Optimize db.iterator()

By using `emplace_back()`, reusing the `std::vector` cache between
`iterator.next()` calls, and not advancing the iterator prematurely.
That last one only matters for single reads (i.e. the first `next()`
call or one made after seeking) and it doesn't improve performance
compared to the previous release, just undoes a mistake in Level/leveldown@b815bea.

(cherry picked from commit Level/leveldown@112906b)

binding.cc

@@ -481,7 +481,7 @@ struct Database {
     }
   }
 
-  bool HasPriorityWork () {
+  bool HasPriorityWork () const {
     return priorityWork_ > 0;
   }
 
@@ -533,8 +533,7 @@ struct BaseIterator {
       gt_(gt),
       gte_(gte),
       limit_(limit),
-      count_(0),
-      eof_(false) {
+      count_(0) {
     options_ = new leveldb::ReadOptions();
     options_->fill_cache = fillCache;
     options_->verify_checksums = false;
@@ -601,40 +600,22 @@ struct BaseIterator {
     didSeek_ = true;
 
     if (OutOfRange(target)) {
-      if (reverse_) {
-        dbIterator_->SeekToFirst();
-        dbIterator_->Prev();
-      } else {
-        dbIterator_->SeekToLast();
-        dbIterator_->Next();
-      }
-
-      return;
+      return SeekToEnd();
     }
 
     dbIterator_->Seek(target);
 
     if (dbIterator_->Valid()) {
       int cmp = dbIterator_->key().compare(target);
-      if (cmp > 0 && reverse_) {
-        dbIterator_->Prev();
-      } else if (cmp < 0 && !reverse_) {
-        dbIterator_->Next();
+      if (reverse_ ? cmp > 0 : cmp < 0) {
+        Next();
       }
     } else {
-      if (reverse_) {
-        dbIterator_->SeekToLast();
-      } else {
-        dbIterator_->SeekToFirst();
-      }
+      SeekToFirst();
       if (dbIterator_->Valid()) {
         int cmp = dbIterator_->key().compare(target);
-        if (cmp > 0 && reverse_) {
-          dbIterator_->SeekToFirst();
-          dbIterator_->Prev();
-        } else if (cmp < 0 && !reverse_) {
-          dbIterator_->SeekToLast();
-          dbIterator_->Next();
+        if (reverse_ ? cmp > 0 : cmp < 0) {
+          SeekToEnd();
         }
       }
     }
@@ -649,24 +630,34 @@ struct BaseIterator {
     }
   }
 
-  bool ReadOne () {
-    if (eof_ || !dbIterator_->Valid()) {
-      return false;
-    }
-
-    if ((limit_ >= 0 && ++count_ > limit_) || OutOfRange(dbIterator_->key())) {
-      eof_ = true;
-      return false;
-    }
+  bool Valid () const {
+    return dbIterator_->Valid() && !OutOfRange(dbIterator_->key());
+  }
 
-    return true;
+  bool Increment () {
+    return limit_ < 0 || ++count_ <= limit_;
   }
 
-  void Advance () {
+  void Next () {
     if (reverse_) dbIterator_->Prev();
     else dbIterator_->Next();
   }
 
+  void SeekToFirst () {
+    if (reverse_) dbIterator_->SeekToLast();
+    else dbIterator_->SeekToFirst();
+  }
+
+  void SeekToLast () {
+    if (reverse_) dbIterator_->SeekToFirst();
+    else dbIterator_->SeekToLast();
+  }
+
+  void SeekToEnd () {
+    SeekToLast();
+    Next();
+  }
+
   leveldb::Slice CurrentKey () const {
     return dbIterator_->key();
   }
@@ -679,7 +670,13 @@ struct BaseIterator {
     return dbIterator_->status();
   }
 
-  bool OutOfRange (const leveldb::Slice& target) {
+  bool OutOfRange (const leveldb::Slice& target) const {
+    // TODO: benchmark to see if this is worth it
+    // if (upperBoundOnly && !reverse_) {
+    //   return ((lt_  != NULL && target.compare(*lt_) >= 0) ||
+    //           (lte_ != NULL && target.compare(*lte_) > 0));
+    // }
+
     return ((lt_  != NULL && target.compare(*lt_) >= 0) ||
             (lte_ != NULL && target.compare(*lte_) > 0) ||
             (gt_  != NULL && target.compare(*gt_) <= 0) ||
@@ -699,7 +696,6 @@ struct BaseIterator {
   std::string* gte_;
   int limit_;
   int count_;
-  bool eof_;
   leveldb::ReadOptions* options_;
 };
 
@@ -747,33 +743,36 @@ struct Iterator final : public BaseIterator {
     if (ref_ != NULL) napi_delete_reference(env, ref_);
   }
 
-  bool ReadMany (uint32_t size, std::vector<std::pair<std::string, std::string>>& result) {
+  bool ReadMany (uint32_t size) {
+    cache_.clear();
     size_t bytesRead = 0;
 
-    while (ReadOne()) {
-      std::string key, value;
+    while (true) {
+      if (landed_) Next();
+      if (!Valid() || !Increment()) break;
 
       if (keys_) {
         leveldb::Slice slice = CurrentKey();
-        key.assign(slice.data(), slice.size());
-        bytesRead += key.size();
+        cache_.emplace_back(slice.data(), slice.size());
+        bytesRead += slice.size();
+      } else {
+        cache_.emplace_back("");
       }
 
       if (values_) {
         leveldb::Slice slice = CurrentValue();
-        value.assign(slice.data(), slice.size());
-        bytesRead += value.size();
+        cache_.emplace_back(slice.data(), slice.size());
+        bytesRead += slice.size();
+      } else {
+        cache_.emplace_back("");
       }
 
-      Advance();
-      result.push_back(std::make_pair(key, value));
-
       if (!landed_) {
         landed_ = true;
         return true;
       }
 
-      if (bytesRead > highWaterMark_ || result.size() >= size) {
+      if (bytesRead > highWaterMark_ || cache_.size() >= size * 2) {
         return true;
       }
     }
@@ -791,6 +790,7 @@ struct Iterator final : public BaseIterator {
   bool nexting_;
   bool isEnding_;
   BaseWorker* endWorker_;
+  std::vector<std::string> cache_;
 
 private:
   napi_ref ref_;
@@ -1188,18 +1188,18 @@ struct ClearWorker final : public PriorityWorker {
                std::string* gt,
                std::string* gte)
     : PriorityWorker(env, database, callback, "leveldown.db.clear") {
-    baseIterator_ = new BaseIterator(database, reverse, lt, lte, gt, gte, limit, false);
+    iterator_ = new BaseIterator(database, reverse, lt, lte, gt, gte, limit, false);
     writeOptions_ = new leveldb::WriteOptions();
     writeOptions_->sync = false;
   }
 
   ~ClearWorker () {
-    delete baseIterator_;
+    delete iterator_;
     delete writeOptions_;
   }
 
   void DoExecute () override {
-    baseIterator_->SeekToRange();
+    iterator_->SeekToRange();
 
     // TODO: add option
     uint32_t hwm = 16 * 1024;
@@ -1208,14 +1208,14 @@ struct ClearWorker final : public PriorityWorker {
     while (true) {
       size_t bytesRead = 0;
 
-      while (bytesRead < hwm && baseIterator_->ReadOne()) {
-        leveldb::Slice key = baseIterator_->CurrentKey();
+      while (bytesRead <= hwm && iterator_->Valid() && iterator_->Increment()) {
+        leveldb::Slice key = iterator_->CurrentKey();
         batch.Delete(key);
         bytesRead += key.size();
-        baseIterator_->Advance();
+        iterator_->Next();
       }
 
-      if (!SetStatus(baseIterator_->Status()) || bytesRead == 0) {
+      if (!SetStatus(iterator_->Status()) || bytesRead == 0) {
         break;
       }
 
@@ -1226,11 +1226,11 @@ struct ClearWorker final : public PriorityWorker {
       batch.Clear();
     }
 
-    baseIterator_->End();
+    iterator_->End();
   }
 
 private:
-  BaseIterator* baseIterator_;
+  BaseIterator* iterator_;
   leveldb::WriteOptions* writeOptions_;
 };
 
@@ -1598,22 +1598,21 @@ struct NextWorker final : public BaseWorker {
 
     // Limit the size of the cache to prevent starving the event loop
     // in JS-land while we're recursively calling process.nextTick().
-    ok_ = iterator_->ReadMany(1000, result_);
+    ok_ = iterator_->ReadMany(1000);
 
     if (!ok_) {
       SetStatus(iterator_->Status());
     }
   }
 
   void HandleOKCallback (napi_env env, napi_value callback) override {
-    size_t arraySize = result_.size() * 2;
+    size_t arraySize = iterator_->cache_.size();
     napi_value jsArray;
     napi_create_array_with_length(env, arraySize, &jsArray);
 
-    for (size_t idx = 0; idx < result_.size(); ++idx) {
-      std::pair<std::string, std::string> row = result_[idx];
-      std::string key = row.first;
-      std::string value = row.second;
+    for (size_t idx = 0; idx < iterator_->cache_.size(); idx += 2) {
+      std::string key = iterator_->cache_[idx];
+      std::string value = iterator_->cache_[idx + 1];
 
       napi_value returnKey;
       if (iterator_->keyAsBuffer_) {
@@ -1630,8 +1629,8 @@ struct NextWorker final : public BaseWorker {
       }
 
       // put the key & value in a descending order, so that they can be .pop:ed in javascript-land
-      napi_set_element(env, jsArray, static_cast<int>(arraySize - idx * 2 - 1), returnKey);
-      napi_set_element(env, jsArray, static_cast<int>(arraySize - idx * 2 - 2), returnValue);
+      napi_set_element(env, jsArray, static_cast<int>(arraySize - idx - 1), returnKey);
+      napi_set_element(env, jsArray, static_cast<int>(arraySize - idx - 2), returnValue);
     }
 
     napi_value argv[3];
@@ -1654,7 +1653,6 @@ struct NextWorker final : public BaseWorker {
   }
 
   Iterator* iterator_;
-  std::vector<std::pair<std::string, std::string> > result_;
   bool ok_;
 };