day(06): solve

Signed-off-by: Matej Focko <me@mfocko.xyz>

inputs/day06/sample.txt

@@ -0,0 +1,10 @@
+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...

src/Day06.kt

@@ -0,0 +1,95 @@
+class Day06(
+    inputType: String,
+) : Day<Int, Int>() {
+    private val map: Array<CharArray> = readInput(6, inputType).map { it.toCharArray() }.toTypedArray()
+
+    private val initialPosition: Pair<Int, Int> =
+        let {
+            for (row in map.withIndex()) {
+                for (cell in row.value.withIndex()) {
+                    if (cell.value == '^') {
+                        return@let cell.index to row.index
+                    }
+                }
+            }
+
+            error("there's always at least one guard")
+        }
+
+    private val route: Set<Pair<Int, Int>> =
+        let {
+            var pos = initialPosition
+            var direction = 0 to -1
+
+            val seen = mutableSetOf<Pair<Int, Int>>()
+            while (inBounds(pos)) {
+                seen.add(pos)
+
+                var next = move(pos, direction)
+                while (!canMoveTo(next)) {
+                    direction = rotate(direction)
+                    next = move(pos, direction)
+                }
+
+                pos = next
+            }
+
+            return@let seen
+        }
+
+    override fun precompute() {
+        // no-op
+    }
+
+    private fun inBounds(p: Pair<Int, Int>): Boolean =
+        p.let { (x, y) ->
+            y >= 0 && y < map.size && x >= 0 && x < map[y].size
+        }
+
+    private fun move(
+        p: Pair<Int, Int>,
+        d: Pair<Int, Int>,
+    ): Pair<Int, Int> {
+        val (x, y) = p
+        val (dx, dy) = d
+        return x + dx to y + dy
+    }
+
+    private fun canMoveTo(p: Pair<Int, Int>): Boolean =
+        p.let { (x, y) ->
+            !inBounds(p) || map[y][x] != '#'
+        }
+
+    private fun rotate(d: Pair<Int, Int>): Pair<Int, Int> =
+        d.let { (dx, dy) ->
+            -dy to dx
+        }
+
+    override fun part1(): Int = route.size
+
+    override fun part2(): Int =
+        route.filter { it != initialPosition }.count { o ->
+            var pos = initialPosition
+            var direction = 0 to -1
+
+            val visited = mutableSetOf<Pair<Pair<Int, Int>, Pair<Int, Int>>>()
+            while (inBounds(pos) && !visited.contains(direction to pos)) {
+                visited.add(direction to pos)
+
+                var next = move(pos, direction)
+                while (next == o || !canMoveTo(next)) {
+                    direction = rotate(direction)
+                    next = move(pos, direction)
+                }
+
+                pos = next
+            }
+
+            inBounds(pos)
+        }
+}
+
+fun main() {
+    Day06("sample").test(41, 6)
+    Day06("input").run()
+}