day(15): add solution

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

src/year2021/day15/Day15.kt

@@ -0,0 +1,126 @@
+package year2021.day15
+
+import readInput
+import java.util.*
+
+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 == y0 || x == x0) && (y >= 0 && y < input.size) && (x >= 0 && x < input[y].size)
+        }
+
+fun validAdjacentIndicesExtended(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 == y0 || x == x0) && (y >= 0 && y < 5 * input.size) && (x >= 0 && x < 5 * input[0].size)
+        }
+
+data class Coordinate(val y: Int, val x: Int)
+
+fun part1(input: List<List<Int>>): Int {
+    val final = Coordinate(input.size - 1, input.last().size - 1)
+
+    val queue = PriorityQueue<Pair<Int, Coordinate>>() { a, b ->
+        a.first - b.first
+    }
+    val distances = mutableMapOf<Coordinate, Int>()
+    val processed = mutableSetOf<Coordinate>()
+
+    queue.add(Pair(0, Coordinate(0, 0)))
+    distances[Coordinate(0, 0)] = 0
+    for (y in input.indices) {
+        for (x in input[y].indices) {
+            queue.add(Pair(Int.MAX_VALUE, Coordinate(y, x)))
+        }
+    }
+
+    while (queue.isNotEmpty()) {
+        val (distance, coordinate) = queue.remove()!!
+        if (processed.contains(coordinate)) {
+            continue
+        }
+        processed.add(coordinate)
+
+        if (coordinate == final) {
+            return distance
+        }
+
+        for ((y, x) in validAdjacentIndices(input, coordinate.y, coordinate.x)) {
+            val neighbour = Coordinate(y, x)
+            val alternative = distance + input[y][x]
+
+            if (alternative < (distances[neighbour] ?: Int.MAX_VALUE)) {
+                distances[neighbour] = alternative
+                queue.add(Pair(alternative, neighbour))
+            }
+        }
+    }
+    return Int.MAX_VALUE
+}
+
+fun getDistance(input: List<List<Int>>, y: Int, x: Int): Int {
+    val riskLevel = input[y % input.size][x % input[0].size]
+    val added = y / input.size + x / input[0].size
+
+    if (riskLevel + added > 9) {
+        return riskLevel + added - 9
+    }
+
+    return riskLevel + added
+}
+
+fun part2(input: List<List<Int>>): Int {
+    val final = Coordinate(5 * input.size - 1, 5 * input.last().size - 1)
+
+    val queue = PriorityQueue<Pair<Int, Coordinate>>() { a, b ->
+        a.first - b.first
+    }
+    val distances = mutableMapOf<Coordinate, Int>()
+    val processed = mutableSetOf<Coordinate>()
+
+    queue.add(Pair(0, Coordinate(0, 0)))
+    distances[Coordinate(0, 0)] = 0
+    for (y in input.indices) {
+        for (x in input[y].indices) {
+            for (i in 0 until 5) {
+                queue.add(Pair(Int.MAX_VALUE, Coordinate(i * y, i * x)))
+            }
+        }
+    }
+
+    while (queue.isNotEmpty()) {
+        val (distance, coordinate) = queue.remove()!!
+        if (processed.contains(coordinate)) {
+            continue
+        }
+        processed.add(coordinate)
+
+        if (coordinate == final) {
+            return distance
+        }
+
+        for ((y, x) in validAdjacentIndicesExtended(input, coordinate.y, coordinate.x)) {
+            val neighbour = Coordinate(y, x)
+            val alternative = distance + getDistance(input, y, x)
+
+            if (alternative < (distances[neighbour] ?: Int.MAX_VALUE)) {
+                distances[neighbour] = alternative
+                queue.add(Pair(alternative, neighbour))
+            }
+        }
+    }
+    return Int.MAX_VALUE
+}
+
+fun main() {
+    val sample = readInput(15, "sample").map { row -> row.map { it.digitToInt() } }
+    val input = readInput(15, "input").map { row -> row.map { it.digitToInt() } }
+
+    check(part1(sample) == 40)
+    println(part1(input))
+
+    check(part2(sample) == 315)
+    println(part2(input))
+}

src/year2021/day15/sample.txt

@@ -0,0 +1,10 @@
+1163751742
+1381373672
+2136511328
+3694931569
+7463417111
+1319128137
+1359912421
+3125421639
+1293138521
+2311944581