add ostream operator

include/action.h

@@ -119,6 +119,11 @@ public:
         return true;
     }
   }
+
+  friend std::ostream& operator<<(std::ostream& os, const Action& action);
+
 };
 
+std::ostream& operator<<(std::ostream& os, const Action& action);
+
 #endif  // CCF_IMMERSIVE_SORTING_ACTION_H_

src/action.cpp

 #include "action.h"
+
+std::ostream& operator<<(std::ostream& os, const Action& action)
+{
+  switch (action.type_) {
+    case Action::PICK_DROP:
+      os << "[pick " << action.source_ << " and drop it into " << action.target_ << "]";
+      break;
+    default:
+      os << "[unknown action with source " << action.source_ << " and target " << action.target_ << "]";
+  }
+  return os;
+}
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