//  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.

#pragma once

#include <cstdint>

#include <functional>

#include <limits>

#include <vector>

#include "memory/arena.h"

#include "port/port.h"

#include "util/autovector.h"

namespace ROCKSDB_NAMESPACE {

class Comparator;

struct FileMetaData;

struct FdWithKeyRange;

struct FileLevel;

// The file tree structure in Version is prebuilt and the range of each file

// is known. On Version::Get(), it uses binary search to find a potential file

// and then check if a target key can be found in the file by comparing the key

// to each file's smallest and largest key. The results of these comparisons

// can be reused beyond checking if a key falls into a file's range.

// With some pre-calculated knowledge, each key comparison that has been done

// can serve as a hint to narrow down further searches: if a key compared to

// be smaller than a file's smallest or largest, that comparison can be used

// to find out the right bound of next binary search. Similarly, if a key

// compared to be larger than a file's smallest or largest, it can be utilized

// to find out the left bound of next binary search.

// With these hints: it can greatly reduce the range of binary search,

// especially for bottom levels, given that one file most likely overlaps with

// only N files from level below (where N is max_bytes_for_level_multiplier).

// So on level L, we will only look at ~N files instead of N^L files on the

// naive approach.

class FileIndexer {

 public:

  explicit FileIndexer(const Comparator* ucmp);

  size_t NumLevelIndex() const;

  size_t LevelIndexSize(size_t level) const;

  // Return a file index range in the next level to search for a key based on

  // smallest and largest key comparison for the current file specified by

  // level and file_index. When *left_index < *right_index, both index should

  // be valid and fit in the vector size.

  void GetNextLevelIndex(const size_t level, const size_t file_index,

                         const int cmp_smallest, const int cmp_largest,

                         int32_t* left_bound, int32_t* right_bound) const;

  void UpdateIndex(Arena* arena, const size_t num_levels,

                   std::vector<FileMetaData*>* const files);

  enum { kLevelMaxIndex = std::numeric_limits<int32_t>::max() };

 private:

  size_t num_levels_;

  const Comparator* ucmp_;

  struct IndexUnit {

    IndexUnit()

        : smallest_lb(0), largest_lb(0), smallest_rb(-1), largest_rb(-1) {}

    // During file search, a key is compared against smallest and largest

    // from a FileMetaData. It can have 3 possible outcomes:

    // (1) key is smaller than smallest, implying it is also smaller than

    //     larger. Precalculated index based on "smallest < smallest" can

    //     be used to provide right bound.

    // (2) key is in between smallest and largest.

    //     Precalculated index based on "smallest > greatest" can be used to

    //     provide left bound.

    //     Precalculated index based on "largest < smallest" can be used to

    //     provide right bound.

    // (3) key is larger than largest, implying it is also larger than smallest.

    //     Precalculated index based on "largest > largest" can be used to

    //     provide left bound.

    //

    // As a result, we will need to do:

    // Compare smallest (<=) and largest keys from upper level file with

    // smallest key from lower level to get a right bound.

    // Compare smallest (>=) and largest keys from upper level file with

    // largest key from lower level to get a left bound.

    //

    // Example:

    //    level 1:              [50 - 60]

    //    level 2:        [1 - 40], [45 - 55], [58 - 80]

    // A key 35, compared to be less than 50, 3rd file on level 2 can be

    // skipped according to rule (1). LB = 0, RB = 1.

    // A key 53, sits in the middle 50 and 60. 1st file on level 2 can be

    // skipped according to rule (2)-a, but the 3rd file cannot be skipped

    // because 60 is greater than 58. LB = 1, RB = 2.

    // A key 70, compared to be larger than 60. 1st and 2nd file can be skipped

    // according to rule (3). LB = 2, RB = 2.

    //

    // Point to a left most file in a lower level that may contain a key,

    // which compares greater than smallest of a FileMetaData (upper level)

    int32_t smallest_lb;

    // Point to a left most file in a lower level that may contain a key,

    // which compares greater than largest of a FileMetaData (upper level)

    int32_t largest_lb;

    // Point to a right most file in a lower level that may contain a key,

    // which compares smaller than smallest of a FileMetaData (upper level)

    int32_t smallest_rb;

    // Point to a right most file in a lower level that may contain a key,

    // which compares smaller than largest of a FileMetaData (upper level)

    int32_t largest_rb;

  };

  // Data structure to store IndexUnits in a whole level

  struct IndexLevel {

    size_t num_index;

    IndexUnit* index_units;

    IndexLevel() : num_index(0), index_units(nullptr) {}

  };

  void CalculateLB(

      const std::vector<FileMetaData*>& upper_files,

      const std::vector<FileMetaData*>& lower_files, IndexLevel* index_level,

      std::function<int(const FileMetaData*, const FileMetaData*)> cmp_op,

      std::function<void(IndexUnit*, int32_t)> set_index);

  void CalculateRB(

      const std::vector<FileMetaData*>& upper_files,

      const std::vector<FileMetaData*>& lower_files, IndexLevel* index_level,

      std::function<int(const FileMetaData*, const FileMetaData*)> cmp_op,

      std::function<void(IndexUnit*, int32_t)> set_index);

  autovector<IndexLevel> next_level_index_;

  int32_t* level_rb_;

};

}  // namespace ROCKSDB_NAMESPACE