implement bfs multi-target distance calculation to nearest winning state

include/distance.hpp

@@ -0,0 +1,32 @@
+#ifndef __DISTANCE_HPP_
+#define __DISTANCE_HPP_
+
+#include "config.hpp"
+
+#include <cstddef>
+#include <vector>
+
+struct DistanceResult {
+  // distances[n] = distance from n to target
+  std::vector<int> distances;
+
+  // parents[n] = next node on the path from n to target
+  std::vector<std::size_t> parents;
+
+  // nearest_target[n] = closest target node to n
+  std::vector<std::size_t> nearest_targets;
+
+  auto Clear() -> void;
+
+  auto Empty() -> bool;
+};
+
+auto CalculateDistances(
+    std::size_t node_count,
+    const std::vector<std::pair<std::size_t, std::size_t>> &edges,
+    const std::vector<std::size_t> &targets) -> DistanceResult;
+
+auto GetPath(const DistanceResult &result, std::size_t source)
+    -> std::vector<std::size_t>;
+
+#endif