day(11): add solution

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

src/year2021/day11/Day11.kt

@@ -0,0 +1,88 @@
+package year2021.day11
+
+import readInput
+import kotlin.collections.ArrayDeque
+
+fun validAdjacentIndices(input: List<List<Int>>, y0: Int, x0: Int): Iterable<Pair<Int, Int>> =
+    (y0 - 1..y0 + 1)
+        .flatMap { y -> (x0 - 1..x0 + 1).map { x -> Pair(y, x) } }
+        .filter { (y, x) ->
+            (y != y0 || x != x0) && (y >= 0 && y < input.size) && (x >= 0 && x < input[y].size)
+        }
+
+fun getFlashes(octopuses: MutableList<MutableList<Int>>): List<Pair<Int, Int>> =
+    (octopuses.indices)
+        .flatMap { y -> octopuses[y].indices.map { x -> Pair(y, x) } }
+        .filter { (y, x) -> octopuses[y][x] > 9 }
+        .toList()
+
+fun step(octopuses: MutableList<MutableList<Int>>): Int {
+    // First, the energy level of each octopus increases by 1.
+    for (y in octopuses.indices) {
+        for (x in octopuses.indices) {
+            octopuses[y][x]++
+        }
+    }
+
+    // Then, any octopus with an energy level greater than 9 flashes.
+    val queue = ArrayDeque(getFlashes(octopuses))
+    while (queue.isNotEmpty()) {
+        val (y, x) = queue.removeFirst()
+
+        for ((y_1, x_1) in validAdjacentIndices(octopuses, y, x)) {
+            octopuses[y_1][x_1]++
+
+            // == 10 means we made it flash
+            if (octopuses[y_1][x_1] == 10) {
+                queue.addLast(Pair(y_1, x_1))
+            }
+        }
+    }
+
+    // Count the flashing octopuses.
+    val flashes = octopuses.sumOf { row -> row.count { it > 9 } }
+
+    // Reset energy level.
+    for (y in octopuses.indices) {
+        for (x in octopuses.indices) {
+            if (octopuses[y][x] > 9) {
+                octopuses[y][x] = 0
+            }
+        }
+    }
+
+    return flashes
+}
+
+fun part1(input: List<List<Int>>): Int {
+    var flashes = 0
+    val octopuses = input.map { row -> row.toMutableList() }.toMutableList()
+
+    repeat(100) {
+        flashes += step(octopuses)
+    }
+
+    return flashes
+}
+
+fun part2(input: List<List<Int>>): Int {
+    var i = 1
+    val octopuses = input.map { row -> row.toMutableList() }.toMutableList()
+    val count = octopuses.size * octopuses[0].size
+
+    while (step(octopuses) != count) {
+        i++
+    }
+    return i
+}
+
+fun main() {
+    val sample = readInput(11, "sample").map { row -> row.map { it.digitToInt() } }
+    val input = readInput(11, "input").map { row -> row.map { it.digitToInt() } }
+
+    check(part1(sample) == 1656)
+    println(part1(input))
+
+    check(part2(sample) == 195)
+    println(part2(input))
+}

src/year2021/day11/sample.txt

@@ -0,0 +1,10 @@
+5483143223
+2745854711
+5264556173
+6141336146
+6357385478
+4167524645
+2176841721
+6882881134
+4846848554
+5283751526