day(09): add solution

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

src/year2021/day09/Day09.kt

@@ -0,0 +1,73 @@
+package year2021.day09
+
+import readInput
+import kotlin.math.max
+
+fun getRiskLevel(input: List<List<Int>>, y: Int, x: Int): Int = listOf(
+    Pair(y - 1, x),
+    Pair(y + 1, x),
+    Pair(y, x + 1),
+    Pair(y, x - 1)
+).filter { (y0, x0) -> y0 >= 0 && y0 < input.size && x0 >= 0 && x0 < input[y0].size }
+    .map { (y0, x0) -> input[y0][x0] }
+    .all { it > input[y][x] }.let { isLowPoint -> if (isLowPoint) input[y][x] + 1 else 0 }
+
+fun part1(input: List<List<Int>>): Int = input
+    .mapIndexed { y, row ->
+        row.mapIndexed { x, col ->
+            getRiskLevel(input, y, x)
+        }
+    }.sumOf { row -> row.sum() }
+
+fun dfs(input: MutableList<MutableList<Int>>, y: Int, x: Int): Int {
+    if (input[y][x] != 0) {
+        return 0
+    }
+
+    input[y][x] = -1
+
+    var total = 1
+    listOf(
+        Pair(y - 1, x),
+        Pair(y + 1, x),
+        Pair(y, x + 1),
+        Pair(y, x - 1)
+    ).filter { (y0, x0) -> y0 >= 0 && y0 < input.size && x0 >= 0 && x0 < input[y0].size }
+        .forEach { (y0, x0) -> total += dfs(input, y0, x0) }
+
+    return total;
+}
+
+fun part2(input: List<List<Int>>): Int {
+    val heatmap = input.map { row ->
+        row.map {
+            max(it - 8, 0)
+        }.toMutableList()
+    }.toMutableList()
+
+    val sizes = mutableListOf<Int>()
+    for (i in 0 until heatmap.size) {
+        for (j in 0 until heatmap[i].size) {
+            if (heatmap[i][j] != 0) {
+                continue
+            }
+            sizes.add(dfs(heatmap, i, j))
+        }
+    }
+
+    sizes.sortDescending()
+    println(sizes)
+
+    return sizes.take(3).fold(1) { product, n -> n * product }
+}
+
+fun main() {
+    val sample = readInput(9, "sample").map { row -> row.map { it.digitToInt() } }
+    val input = readInput(9, "input").map { row -> row.map { it.digitToInt() } }
+
+    check(part1(sample) == 15)
+    println(part1(input))
+
+    check(part2(sample) == 1134)
+    println(part2(input))
+}

src/year2021/day09/sample.txt

@@ -0,0 +1,5 @@
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678