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tdutils/td/utils/BufferedReader.h

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+/*
+    This file is part of TON Blockchain Library.
+
+    TON Blockchain Library is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License as published by
+    the Free Software Foundation, either version 2 of the License, or
+    (at your option) any later version.
+
+    TON Blockchain Library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public License
+    along with TON Blockchain Library.  If not, see <http://www.gnu.org/licenses/>.
+
+    Copyright 2017-2019 Telegram Systems LLP
+*/
+#pragma once
+
+#include "td/utils/common.h"
+#include "td/utils/port/FileFd.h"
+#include "td/utils/Slice.h"
+#include "td/utils/Status.h"
+
+namespace td {
+
+class BufferedReader {
+ public:
+  explicit BufferedReader(FileFd &file, size_t buff_size = 8152)
+      : file_(file), buff_(buff_size), begin_pos_(0), end_pos_(0) {
+  }
+
+  Result<size_t> read(MutableSlice slice) TD_WARN_UNUSED_RESULT;
+
+ private:
+  FileFd &file_;
+  vector<char> buff_;
+  size_t begin_pos_;
+  size_t end_pos_;
+};
+
+inline Result<size_t> BufferedReader::read(MutableSlice slice) {
+  size_t available = end_pos_ - begin_pos_;
+  if (available >= slice.size()) {
+    // have enough data in buffer
+    slice.copy_from({&buff_[begin_pos_], slice.size()});
+    begin_pos_ += slice.size();
+    return slice.size();
+  }
+
+  if (available) {
+    slice.copy_from({&buff_[begin_pos_], available});
+    begin_pos_ += available;
+    slice.remove_prefix(available);
+  }
+
+  if (slice.size() > buff_.size() / 2) {
+    TRY_RESULT(result, file_.read(slice));
+    return result + available;
+  }
+
+  TRY_RESULT(result, file_.read({&buff_[0], buff_.size()}));
+  begin_pos_ = 0;
+  end_pos_ = result;
+
+  size_t left = min(end_pos_, slice.size());
+  slice.copy_from({&buff_[0], left});
+  begin_pos_ = left;
+  return left + available;
+}
+
+}  // namespace td