diff --git a/src/learn/gensfen.cpp b/src/learn/gensfen.cpp
index 1dddac5a..4accb882 100644
--- a/src/learn/gensfen.cpp
+++ b/src/learn/gensfen.cpp
@@ -1,7 +1,7 @@
 #include "gensfen.h"
 
+#include "sfen_writer.h"
 #include "packed_sfen.h"
-#include "sfen_stream.h"
 
 #include "misc.h"
 #include "position.h"
@@ -16,6 +16,7 @@
 
 #include "syzygy/tbprobe.h"
 
+#include <atomic>
 #include <chrono>
 #include <climits>
 #include <cmath>
@@ -28,7 +29,6 @@
 #include <memory>
 #include <optional>
 #include <random>
-#include <regex>
 #include <shared_mutex>
 #include <sstream>
 #include <unordered_set>
@@ -37,188 +37,6 @@ using namespace std;
 
 namespace Learner
 {
-    // Helper class for exporting Sfen
-    struct SfenWriter
-    {
-        // Amount of sfens required to flush the buffer.
-        static constexpr size_t SFEN_WRITE_SIZE = 5000;
-
-        // File name to write and number of threads to create
-        SfenWriter(string filename_, int thread_num, uint64_t save_count, SfenOutputType sfen_output_type)
-        {
-            sfen_buffers_pool.reserve((size_t)thread_num * 10);
-            sfen_buffers.resize(thread_num);
-
-            auto out = sync_region_cout.new_region();
-            out << "INFO (sfen_writer): Creating new data file at " << filename_ << endl;
-
-            sfen_format = sfen_output_type;
-            output_file_stream = create_new_sfen_output(filename_, sfen_format);
-            filename = filename_;
-            save_every = save_count;
-
-            finished = false;
-
-            file_worker_thread = std::thread([&] { this->file_write_worker(); });
-        }
-
-        ~SfenWriter()
-        {
-            flush();
-
-            finished = true;
-            file_worker_thread.join();
-            output_file_stream.reset();
-
-#if !defined(NDEBUG)
-            {
-                // All buffers should be empty since file_worker_thread
-                // should have written everything before exiting.
-                for (const auto& p : sfen_buffers) { assert(p == nullptr); (void)p ; }
-                assert(sfen_buffers_pool.empty());
-            }
-#endif
-        }
-
-        void write(size_t thread_id, const PackedSfenValue& psv)
-        {
-            // We have a buffer for each thread and add it there.
-            // If the buffer overflows, write it to a file.
-
-            // This buffer is prepared for each thread.
-            auto& buf = sfen_buffers[thread_id];
-
-            // Secure since there is no buf at the first time
-            // and immediately after writing the thread buffer.
-            if (!buf)
-            {
-                buf = std::make_unique<PSVector>();
-                buf->reserve(SFEN_WRITE_SIZE);
-            }
-
-            // Buffer is exclusive to this thread.
-            // There is no need for a critical section.
-            buf->push_back(psv);
-
-            if (buf->size() >= SFEN_WRITE_SIZE)
-            {
-                // If you load it in sfen_buffers_pool, the worker will do the rest.
-
-                // Critical section since sfen_buffers_pool is shared among threads.
-                std::unique_lock<std::mutex> lk(mutex);
-                sfen_buffers_pool.emplace_back(std::move(buf));
-            }
-        }
-
-        void flush()
-        {
-            for (size_t i = 0; i < sfen_buffers.size(); ++i)
-            {
-                flush(i);
-            }
-        }
-
-        // Move what remains in the buffer for your thread to a buffer for writing to a file.
-        void flush(size_t thread_id)
-        {
-            std::unique_lock<std::mutex> lk(mutex);
-
-            auto& buf = sfen_buffers[thread_id];
-
-            // There is a case that buf==nullptr, so that check is necessary.
-            if (buf && buf->size() != 0)
-            {
-                sfen_buffers_pool.emplace_back(std::move(buf));
-            }
-        }
-
-        // Dedicated thread to write to file
-        void file_write_worker()
-        {
-            while (!finished || sfen_buffers_pool.size())
-            {
-                vector<std::unique_ptr<PSVector>> buffers;
-                {
-                    std::unique_lock<std::mutex> lk(mutex);
-
-                    // Atomically swap take the filled buffers and
-                    // create a new buffer pool for threads to fill.
-                    buffers = std::move(sfen_buffers_pool);
-                    sfen_buffers_pool = std::vector<std::unique_ptr<PSVector>>();
-                }
-
-                if (!buffers.size())
-                {
-                    // Poor man's condition variable.
-                    sleep(100);
-                }
-                else
-                {
-                    for (auto& buf : buffers)
-                    {
-                        output_file_stream->write(*buf);
-
-                        sfen_write_count += buf->size();
-
-                        // Add the processed number here, and if it exceeds save_every,
-                        // change the file name and reset this counter.
-                        sfen_write_count_current_file += buf->size();
-                        if (sfen_write_count_current_file >= save_every)
-                        {
-                            sfen_write_count_current_file = 0;
-
-                            // Sequential number attached to the file
-                            int n = (int)(sfen_write_count / save_every);
-
-                            // Rename the file and open it again.
-                            // Add ios::app in consideration of overwriting.
-                            // (Depending on the operation, it may not be necessary.)
-                            string new_filename = filename + "_" + std::to_string(n);
-                            output_file_stream = create_new_sfen_output(new_filename, sfen_format);
-
-                            auto out = sync_region_cout.new_region();
-                            out << "INFO (sfen_writer): Creating new data file at " << new_filename << endl;
-                        }
-                    }
-                }
-            }
-        }
-
-    private:
-
-        std::unique_ptr<BasicSfenOutputStream> output_file_stream;
-
-        // A new net is saved after every save_every sfens are processed.
-        uint64_t save_every = std::numeric_limits<uint64_t>::max();
-
-        // File name passed in the constructor
-        std::string filename;
-
-        // Thread to write to the file
-        std::thread file_worker_thread;
-
-        // Flag that all threads have finished
-        atomic<bool> finished;
-
-        SfenOutputType sfen_format;
-
-        // buffer before writing to file
-        // sfen_buffers is the buffer for each thread
-        // sfen_buffers_pool is a buffer for writing.
-        // After loading the phase in the former buffer by SFEN_WRITE_SIZE,
-        // transfer it to the latter.
-        std::vector<std::unique_ptr<PSVector>> sfen_buffers;
-        std::vector<std::unique_ptr<PSVector>> sfen_buffers_pool;
-
-        // Mutex required to access sfen_buffers_pool
-        std::mutex mutex;
-
-        // Number of sfens written in total, and the
-        // number of sfens written in the current file.
-        uint64_t sfen_write_count = 0;
-        uint64_t sfen_write_count_current_file = 0;
-    };
-
     // Class to generate sfen with multiple threads
     struct Gensfen
     {
diff --git a/src/learn/sfen_writer.h b/src/learn/sfen_writer.h
new file mode 100644
index 00000000..1bbd916c
--- /dev/null
+++ b/src/learn/sfen_writer.h
@@ -0,0 +1,206 @@
+#include "packed_sfen.h"
+#include "sfen_stream.h"
+
+#include "misc.h"
+
+#include "extra/nnue_data_binpack_format.h"
+
+#include "syzygy/tbprobe.h"
+
+#include <cstring>
+#include <filesystem>
+#include <fstream>
+#include <limits>
+#include <list>
+#include <memory>
+#include <optional>
+#include <shared_mutex>
+#include <thread>
+#include <atomic>
+
+using namespace std;
+
+namespace Learner {
+
+    // Helper class for exporting Sfen
+    struct SfenWriter
+    {
+        // Amount of sfens required to flush the buffer.
+        static constexpr size_t SFEN_WRITE_SIZE = 5000;
+
+        // File name to write and number of threads to create
+        SfenWriter(string filename_, int thread_num, uint64_t save_count, SfenOutputType sfen_output_type)
+        {
+            sfen_buffers_pool.reserve((size_t)thread_num * 10);
+            sfen_buffers.resize(thread_num);
+
+            auto out = sync_region_cout.new_region();
+            out << "INFO (sfen_writer): Creating new data file at " << filename_ << endl;
+
+            sfen_format = sfen_output_type;
+            output_file_stream = create_new_sfen_output(filename_, sfen_format);
+            filename = filename_;
+            save_every = save_count;
+
+            finished = false;
+
+            file_worker_thread = std::thread([&] { this->file_write_worker(); });
+        }
+
+        ~SfenWriter()
+        {
+            flush();
+
+            finished = true;
+            file_worker_thread.join();
+            output_file_stream.reset();
+
+#if !defined(NDEBUG)
+            {
+                // All buffers should be empty since file_worker_thread
+                // should have written everything before exiting.
+                for (const auto& p : sfen_buffers) { assert(p == nullptr); (void)p ; }
+                assert(sfen_buffers_pool.empty());
+            }
+#endif
+        }
+
+        void write(size_t thread_id, const PackedSfenValue& psv)
+        {
+            // We have a buffer for each thread and add it there.
+            // If the buffer overflows, write it to a file.
+
+            // This buffer is prepared for each thread.
+            auto& buf = sfen_buffers[thread_id];
+
+            // Secure since there is no buf at the first time
+            // and immediately after writing the thread buffer.
+            if (!buf)
+            {
+                buf = std::make_unique<PSVector>();
+                buf->reserve(SFEN_WRITE_SIZE);
+            }
+
+            // Buffer is exclusive to this thread.
+            // There is no need for a critical section.
+            buf->push_back(psv);
+
+            if (buf->size() >= SFEN_WRITE_SIZE)
+            {
+                // If you load it in sfen_buffers_pool, the worker will do the rest.
+
+                // Critical section since sfen_buffers_pool is shared among threads.
+                std::unique_lock<std::mutex> lk(mutex);
+                sfen_buffers_pool.emplace_back(std::move(buf));
+            }
+        }
+
+        void flush()
+        {
+            for (size_t i = 0; i < sfen_buffers.size(); ++i)
+            {
+                flush(i);
+            }
+        }
+
+        // Move what remains in the buffer for your thread to a buffer for writing to a file.
+        void flush(size_t thread_id)
+        {
+            std::unique_lock<std::mutex> lk(mutex);
+
+            auto& buf = sfen_buffers[thread_id];
+
+            // There is a case that buf==nullptr, so that check is necessary.
+            if (buf && buf->size() != 0)
+            {
+                sfen_buffers_pool.emplace_back(std::move(buf));
+            }
+        }
+
+        // Dedicated thread to write to file
+        void file_write_worker()
+        {
+            while (!finished || sfen_buffers_pool.size())
+            {
+                vector<std::unique_ptr<PSVector>> buffers;
+                {
+                    std::unique_lock<std::mutex> lk(mutex);
+
+                    // Atomically swap take the filled buffers and
+                    // create a new buffer pool for threads to fill.
+                    buffers = std::move(sfen_buffers_pool);
+                    sfen_buffers_pool = std::vector<std::unique_ptr<PSVector>>();
+                }
+
+                if (!buffers.size())
+                {
+                    // Poor man's condition variable.
+                    sleep(100);
+                }
+                else
+                {
+                    for (auto& buf : buffers)
+                    {
+                        output_file_stream->write(*buf);
+
+                        sfen_write_count += buf->size();
+
+                        // Add the processed number here, and if it exceeds save_every,
+                        // change the file name and reset this counter.
+                        sfen_write_count_current_file += buf->size();
+                        if (sfen_write_count_current_file >= save_every)
+                        {
+                            sfen_write_count_current_file = 0;
+
+                            // Sequential number attached to the file
+                            int n = (int)(sfen_write_count / save_every);
+
+                            // Rename the file and open it again.
+                            // Add ios::app in consideration of overwriting.
+                            // (Depending on the operation, it may not be necessary.)
+                            string new_filename = filename + "_" + std::to_string(n);
+                            output_file_stream = create_new_sfen_output(new_filename, sfen_format);
+
+                            auto out = sync_region_cout.new_region();
+                            out << "INFO (sfen_writer): Creating new data file at " << new_filename << endl;
+                        }
+                    }
+                }
+            }
+        }
+
+    private:
+
+        std::unique_ptr<BasicSfenOutputStream> output_file_stream;
+
+        // A new net is saved after every save_every sfens are processed.
+        uint64_t save_every = std::numeric_limits<uint64_t>::max();
+
+        // File name passed in the constructor
+        std::string filename;
+
+        // Thread to write to the file
+        std::thread file_worker_thread;
+
+        // Flag that all threads have finished
+        atomic<bool> finished;
+
+        SfenOutputType sfen_format;
+
+        // buffer before writing to file
+        // sfen_buffers is the buffer for each thread
+        // sfen_buffers_pool is a buffer for writing.
+        // After loading the phase in the former buffer by SFEN_WRITE_SIZE,
+        // transfer it to the latter.
+        std::vector<std::unique_ptr<PSVector>> sfen_buffers;
+        std::vector<std::unique_ptr<PSVector>> sfen_buffers_pool;
+
+        // Mutex required to access sfen_buffers_pool
+        std::mutex mutex;
+
+        // Number of sfens written in total, and the
+        // number of sfens written in the current file.
+        uint64_t sfen_write_count = 0;
+        uint64_t sfen_write_count_current_file = 0;
+    };
+}