Write a util function to determine the valid next steps, and share it…

… between parts 1 and 2

lib/day10/part_1.dart

@@ -39,25 +39,12 @@ Set<Point> _summitsReachableFromPoint(
   Set<Point> summits = {};
   visitedPoints.add(point);
 
-  for (final record in [(1, 0), (-1, 0), (0, 1), (0, -1)]) {
-    final newX = point.x + record.$1;
-    final newY = point.y + record.$2;
-
-    if (newX < 0 ||
-        newY < 0 ||
-        newX >= topoMap.height ||
-        newY >= topoMap.height) {
-      continue;
-    }
-
-    final nextPoint = topoMap.getPoint(newX, newY);
-    if (nextPoint.isGradualStepFromPoint(point)) {
-      if (nextPoint.isSummit) {
-        summits.add(nextPoint);
-      } else {
-        summits.addAll(
-            _summitsReachableFromPoint(topoMap, nextPoint, visitedPoints));
-      }
+  for (final nextPoint in topoMap.getValidNextSteps(point)) {
+    if (nextPoint.isSummit) {
+      summits.add(nextPoint);
+    } else {
+      summits.addAll(
+          _summitsReachableFromPoint(topoMap, nextPoint, visitedPoints));
     }
   }
 

lib/day10/part_2.dart

@@ -13,7 +13,7 @@ Future<int> calculate(File file) async {
 
   int sum = 0;
   for (final trailhead in topoMap.trailheads) {
-    final summits = _summitsReachableFromPoint(topoMap, trailhead, {});
+    final summits = _summitsReachableFromPoint(topoMap, trailhead);
     sum += summits.length;
   }
 
@@ -23,31 +23,16 @@ Future<int> calculate(File file) async {
 /// Finds all of the summits reachable from this point. Because we are not
 /// tracking visited points in this DFS algorithm, this will return the same
 /// point multiple times if it is reachable via multiple paths. Since there is a
-/// requirement of the next elevation increasing by exactly one, there is no risk
-/// of backtracking and hitting endless cycles.
-List<Point> _summitsReachableFromPoint(
-    TopographicMap topoMap, Point point, Set<Point> visitedPoints) {
+/// requirement of the next elevation increasing by exactly one, there is no
+/// risk of backtracking and hitting endless cycles.
+List<Point> _summitsReachableFromPoint(TopographicMap topoMap, Point point) {
   List<Point> summits = [];
 
-  for (final record in [(1, 0), (-1, 0), (0, 1), (0, -1)]) {
-    final newX = point.x + record.$1;
-    final newY = point.y + record.$2;
-
-    if (newX < 0 ||
-        newY < 0 ||
-        newX >= topoMap.height ||
-        newY >= topoMap.height) {
-      continue;
-    }
-
-    final nextPoint = topoMap.getPoint(newX, newY);
-    if (nextPoint.isGradualStepFromPoint(point)) {
-      if (nextPoint.isSummit) {
-        summits.add(nextPoint);
-      } else {
-        summits.addAll(
-            _summitsReachableFromPoint(topoMap, nextPoint, visitedPoints));
-      }
+  for (final nextPoint in topoMap.getValidNextSteps(point)) {
+    if (nextPoint.isSummit) {
+      summits.add(nextPoint);
+    } else {
+      summits.addAll(_summitsReachableFromPoint(topoMap, nextPoint));
     }
   }
 

lib/day10/shared.dart

@@ -61,6 +61,30 @@ final class TopographicMap {
     return map[r][c];
   }
 
+  /// Returns the set of valid next steps from the given point.
+  ///
+  /// A valid step is one step along a cardinal directions, in the bounds of
+  /// the map, and at an elevation exactly 1 greater than the given point.
+  Set<Point> getValidNextSteps(Point point) {
+    Set<Point> nextSteps = {};
+
+    for (final record in [(1, 0), (-1, 0), (0, 1), (0, -1)]) {
+      final newX = point.x + record.$1;
+      final newY = point.y + record.$2;
+
+      if (newX < 0 || newY < 0 || newX >= height || newY >= height) {
+        continue;
+      }
+
+      final nextPoint = getPoint(newX, newY);
+      if (nextPoint.isGradualStepFromPoint(point)) {
+        nextSteps.add(nextPoint);
+      }
+    }
+
+    return nextSteps;
+  }
+
   /// Returns all of the trailheads in this map.
   Iterable<Point> get trailheads => map.flattened.where((p) => p.isTrailhead);
 }