Solution Day18, part one - small

With the A Star algorithm of Day 16 (ReindeerMaze), the small example runs
in reasonable time. But the full input takes very long. Since the grid is
filled only very sparsely, there are probably too many options for the
shortest path. We should not calculate all of them.

Author: rhaendel

src/main/kotlin/Day18.kt

@@ -0,0 +1,74 @@
+import de.ronny_h.extensions.Coordinates
+import de.ronny_h.extensions.Direction
+import de.ronny_h.extensions.Direction.EAST
+import de.ronny_h.extensions.Direction.NORTH
+import de.ronny_h.extensions.Direction.SOUTH
+import de.ronny_h.extensions.Direction.WEST
+import de.ronny_h.extensions.Grid
+import de.ronny_h.extensions.ShortestPath
+import de.ronny_h.extensions.aStar
+
+fun main() {
+    val day = "Day18"
+
+    fun part1(input: List<String>, width: Int): Int {
+        val memorySpace = MemorySpace(width, input.map {
+            val (x, y) = it.split(',')
+            Coordinates(x.toInt(), y.toInt())
+        })
+        memorySpace.printGrid()
+        println("-----------------")
+        val shortestPaths = memorySpace.shortestPath(Coordinates(0,0), Coordinates(width-1, width-1))
+        memorySpace.printGrid(path = shortestPaths.first().path.associateWith { 'O' })
+        return shortestPaths.first().distance
+    }
+
+    fun part1Small(input: List<String>) = part1(input, 7) // 0..6
+    fun part1Big(input: List<String>) = part1(input, 71)  // 0..70
+
+    fun part2(input: List<String>): Int {
+        return input.size
+    }
+
+    println("$day part 1")
+
+    val testInput = readInput("${day}_test")
+    printAndCheck(testInput.subList(0, 12), ::part1Small, 22)
+
+    val input = readInput(day)
+    printAndCheck(input.subList(0, 1024), ::part1Big, 3714264)
+
+
+    println("$day part 2")
+
+    printAndCheck(
+        listOf(
+            "1   6"
+        ), ::part2, 1
+    )
+    printAndCheck(testInput, ::part2, 31)
+    printAndCheck(input, ::part2, 18805872)
+}
+
+private class MemorySpace(width: Int, corrupted: List<Coordinates>) : Grid<Char>(width, width, '.', '#', corrupted) {
+    private val corrupted = '#'
+    override fun Char.toElementType() = this
+
+    fun shortestPath(start: Coordinates, goal: Coordinates): List<ShortestPath<Coordinates>> {
+        val neighbours: (Coordinates) -> List<Coordinates> = { node ->
+            Direction
+                .entries
+                .map { node + it }
+                .filter { getAt(node) != corrupted }
+        }
+
+        val d: (Coordinates, Coordinates) -> Int = { a, b ->
+            // pre-condition: a and b are neighbours
+            1
+        }
+
+        val h: (Coordinates) -> Int = { it taxiDistanceTo goal}
+
+        return aStar(start, goal::equals, neighbours, d, h)
+    }
+}