//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "file/file_prefetch_buffer.h"

#include <algorithm>
#include <mutex>

#include "file/random_access_file_reader.h"
#include "monitoring/histogram.h"
#include "monitoring/iostats_context_imp.h"
#include "port/port.h"
#include "test_util/sync_point.h"
#include "util/random.h"
#include "util/rate_limiter.h"

namespace ROCKSDB_NAMESPACE {
Status FilePrefetchBuffer::Prefetch(RandomAccessFileReader* reader,
                                    uint64_t offset, size_t n,
                                    bool for_compaction) {
  if (!enable_ || reader == nullptr) {
    return Status::OK();
  }
  size_t alignment = reader->file()->GetRequiredBufferAlignment();
  size_t offset_ = static_cast<size_t>(offset);
  uint64_t rounddown_offset = Rounddown(offset_, alignment);
  uint64_t roundup_end = Roundup(offset_ + n, alignment);
  uint64_t roundup_len = roundup_end - rounddown_offset;
  assert(roundup_len >= alignment);
  assert(roundup_len % alignment == 0);

  // Check if requested bytes are in the existing buffer_.
  // If all bytes exist -- return.
  // If only a few bytes exist -- reuse them & read only what is really needed.
  //     This is typically the case of incremental reading of data.
  // If no bytes exist in buffer -- full pread.

  Status s;
  uint64_t chunk_offset_in_buffer = 0;
  uint64_t chunk_len = 0;
  bool copy_data_to_new_buffer = false;
  if (buffer_.CurrentSize() > 0 && offset >= buffer_offset_ &&
      offset <= buffer_offset_ + buffer_.CurrentSize()) {
    if (offset + n <= buffer_offset_ + buffer_.CurrentSize()) {
      // All requested bytes are already in the buffer. So no need to Read
      // again.
      return s;
    } else {
      // Only a few requested bytes are in the buffer. memmove those chunk of
      // bytes to the beginning, and memcpy them back into the new buffer if a
      // new buffer is created.
      chunk_offset_in_buffer =
          Rounddown(static_cast<size_t>(offset - buffer_offset_), alignment);
      chunk_len = buffer_.CurrentSize() - chunk_offset_in_buffer;
      assert(chunk_offset_in_buffer % alignment == 0);
      assert(chunk_len % alignment == 0);
      assert(chunk_offset_in_buffer + chunk_len <=
             buffer_offset_ + buffer_.CurrentSize());
      if (chunk_len > 0) {
        copy_data_to_new_buffer = true;
      } else {
        // this reset is not necessary, but just to be safe.
        chunk_offset_in_buffer = 0;
      }
    }
  }

  // Create a new buffer only if current capacity is not sufficient, and memcopy
  // bytes from old buffer if needed (i.e., if chunk_len is greater than 0).
  if (buffer_.Capacity() < roundup_len) {
    buffer_.Alignment(alignment);
    buffer_.AllocateNewBuffer(static_cast<size_t>(roundup_len),
                              copy_data_to_new_buffer, chunk_offset_in_buffer,
                              static_cast<size_t>(chunk_len));
  } else if (chunk_len > 0) {
    // New buffer not needed. But memmove bytes from tail to the beginning since
    // chunk_len is greater than 0.
    buffer_.RefitTail(static_cast<size_t>(chunk_offset_in_buffer),
                      static_cast<size_t>(chunk_len));
  }

  Slice result;
  s = reader->Read(rounddown_offset + chunk_len,
                   static_cast<size_t>(roundup_len - chunk_len), &result,
                   buffer_.BufferStart() + chunk_len, for_compaction);
  if (s.ok()) {
    buffer_offset_ = rounddown_offset;
    buffer_.Size(static_cast<size_t>(chunk_len) + result.size());
  }
  return s;
}

bool FilePrefetchBuffer::TryReadFromCache(uint64_t offset, size_t n,
                                          Slice* result, bool for_compaction) {
  if (track_min_offset_ && offset < min_offset_read_) {
    min_offset_read_ = static_cast<size_t>(offset);
  }
  if (!enable_ || offset < buffer_offset_) {
    return false;
  }

  // If the buffer contains only a few of the requested bytes:
  //    If readahead is enabled: prefetch the remaining bytes + readadhead bytes
  //        and satisfy the request.
  //    If readahead is not enabled: return false.
  if (offset + n > buffer_offset_ + buffer_.CurrentSize()) {
    if (readahead_size_ > 0) {
      assert(file_reader_ != nullptr);
      assert(max_readahead_size_ >= readahead_size_);
      Status s;
      if (for_compaction) {
        s = Prefetch(file_reader_, offset, std::max(n, readahead_size_),
                     for_compaction);
      } else {
        s = Prefetch(file_reader_, offset, n + readahead_size_, for_compaction);
      }
      if (!s.ok()) {
        return false;
      }
      readahead_size_ = std::min(max_readahead_size_, readahead_size_ * 2);
    } else {
      return false;
    }
  }

  uint64_t offset_in_buffer = offset - buffer_offset_;
  *result = Slice(buffer_.BufferStart() + offset_in_buffer, n);
  return true;
}
}  // namespace ROCKSDB_NAMESPACE