text: // 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.
  //
  // The representation of a DBImpl consists of a set of Versions.  The
  // newest version is called "current".  Older versions may be kept
  // around to provide a consistent view to live iterators.
  //
  // Each Version keeps track of a set of Table files per level.  The
  // entire set of versions is maintained in a VersionSet.
  //
  // Version,VersionSet are thread-compatible, but require external
  // synchronization on all accesses.
  
  #ifndef STORAGE_LEVELDB_DB_VERSION_SET_H_
  #define STORAGE_LEVELDB_DB_VERSION_SET_H_
  
  #include <map>
  #include <set>
  #include <vector>
  
  #include "db/dbformat.h"
  #include "db/version_edit.h"
  #include "port/port.h"
  #include "port/thread_annotations.h"
  
  namespace leveldb {
  
  namespace log {
  class Writer;
  }
  
  class Compaction;
  class Iterator;
  class MemTable;
  class TableBuilder;
  class TableCache;
  class Version;
  class VersionSet;
  class WritableFile;
  
  // Return the smallest index i such that files[i]->largest >= key.
  // Return files.size() if there is no such file.
  // REQUIRES: "files" contains a sorted list of non-overlapping files.
  int FindFile(const InternalKeyComparator& icmp,
               const std::vector<FileMetaData*>& files, const Slice& key);
  
  // Returns true iff some file in "files" overlaps the user key range
  // [*smallest,*largest].
  // smallest==nullptr represents a key smaller than all keys in the DB.
  // largest==nullptr represents a key largest than all keys in the DB.
  // REQUIRES: If disjoint_sorted_files, files[] contains disjoint ranges
  //           in sorted order.
  bool SomeFileOverlapsRange(const InternalKeyComparator& icmp,
                             bool disjoint_sorted_files,
                             const std::vector<FileMetaData*>& files,
                             const Slice* smallest_user_key,
                             const Slice* largest_user_key);
  
  class Version {
   public:
    // Lookup the value for key.  If found, store it in *val and
    // return OK.  Else return a non-OK status.  Fills *stats.
    // REQUIRES: lock is not held
    struct GetStats {
      FileMetaData* seek_file;
      int seek_file_level;
      Status s;
    };
  
    // Append to *iters a sequence of iterators that will
    // yield the contents of this Version when merged together.
    // REQUIRES: This version has been saved (see VersionSet::SaveTo)
    void AddIterators(const ReadOptions&, std::vector<Iterator*>* iters);
  
    Status Get(const ReadOptions&, const LookupKey& key, std::string* val,
               GetStats* stats);
  
    // Adds "stats" into the current state.  Returns true if a new
    // compaction may need to be triggered, false otherwise.
    // REQUIRES: lock is held
    bool UpdateStats(const GetStats& stats);
  
    // Record a sample of bytes read at the specified internal key.
    // Samples are taken approximately once every config::kReadBytesPeriod
    // bytes.  Returns true if a new compaction may need to be triggered.
    // REQUIRES: lock is held
    bool RecordReadSample(Slice key);
  
    // Reference count management (so Versions do not disappear out from
    // under live iterators)
    void Ref();
    void Unref();
  
    void GetOverlappingInputs(
        int level,
        const InternalKey* begin,  // nullptr means before all keys
        const InternalKey* end,    // nullptr means after all keys
        std::vector<FileMetaData*>* inputs);
  
    // Returns true iff some file in the specified level overlaps
    // some part of [*smallest_user_key,*largest_user_key].
    // smallest_user_key==nullptr represents a key smaller than all the DB's keys.
    // largest_user_key==nullptr represents a key largest than all the DB's keys.
    bool OverlapInLevel(int level, const Slice* smallest_user_key,
                        const Slice* largest_user_key);
  
    // Return the level at which we should place a new memtable compaction
    // result that covers the range [smallest_user_key,largest_user_key].
    int PickLevelForMemTableOutput(const Slice& smallest_user_key,
                                   const Slice& largest_user_key);
  
    int NumFiles(int level) const { return files_[level].size(); }
  
    // Return a human readable string that describes this version's contents.
    std::string DebugString() const;
  
   private:
    friend class Compaction;
    friend class VersionSet;
  
    class LevelFileNumIterator;
  
    explicit Version(VersionSet* vset)
        : vset_(vset),
          next_(this),
          prev_(this),
          refs_(0),
          file_to_compact_(nullptr),
          file_to_compact_level_(-1),
          compaction_score_(-1),
          compaction_level_(-1) {}
  
    Version(const Version&) = delete;
    Version& operator=(const Version&) = delete;
  
    ~Version();
  
    Iterator* NewConcatenatingIterator(const ReadOptions&, int level) const;
  
    // Call func(arg, level, f) for every file that overlaps user_key in
    // order from newest to oldest.  If an invocation of func returns
    // false, makes no more calls.
    //
    // REQUIRES: user portion of internal_key == user_key.
    void ForEachOverlapping(Slice user_key, Slice internal_key, void* arg,
                            bool (*func)(void*, int, FileMetaData*));
  
    VersionSet* vset_;  // VersionSet to which this Version belongs
    Version* next_;     // Next version in linked list
    Version* prev_;     // Previous version in linked list
    int refs_;          // Number of live refs to this version
  
    // List of files per level
    std::vector<FileMetaData*> files_[config::kNumLevels];
  
    // Next file to compact based on seek stats.
    FileMetaData* file_to_compact_;
    int file_to_compact_level_;
  
    // Level that should be compacted next and its compaction score.
    // Score < 1 means compaction is not strictly needed.  These fields
    // are initialized by Finalize().
    double compaction_score_;
    int compaction_level_;
  };
  
  class VersionSet {
   public:
    VersionSet(const std::string& dbname, const Options* options,
               TableCache* table_cache, const InternalKeyComparator*);
    VersionSet(const VersionSet&) = delete;
    VersionSet& operator=(const VersionSet&) = delete;
  
    ~VersionSet();
  
    // Apply *edit to the current version to form a new descriptor that
    // is both saved to persistent state and installed as the new
    // current version.  Will release *mu while actually writing to the file.
    // REQUIRES: *mu is held on entry.
    // REQUIRES: no other thread concurrently calls LogAndApply()
    Status LogAndApply(VersionEdit* edit, port::Mutex* mu)
        EXCLUSIVE_LOCKS_REQUIRED(mu);
  
    // Recover the last saved descriptor from persistent storage.
    Status Recover(bool* save_manifest);
  
    // Return the current version.
    Version* current() const { return current_; }
  
    // Return the current manifest file number
    uint64_t ManifestFileNumber() const { return manifest_file_number_; }
  
    // Allocate and return a new file number
    uint64_t NewFileNumber() { return next_file_number_++; }
  
    // Arrange to reuse "file_number" unless a newer file number has
    // already been allocated.
    // REQUIRES: "file_number" was returned by a call to NewFileNumber().
    void ReuseFileNumber(uint64_t file_number) {
      if (next_file_number_ == file_number + 1) {
        next_file_number_ = file_number;
      }
    }
  
    // Return the number of Table files at the specified level.
    int NumLevelFiles(int level) const;
  
    // Return the combined file size of all files at the specified level.
    int64_t NumLevelBytes(int level) const;
  
    // Return the last sequence number.
    uint64_t LastSequence() const { return last_sequence_; }
  
    // Set the last sequence number to s.
    void SetLastSequence(uint64_t s) {
      assert(s >= last_sequence_);
      last_sequence_ = s;
    }
  
    // Mark the specified file number as used.
    void MarkFileNumberUsed(uint64_t number);
  
    // Return the current log file number.
    uint64_t LogNumber() const { return log_number_; }
  
    // Return the log file number for the log file that is currently
    // being compacted, or zero if there is no such log file.
    uint64_t PrevLogNumber() const { return prev_log_number_; }
  
    // Pick level and inputs for a new compaction.
    // Returns nullptr if there is no compaction to be done.
    // Otherwise returns a pointer to a heap-allocated object that
    // describes the compaction.  Caller should delete the result.
    Compaction* PickCompaction();
  
    // Return a compaction object for compacting the range [begin,end] in
    // the specified level.  Returns nullptr if there is nothing in that
    // level that overlaps the specified range.  Caller should delete
    // the result.
    Compaction* CompactRange(int level, const InternalKey* begin,
                             const InternalKey* end);
  
    // Return the maximum overlapping data (in bytes) at next level for any
    // file at a level >= 1.
    int64_t MaxNextLevelOverlappingBytes();
  
    // Create an iterator that reads over the compaction inputs for "*c".
    // The caller should delete the iterator when no longer needed.
    Iterator* MakeInputIterator(Compaction* c);
  
    // Returns true iff some level needs a compaction.
    bool NeedsCompaction() const {
      Version* v = current_;
      return (v->compaction_score_ >= 1) || (v->file_to_compact_ != nullptr);
    }
  
    // Add all files listed in any live version to *live.
    // May also mutate some internal state.
    void AddLiveFiles(std::set<uint64_t>* live);
  
    // Return the approximate offset in the database of the data for
    // "key" as of version "v".
    uint64_t ApproximateOffsetOf(Version* v, const InternalKey& key);
  
    // Return a human-readable short (single-line) summary of the number
    // of files per level.  Uses *scratch as backing store.
    struct LevelSummaryStorage {
      char buffer[100];
    };
    const char* LevelSummary(LevelSummaryStorage* scratch) const;
  
   private:
    class Builder;
  
    friend class Compaction;
    friend class Version;
  
    bool ReuseManifest(const std::string& dscname, const std::string& dscbase);
  
    void Finalize(Version* v);
  
    void GetRange(const std::vector<FileMetaData*>& inputs, InternalKey* smallest,
                  InternalKey* largest);
  
    void GetRange2(const std::vector<FileMetaData*>& inputs1,
                   const std::vector<FileMetaData*>& inputs2,
                   InternalKey* smallest, InternalKey* largest);
  
    void SetupOtherInputs(Compaction* c);
  
    // Save current contents to *log
    Status WriteSnapshot(log::Writer* log);
  
    void AppendVersion(Version* v);
  
    Env* const env_;
    const std::string dbname_;
    const Options* const options_;
    TableCache* const table_cache_;
    const InternalKeyComparator icmp_;
    uint64_t next_file_number_;
    uint64_t manifest_file_number_;
    uint64_t last_sequence_;
    uint64_t log_number_;
    uint64_t prev_log_number_;  // 0 or backing store for memtable being compacted
  
    // Opened lazily
    WritableFile* descriptor_file_;
    log::Writer* descriptor_log_;
    Version dummy_versions_;  // Head of circular doubly-linked list of versions.
    Version* current_;        // == dummy_versions_.prev_
  
    // Per-level key at which the next compaction at that level should start.
    // Either an empty string, or a valid InternalKey.
    std::string compact_pointer_[config::kNumLevels];
  };
  
  // A Compaction encapsulates information about a compaction.
  class Compaction {
   public:
    ~Compaction();
  
    // Return the level that is being compacted.  Inputs from "level"
    // and "level+1" will be merged to produce a set of "level+1" files.
    int level() const { return level_; }
  
    // Return the object that holds the edits to the descriptor done
    // by this compaction.
    VersionEdit* edit() { return &edit_; }
  
    // "which" must be either 0 or 1
    int num_input_files(int which) const { return inputs_[which].size(); }
  
    // Return the ith input file at "level()+which" ("which" must be 0 or 1).
    FileMetaData* input(int which, int i) const { return inputs_[which][i]; }
  
    // Maximum size of files to build during this compaction.
    uint64_t MaxOutputFileSize() const { return max_output_file_size_; }
  
    // Is this a trivial compaction that can be implemented by just
    // moving a single input file to the next level (no merging or splitting)
    bool IsTrivialMove() const;
  
    // Add all inputs to this compaction as delete operations to *edit.
    void AddInputDeletions(VersionEdit* edit);
  
    // Returns true if the information we have available guarantees that
    // the compaction is producing data in "level+1" for which no data exists
    // in levels greater than "level+1".
    bool IsBaseLevelForKey(const Slice& user_key);
  
    // Returns true iff we should stop building the current output
    // before processing "internal_key".
    bool ShouldStopBefore(const Slice& internal_key);
  
    // Release the input version for the compaction, once the compaction
    // is successful.
    void ReleaseInputs();
  
   private:
    friend class Version;
    friend class VersionSet;
  
    Compaction(const Options* options, int level);
  
    int level_;
    uint64_t max_output_file_size_;
    Version* input_version_;
    VersionEdit edit_;
  
    // Each compaction reads inputs from "level_" and "level_+1"
    std::vector<FileMetaData*> inputs_[2];  // The two sets of inputs
  
    // State used to check for number of overlapping grandparent files
    // (parent == level_ + 1, grandparent == level_ + 2)
    std::vector<FileMetaData*> grandparents_;
    size_t grandparent_index_;  // Index in grandparent_starts_
    bool seen_key_;             // Some output key has been seen
    int64_t overlapped_bytes_;  // Bytes of overlap between current output
                                // and grandparent files
  
    // State for implementing IsBaseLevelForKey
  
    // level_ptrs_ holds indices into input_version_->levels_: our state
    // is that we are positioned at one of the file ranges for each
    // higher level than the ones involved in this compaction (i.e. for
    // all L >= level_ + 2).
    size_t level_ptrs_[config::kNumLevels];
  };
  
  }  // namespace leveldb
  
  #endif  // STORAGE_LEVELDB_DB_VERSION_SET_H_
  