chore: import upstream snapshot with attribution

src/io/file_io.cpp

@@ -0,0 +1,80 @@
+/*!
+ * Copyright (c) 2018-2026 Microsoft Corporation. All rights reserved.
+ * Copyright (c) 2018-2026 The LightGBM developers. All rights reserved.
+ * Licensed under the MIT License. See LICENSE file in the project root for
+ * license information.
+ */
+#include <LightGBM/utils/file_io.h>
+
+#include <LightGBM/utils/log.h>
+
+#include <algorithm>
+#include <cstdio>
+#include <memory>
+#include <sstream>
+#include <string>
+#include <unordered_map>
+
+namespace LightGBM {
+
+struct LocalFile : VirtualFileReader, VirtualFileWriter {
+  LocalFile(const std::string& filename, const std::string& mode)
+      : filename_(filename), mode_(mode) {}
+  virtual ~LocalFile() {
+    if (file_ != NULL) {
+      fclose(file_);
+    }
+  }
+
+  bool Init() {
+    if (file_ == NULL) {
+#if _MSC_VER
+      fopen_s(&file_, filename_.c_str(), mode_.c_str());
+#else
+      file_ = fopen(filename_.c_str(), mode_.c_str());
+#endif
+    }
+    return file_ != NULL;
+  }
+
+  bool Exists() const {
+    LocalFile file(filename_, "rb");
+    return file.Init();
+  }
+
+  size_t Read(void* buffer, size_t bytes) const {
+    return fread(buffer, 1, bytes, file_);
+  }
+
+  size_t Write(const void* buffer, size_t bytes) {
+    size_t bytes_written = fwrite(buffer, 1, bytes, file_);
+    if (bytes_written != bytes) {
+      Log::Fatal(
+          "Cannot write binary data to %s, wrote %zu of %zu bytes",
+          filename_.c_str(), bytes_written, bytes);
+    }
+    return bytes_written;
+  }
+
+ private:
+  FILE* file_ = NULL;
+  const std::string filename_;
+  const std::string mode_;
+};
+
+std::unique_ptr<VirtualFileReader> VirtualFileReader::Make(
+    const std::string& filename) {
+  return std::unique_ptr<VirtualFileReader>(new LocalFile(filename, "rb"));
+}
+
+std::unique_ptr<VirtualFileWriter> VirtualFileWriter::Make(
+    const std::string& filename) {
+  return std::unique_ptr<VirtualFileWriter>(new LocalFile(filename, "wb"));
+}
+
+bool VirtualFileWriter::Exists(const std::string& filename) {
+  LocalFile file(filename, "rb");
+  return file.Exists();
+}
+
+}  // namespace LightGBM