Day 9

Author: yev-kanivets

src/Day09.kt

@@ -0,0 +1,82 @@
+import java.util.*
+
+fun main() {
+
+    data class Point(
+        val row: Int,
+        val column: Int,
+        val value: Int
+    )
+
+    fun List<String>.parse() = map { row -> row.map { it.digitToInt() } }
+
+    fun List<List<Int>>.getOrMaxInt(
+        rowIndex: Int,
+        columnIndex: Int
+    ) = getOrElse(rowIndex) { emptyList() }.getOrElse(columnIndex) { Int.MAX_VALUE }
+
+    fun List<List<Int>>.getOrMinInt(
+        rowIndex: Int,
+        columnIndex: Int
+    ) = getOrElse(rowIndex) { emptyList() }.getOrElse(columnIndex) { Int.MIN_VALUE }.takeUnless { it == 9 }
+        ?: Int.MIN_VALUE
+
+    fun List<List<Int>>.findLowestPoints() = mapIndexed { rowIndex, row ->
+        row.mapIndexedNotNull { columnIndex, value ->
+            if (value < getOrMaxInt(rowIndex - 1, columnIndex)
+                && value < getOrMaxInt(rowIndex + 1, columnIndex)
+                && value < getOrMaxInt(rowIndex, columnIndex - 1)
+                && value < getOrMaxInt(rowIndex, columnIndex + 1)
+            ) {
+                Point(rowIndex, columnIndex, value)
+            } else null
+        }
+    }.flatten()
+
+    fun List<List<Int>>.findBasin(lowestPoint: Point): List<Point> {
+        val visited = Array(size) { BooleanArray(first().size) { false } }
+        val basin = mutableListOf<Point>()
+
+        val queue = LinkedList<Point>()
+        queue.add(lowestPoint)
+        basin.add(lowestPoint)
+        visited[lowestPoint.row][lowestPoint.column] = true
+
+        while (queue.isNotEmpty()) {
+            val point = queue.removeFirst()
+
+            listOf(
+                Point(point.row - 1, point.column, getOrMinInt(point.row - 1, point.column)),
+                Point(point.row + 1, point.column, getOrMinInt(point.row + 1, point.column)),
+                Point(point.row, point.column - 1, getOrMinInt(point.row, point.column - 1)),
+                Point(point.row, point.column + 1, getOrMinInt(point.row, point.column + 1))
+            )
+                .filter { point.value < getOrMinInt(it.row, it.column) && !visited[it.row][it.column] }
+                .forEach {
+                    queue.add(it)
+                    basin.add(it)
+                    visited[it.row][it.column] = true
+                }
+        }
+
+        return basin
+    }
+
+    fun part1(
+        input: List<String>
+    ) = input.parse().let { array -> array.findLowestPoints().sumOf { it.value + 1 } }
+
+    fun part2(
+        input: List<String>
+    ) = input.parse().let { array ->
+        array.findLowestPoints()
+            .map { array.findBasin(it).count() }
+            .sortedDescending()
+            .take(3)
+            .reduce { acc, value -> acc * value }
+    }
+
+    val input = readInput("Day09")
+    println(part1(input))
+    println(part2(input))
+}