().toInt()
            }
            counts
        }

    /* Part 2 */
    private val co2ScrubberRating = filterCommonBit(data, 0, '1', '0').toInt(2)
    private val oxygenGeneratorRating = filterCommonBit(data, 0, '0', '1').toInt(2)
    val lifeSupportRating = co2ScrubberRating * oxygenGeneratorRating

    private fun filterCommonBit(data: List<String>, i: Int, least: Char, most: Char): String {
        if (data.size == 1) return data[0]
        val numberOfOnes = data.count { binaryString -> binaryString[i] == '1' }
        val toKeep = if (numberOfOnes > (data.size - 1) / 2) most else least
        val filteredData = data.filter { binaryString -> binaryString[i] == toKeep }
        return filterCommonBit(filteredData, i + 1, least, most)
    }
}