graph_traversal: change how unneeded handlers are passed

Instead of having N! overloads for each function that can take N
optional callbacks (`is_target`, `visit`, and `visit_with_parent`), each
with their own copy of the default implementation, I now use a stub
struct (`detail::optional_func`) that implements the default behavior
which the templated function type defaults to.

Callers now have to pass `{}` in place of the previously-omitted
callback argument, which I think is cleaner and safer.

2023/src/day16.hpp

@@ -258,7 +258,7 @@ int GraphHelper::count_energized(const GraphHelper::Key &source) const {
         [this](int id, auto &&handler) {
             process_component_neighbors(id, handler);
         },
-        visit_with_parent);
+        /*is_target*/ {}, visit_with_parent);
 
     return std::count(energized.data().begin(), energized.data().end(), true);
 }

2023/src/day19.hpp

@@ -293,6 +293,7 @@ long Part2Solver::solve() {
     aoc::graph::dfs<use_seen>(
         source,
         [this](const Key &key, auto &&visit) { process_neighbors(key, visit); },
+        /*is_target*/ {},
         std::bind_front(&Part2Solver::visit_with_parent, this));
 
     if constexpr (aoc::DEBUG) {

2023/src/day21.hpp

@@ -62,7 +62,8 @@ struct Garden {
             return true;
         };
 
-        aoc::graph::bfs_manual_dedupe(source, process_neighbors, visit);
+        aoc::graph::bfs_manual_dedupe(source, process_neighbors,
+                                      /*is_target*/ {}, visit);
 
         return distances;
     }

2024/src/day10.hpp

@@ -102,7 +102,7 @@ aoc::ds::Grid<int> IslandMap::calc_scores() const {
             [this](const Pos &pos, auto &&visit) {
                 this->process_neighbors(pos, visit);
             },
-            visit_with_parent);
+            /*is_target*/ {}, visit_with_parent);
     }
 
     return scores;

2024/src/day12.hpp

@@ -114,7 +114,8 @@ void Garden::process_plot(const Pos &pos, char plant_type) {
         ++region.area;
         return true;
     };
-    aoc::graph::bfs_manual_dedupe(pos, process_neighbors, visit);
+    aoc::graph::bfs_manual_dedupe(pos, process_neighbors, /*is_target*/ {},
+                                  visit);
 }
 
 template <aoc::Part part>

2024/src/day16.hpp

@@ -88,7 +88,7 @@ int Maze::find_shortest_path() const {
         [this](const Key &key, auto &&visit_neighbor) {
             this->process_neighbors(key, visit_neighbor);
         },
-        std::bind_front(&Maze::get_distance, this), is_target);
+        std::bind_front(&Maze::get_distance, this), is_target, /*visit*/ {});
 
     if constexpr (aoc::DEBUG) {
         if (distance >= 0) {