day 12

src/main/kotlin/me/gergo/Aoc12.kt

@@ -0,0 +1,115 @@
+package me.gergo
+
+import java.io.File
+import kotlin.Double.Companion.POSITIVE_INFINITY
+import kotlin.math.abs
+
+fun main() {
+    val terrain = File("src/main/resources/input12.txt").readLines()
+        .map { it.toCharArray() }.toTypedArray()
+
+    val start = terrain.findAll('S').first()
+    val goal = terrain.findAll('E').first()
+
+    terrain[start] = 'a'
+    terrain[goal] = 'z'
+
+    fun distance(a: Coordinate, b: Coordinate): Double {
+        if (manhattanDistance(a, b) > 1) return POSITIVE_INFINITY // Can only move 1 step up, down, left or right
+        val valueA = terrain[a].code
+        val valueB = terrain[b].code
+        if (valueB - valueA > 1) return POSITIVE_INFINITY // Can only move at most one step up
+        return 1.0
+    }
+
+    // Part One
+    val result1 = aStar(
+        start, goal,
+        terrain::neighbors,
+        { c -> manhattanDistance(c, goal) },
+        ::distance
+    ).size - 1
+    println("Fewest steps required from S: $result1")
+
+    // Part Two
+
+    val result2 = terrain.findAll('a')
+        .map {
+            val path = aStar(it, goal, terrain::neighbors, { c -> manhattanDistance(c, goal) }, ::distance)
+            if (path.isEmpty()) Int.MAX_VALUE else path.size - 1
+        }
+        .min()
+    println("Fewest steps required from any 'a': $result2")
+}
+
+private fun Array<CharArray>.findAll(value: Char) = sequence<Coordinate> {
+    val matrix = this@findAll
+    for (y in matrix.indices) {
+        for (x in matrix[y].indices) {
+            if (matrix[y][x] == value) yield(Coordinate(x, y))
+        }
+    }
+}
+
+private fun Array<CharArray>.neighbors(c: Coordinate) = neighbors(this[0].size, this.size, c)
+
+private operator fun Array<CharArray>.get(c: Coordinate) = this[c.y][c.x]
+private operator fun Array<CharArray>.set(c: Coordinate, value: Char) {
+    this[c.y][c.x] = value
+}
+
+data class Coordinate(val x: Int, val y: Int)
+
+fun aStar(
+    start: Coordinate, goal: Coordinate,
+    neighborFn: (Coordinate) -> Sequence<Coordinate>,
+    heuristicFn: (Coordinate) -> Double,
+    distanceFn: (Coordinate, Coordinate) -> Double
+): List<Coordinate> {
+    val openSet = mutableSetOf(start)
+    val cameFrom = mutableMapOf<Coordinate, Coordinate>()
+
+    val gScore = mutableMapOf<Coordinate, Double>()
+    gScore[start] = 0.0
+
+    val fScore = mutableMapOf<Coordinate, Double>()
+    fScore[start] = heuristicFn(start)
+
+    while (openSet.isNotEmpty()) {
+        val current = openSet.minBy { fScore.getOrDefault(it, POSITIVE_INFINITY) }
+        if (current == goal) {
+            // Path found, reconstructing
+            val result = mutableListOf(current)
+            var c: Coordinate? = current
+            while (true) {
+                c = cameFrom[c]
+                if (c == null) break
+                result.add(0, c)
+            }
+            return result
+        }
+
+        openSet.remove(current)
+        for (neighbor in neighborFn(current)) {
+            val tentativeGScore = gScore.getOrDefault(current, POSITIVE_INFINITY) + distanceFn(current, neighbor)
+            if (tentativeGScore < gScore.getOrDefault(neighbor, POSITIVE_INFINITY)) {
+                cameFrom[neighbor] = current
+                gScore[neighbor] = tentativeGScore
+                fScore[neighbor] = tentativeGScore + heuristicFn(neighbor)
+                openSet.add(neighbor)
+            }
+        }
+    }
+    return emptyList() // Path not found
+}
+
+fun neighbors(width: Int, height: Int, c: Coordinate) = sequence {
+    val x = c.x
+    val y = c.y
+    if (x > 0) yield(Coordinate(x - 1, y))
+    if (y + 1 < height) yield(Coordinate(x, y + 1))
+    if (x + 1 < width) yield(Coordinate(x + 1, y))
+    if (y > 0) yield(Coordinate(x, y - 1))
+}
+
+fun manhattanDistance(a: Coordinate, b: Coordinate): Double = (abs(b.x - a.x) + abs(b.y - a.y)).toDouble()

src/main/resources/input12.txt

@@ -0,0 +1,41 @@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