Add output_file option to gather_statistics.

It is optional. When specified it will also forward the final results output to the provided file.

docs/stats.md

@@ -10,6 +10,8 @@ Any name that doesn't designate an argument name or is not an argument will be i
 
 `input_file` - the path to the .bin or .binpack input file to read
 
+`output_file` - optional path to the output file to write the results too. Results are always written on the console, so if this is specified the results will be written in both places.
+
 `max_count` - the maximum number of positions to process. Default: no limit.
 
 ## Groups

src/tools/stats.cpp

@@ -1130,7 +1130,8 @@ namespace Stockfish::Tools::Stats
     void do_gather_statistics(
         const std::string& filename,
         StatisticGathererSet& statistic_gatherers,
-        std::uint64_t max_count)
+        std::uint64_t max_count,
+        const std::optional<std::string>& output_filename)
     {
         Thread* th = Threads.main();
         Position& pos = th->rootPos;
@@ -1171,7 +1172,13 @@ namespace Stockfish::Tools::Stats
         std::cout << "Finished gathering statistics.\n\n";
         std::cout << "Results:\n\n";
 
-        std::cout << statistic_gatherers.get_output().to_string();
+        const auto output_str = statistic_gatherers.get_output().to_string();
+        std::cout << output_str;
+        if (output_filename.has_value())
+        {
+            std::ofstream out_file(*output_filename);
+            out_file << output_str;
+        }
     }
 
     void gather_statistics(std::istringstream& is)
@@ -1183,6 +1190,7 @@ namespace Stockfish::Tools::Stats
         StatisticGathererSet statistic_gatherers;
 
         std::string input_file;
+        std::optional<std::string> output_file;
         std::uint64_t max_count = std::numeric_limits<std::uint64_t>::max();
 
         while(true)
@@ -1195,13 +1203,19 @@ namespace Stockfish::Tools::Stats
 
             if (token == "input_file")
                 is >> input_file;
+            else if (token == "output_file")
+            {
+                std::string s;
+                is >> s;
+                output_file = s;
+            }
             else if (token == "max_count")
                 is >> max_count;
             else
                 registry.add_statistic_gatherers_by_group(statistic_gatherers, token);
         }
 
-        do_gather_statistics(input_file, statistic_gatherers, max_count);
+        do_gather_statistics(input_file, statistic_gatherers, max_count, output_file);
     }
 
 }