day(03): add solution

Signed-off-by: Matej Focko <mfocko@redhat.com>

src/Day03.kt

@@ -0,0 +1,59 @@
+fun main() {
+    fun toBits(input: List<String>): List<List<Int>> {
+        return input.map { row ->
+            row.map { if (it == '1') 1 else 0 }
+        }
+    }
+
+    fun part1(input: List<List<Int>>): Int {
+        var gamma = 0
+        var epsilon = 0
+
+        for (i in input[0].indices) {
+            gamma = gamma.shl(1)
+            epsilon = epsilon.shl(1)
+
+            val ones = input.indices.map { input[it][i] }.sum()
+
+            if (2 * ones > input.size) {
+                gamma++
+            } else {
+                epsilon++
+            }
+        }
+
+        return epsilon * gamma
+    }
+
+    fun part2(input: List<List<Int>>): Int {
+        fun part2Helper(predicate: (Int, Int) -> Boolean): Int {
+            var wanted = input.indices.toList()
+            var i = 0
+
+            while (wanted.size > 1) {
+                val ones = wanted.sumOf { input[it][i] }
+                val wantedBit = if (predicate(ones, wanted.size)) 1 else 0
+
+                wanted = wanted.filter { input[it][i] == wantedBit }.toList()
+                i++
+            }
+
+            return input[wanted[0]].fold(0) { n, d -> 2 * n + d }
+        }
+
+        val oxygenGeneratorRating = part2Helper { ones, length -> 2 * ones >= length }
+        val co2ScrubberRating = part2Helper { ones, length -> 2 * ones < length }
+
+        return oxygenGeneratorRating * co2ScrubberRating
+    }
+
+
+    val testInput = toBits(readInput("Day03_test"))
+    val input = toBits(readInput("Day03"))
+
+    check(part1(testInput) == 198)
+    println(part1(input))
+
+    check(part2(testInput) == 230)
+    println(part2(input))
+}

src/Day03_test.txt

@@ -0,0 +1,12 @@
+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010