// 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 "tensorflow/core/lib/io/two_level_iterator.h"

#include "tensorflow/core/lib/io/table.h"
#include "tensorflow/core/lib/io/block.h"
#include "tensorflow/core/lib/io/format.h"
#include "tensorflow/core/lib/io/iterator.h"

namespace tensorflow {
namespace table {

namespace {

typedef Iterator* (*BlockFunction)(void*, const StringPiece&);

class TwoLevelIterator : public Iterator {
 public:
  TwoLevelIterator(Iterator* index_iter, BlockFunction block_function,
                   void* arg);

  virtual ~TwoLevelIterator();

  virtual void Seek(const StringPiece& target);
  virtual void SeekToFirst();
  virtual void Next();

  virtual bool Valid() const {
    return (data_iter_ == nullptr) ? false : data_iter_->Valid();
  }
  virtual StringPiece key() const {
    assert(Valid());
    return data_iter_->key();
  }
  virtual StringPiece value() const {
    assert(Valid());
    return data_iter_->value();
  }
  virtual Status status() const {
    // 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_ != NULL && !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 SetDataIterator(Iterator* data_iter);
  void InitDataBlock();

  BlockFunction block_function_;
  void* arg_;
  Status status_;
  Iterator* index_iter_;
  Iterator* data_iter_;  // May be NULL
  // If data_iter_ is non-NULL, then "data_block_handle_" holds the
  // "index_value" passed to block_function_ to create the data_iter_.
  string data_block_handle_;
};

TwoLevelIterator::TwoLevelIterator(Iterator* index_iter,
                                   BlockFunction block_function, void* arg)
    : block_function_(block_function),
      arg_(arg),
      index_iter_(index_iter),
      data_iter_(NULL) {}

TwoLevelIterator::~TwoLevelIterator() {
  delete index_iter_;
  delete data_iter_;
}

void TwoLevelIterator::Seek(const StringPiece& target) {
  index_iter_->Seek(target);
  InitDataBlock();
  if (data_iter_ != NULL) data_iter_->Seek(target);
  SkipEmptyDataBlocksForward();
}

void TwoLevelIterator::SeekToFirst() {
  index_iter_->SeekToFirst();
  InitDataBlock();
  if (data_iter_ != NULL) data_iter_->SeekToFirst();
  SkipEmptyDataBlocksForward();
}

void TwoLevelIterator::Next() {
  assert(Valid());
  data_iter_->Next();
  SkipEmptyDataBlocksForward();
}

void TwoLevelIterator::SkipEmptyDataBlocksForward() {
  while (data_iter_ == NULL || !data_iter_->Valid()) {
    // Move to next block
    if (!index_iter_->Valid()) {
      SetDataIterator(NULL);
      return;
    }
    index_iter_->Next();
    InitDataBlock();
    if (data_iter_ != NULL) data_iter_->SeekToFirst();
  }
}

void TwoLevelIterator::SetDataIterator(Iterator* data_iter) {
  if (data_iter_ != NULL) {
    SaveError(data_iter_->status());
    delete data_iter_;
  }
  data_iter_ = data_iter;
}

void TwoLevelIterator::InitDataBlock() {
  if (!index_iter_->Valid()) {
    SetDataIterator(NULL);
  } else {
    StringPiece handle = index_iter_->value();
    if (data_iter_ != NULL && 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_, handle);
      data_block_handle_.assign(handle.data(), handle.size());
      SetDataIterator(iter);
    }
  }
}

}  // namespace

Iterator* NewTwoLevelIterator(Iterator* index_iter,
                              BlockFunction block_function, void* arg) {
  return new TwoLevelIterator(index_iter, block_function, arg);
}

}  // namespace table
}  // namespace tensorflow