day(06): refactor

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

src/Day06.kt

@@ -1,9 +1,11 @@
+private typealias Vector = Pair<Int, Int>
+
 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> =
+    private val initialPosition: Vector =
         let {
             for (row in map.withIndex()) {
                 for (cell in row.value.withIndex()) {
@@ -16,76 +18,61 @@ class Day06(
             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
-        }
+    private val route: Set<Vector> = guard(null).let { (visited, _) -> visited.map { (_, p) -> p }.toSet() }
 
     override fun precompute() {
         // no-op
     }
 
-    private fun inBounds(p: Pair<Int, Int>): Boolean =
+    private fun inBounds(p: Vector): 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> {
+        p: Vector,
+        d: Vector,
+    ): Vector {
         val (x, y) = p
         val (dx, dy) = d
         return x + dx to y + dy
     }
 
-    private fun canMoveTo(p: Pair<Int, Int>): Boolean =
+    private fun canMoveTo(p: Vector): Boolean =
         p.let { (x, y) ->
             !inBounds(p) || map[y][x] != '#'
         }
 
-    private fun rotate(d: Pair<Int, Int>): Pair<Int, Int> =
+    private fun rotate(d: Vector): Vector =
         d.let { (dx, dy) ->
             -dy to dx
         }
 
+    private fun guard(obstacle: Vector?): Pair<Set<Pair<Vector, Vector>>, Vector> {
+        var pos = initialPosition
+        var direction = 0 to -1
+
+        val visited = mutableSetOf<Pair<Vector, Vector>>()
+        while (inBounds(pos) && !visited.contains(direction to pos)) {
+            visited.add(direction to pos)
+
+            var next = move(pos, direction)
+            while (next == obstacle || !canMoveTo(next)) {
+                direction = rotate(direction)
+                next = move(pos, direction)
+            }
+
+            pos = next
+        }
+
+        return visited to pos
+    }
+
     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)
+        route.filter { it != initialPosition }.count {
+            guard(it).let { (_, pos) -> inBounds(pos) }
         }
 }