// 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 "table/two_level_iterator.h"

#include "leveldb/table.h"

#include "table/block.h"

#include "table/format.h"

#include "table/iterator_wrapper.h"

namespace leveldb {

namespace {

typedef Iterator* (*BlockFunction)(void*, const ReadOptions&, const Slice&);

class TwoLevelIterator : public Iterator {

 public:

  TwoLevelIterator(Iterator* index_iter, BlockFunction block_function,

                   void* arg, const ReadOptions& options);

  ~TwoLevelIterator() override;

  void Seek(const Slice& target) override;

  void SeekToFirst() override;

  void SeekToLast() override;

  void Next() override;

  void Prev() override;

  bool Valid() const override { return data_iter_.Valid(); }

  Slice key() const override {

    assert(Valid());

    return data_iter_.key();

  }

  Slice value() const override {

    assert(Valid());

    return data_iter_.value();

  }

  Status status() const override {

    // It'd be nice if status() returned a const Status& instead of a Status

    if (!index_iter_.status().ok()) {

      return index_iter_.status();

    } else if (data_iter_.iter() != nullptr && !data_iter_.status().ok()) {

      return data_iter_.status();

    } else {

      return status_;

    }

  }

 private:

  void SaveError(const Status& s) {

    if (status_.ok() && !s.ok()) status_ = s;

  }

  void SkipEmptyDataBlocksForward();

  void SkipEmptyDataBlocksBackward();

  void SetDataIterator(Iterator* data_iter);

  void InitDataBlock();

  BlockFunction block_function_;

  void* arg_;

  const ReadOptions options_;

  Status status_;

  IteratorWrapper index_iter_;

  IteratorWrapper data_iter_;  // May be nullptr

  // If data_iter_ is non-null, then "data_block_handle_" holds the

  // "index_value" passed to block_function_ to create the data_iter_.

  std::string data_block_handle_;

};

TwoLevelIterator::TwoLevelIterator(Iterator* index_iter,

                                   BlockFunction block_function, void* arg,

                                   const ReadOptions& options)

    : block_function_(block_function),

      arg_(arg),

      options_(options),

      index_iter_(index_iter),

      data_iter_(nullptr) {}

TwoLevelIterator::~TwoLevelIterator() = default;

void TwoLevelIterator::Seek(const Slice& target) {

  index_iter_.Seek(target);

  InitDataBlock();

  if (data_iter_.iter() != nullptr) data_iter_.Seek(target);

  SkipEmptyDataBlocksForward();

}

void TwoLevelIterator::SeekToFirst() {

  index_iter_.SeekToFirst();

  InitDataBlock();

  if (data_iter_.iter() != nullptr) data_iter_.SeekToFirst();

  SkipEmptyDataBlocksForward();

}

void TwoLevelIterator::SeekToLast() {

  index_iter_.SeekToLast();

  InitDataBlock();

  if (data_iter_.iter() != nullptr) data_iter_.SeekToLast();

  SkipEmptyDataBlocksBackward();

}

void TwoLevelIterator::Next() {

  assert(Valid());

  data_iter_.Next();

  SkipEmptyDataBlocksForward();

}

void TwoLevelIterator::Prev() {

  assert(Valid());

  data_iter_.Prev();

  SkipEmptyDataBlocksBackward();

}

void TwoLevelIterator::SkipEmptyDataBlocksForward() {

  while (data_iter_.iter() == nullptr || !data_iter_.Valid()) {

    // Move to next block

    if (!index_iter_.Valid()) {

      SetDataIterator(nullptr);

      return;

    }

    index_iter_.Next();

    InitDataBlock();

    if (data_iter_.iter() != nullptr) data_iter_.SeekToFirst();

  }

}

void TwoLevelIterator::SkipEmptyDataBlocksBackward() {

  while (data_iter_.iter() == nullptr || !data_iter_.Valid()) {

    // Move to next block

    if (!index_iter_.Valid()) {

      SetDataIterator(nullptr);

      return;

    }

    index_iter_.Prev();

    InitDataBlock();

    if (data_iter_.iter() != nullptr) data_iter_.SeekToLast();

  }

}

void TwoLevelIterator::SetDataIterator(Iterator* data_iter) {

  if (data_iter_.iter() != nullptr) SaveError(data_iter_.status());

  data_iter_.Set(data_iter);

}

void TwoLevelIterator::InitDataBlock() {

  if (!index_iter_.Valid()) {

    SetDataIterator(nullptr);

  } else {

    Slice handle = index_iter_.value();

    if (data_iter_.iter() != nullptr &&

        handle.compare(data_block_handle_) == 0) {

      // data_iter_ is already constructed with this iterator, so

      // no need to change anything

    } else {

      Iterator* iter = (*block_function_)(arg_, options_, handle);

      data_block_handle_.assign(handle.data(), handle.size());

      SetDataIterator(iter);

    }

  }

}

}  // namespace

Iterator* NewTwoLevelIterator(Iterator* index_iter,

                              BlockFunction block_function, void* arg,

                              const ReadOptions& options) {

  return new TwoLevelIterator(index_iter, block_function, arg, options);

}

}  // namespace leveldb