Add dummy and LevelDB database backends

src/leveldb/table/merger.cc

@@ -0,0 +1,197 @@
+// 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/merger.h"
+
+#include "leveldb/comparator.h"
+#include "leveldb/iterator.h"
+#include "table/iterator_wrapper.h"
+
+namespace leveldb {
+
+namespace {
+class MergingIterator : public Iterator {
+ public:
+  MergingIterator(const Comparator* comparator, Iterator** children, int n)
+      : comparator_(comparator),
+        children_(new IteratorWrapper[n]),
+        n_(n),
+        current_(NULL),
+        direction_(kForward) {
+    for (int i = 0; i < n; i++) {
+      children_[i].Set(children[i]);
+    }
+  }
+
+  virtual ~MergingIterator() {
+    delete[] children_;
+  }
+
+  virtual bool Valid() const {
+    return (current_ != NULL);
+  }
+
+  virtual void SeekToFirst() {
+    for (int i = 0; i < n_; i++) {
+      children_[i].SeekToFirst();
+    }
+    FindSmallest();
+    direction_ = kForward;
+  }
+
+  virtual void SeekToLast() {
+    for (int i = 0; i < n_; i++) {
+      children_[i].SeekToLast();
+    }
+    FindLargest();
+    direction_ = kReverse;
+  }
+
+  virtual void Seek(const Slice& target) {
+    for (int i = 0; i < n_; i++) {
+      children_[i].Seek(target);
+    }
+    FindSmallest();
+    direction_ = kForward;
+  }
+
+  virtual void Next() {
+    assert(Valid());
+
+    // Ensure that all children are positioned after key().
+    // If we are moving in the forward direction, it is already
+    // true for all of the non-current_ children since current_ is
+    // the smallest child and key() == current_->key().  Otherwise,
+    // we explicitly position the non-current_ children.
+    if (direction_ != kForward) {
+      for (int i = 0; i < n_; i++) {
+        IteratorWrapper* child = &children_[i];
+        if (child != current_) {
+          child->Seek(key());
+          if (child->Valid() &&
+              comparator_->Compare(key(), child->key()) == 0) {
+            child->Next();
+          }
+        }
+      }
+      direction_ = kForward;
+    }
+
+    current_->Next();
+    FindSmallest();
+  }
+
+  virtual void Prev() {
+    assert(Valid());
+
+    // Ensure that all children are positioned before key().
+    // If we are moving in the reverse direction, it is already
+    // true for all of the non-current_ children since current_ is
+    // the largest child and key() == current_->key().  Otherwise,
+    // we explicitly position the non-current_ children.
+    if (direction_ != kReverse) {
+      for (int i = 0; i < n_; i++) {
+        IteratorWrapper* child = &children_[i];
+        if (child != current_) {
+          child->Seek(key());
+          if (child->Valid()) {
+            // Child is at first entry >= key().  Step back one to be < key()
+            child->Prev();
+          } else {
+            // Child has no entries >= key().  Position at last entry.
+            child->SeekToLast();
+          }
+        }
+      }
+      direction_ = kReverse;
+    }
+
+    current_->Prev();
+    FindLargest();
+  }
+
+  virtual Slice key() const {
+    assert(Valid());
+    return current_->key();
+  }
+
+  virtual Slice value() const {
+    assert(Valid());
+    return current_->value();
+  }
+
+  virtual Status status() const {
+    Status status;
+    for (int i = 0; i < n_; i++) {
+      status = children_[i].status();
+      if (!status.ok()) {
+        break;
+      }
+    }
+    return status;
+  }
+
+ private:
+  void FindSmallest();
+  void FindLargest();
+
+  // We might want to use a heap in case there are lots of children.
+  // For now we use a simple array since we expect a very small number
+  // of children in leveldb.
+  const Comparator* comparator_;
+  IteratorWrapper* children_;
+  int n_;
+  IteratorWrapper* current_;
+
+  // Which direction is the iterator moving?
+  enum Direction {
+    kForward,
+    kReverse
+  };
+  Direction direction_;
+};
+
+void MergingIterator::FindSmallest() {
+  IteratorWrapper* smallest = NULL;
+  for (int i = 0; i < n_; i++) {
+    IteratorWrapper* child = &children_[i];
+    if (child->Valid()) {
+      if (smallest == NULL) {
+        smallest = child;
+      } else if (comparator_->Compare(child->key(), smallest->key()) < 0) {
+        smallest = child;
+      }
+    }
+  }
+  current_ = smallest;
+}
+
+void MergingIterator::FindLargest() {
+  IteratorWrapper* largest = NULL;
+  for (int i = n_-1; i >= 0; i--) {
+    IteratorWrapper* child = &children_[i];
+    if (child->Valid()) {
+      if (largest == NULL) {
+        largest = child;
+      } else if (comparator_->Compare(child->key(), largest->key()) > 0) {
+        largest = child;
+      }
+    }
+  }
+  current_ = largest;
+}
+}  // namespace
+
+Iterator* NewMergingIterator(const Comparator* cmp, Iterator** list, int n) {
+  assert(n >= 0);
+  if (n == 0) {
+    return NewEmptyIterator();
+  } else if (n == 1) {
+    return list[0];
+  } else {
+    return new MergingIterator(cmp, list, n);
+  }
+}
+
+}  // namespace leveldb