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// 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_(nullptr),
direction_(kForward) {
for (int i = 0; i < n; i++) {
children_[i].Set(children[i]);
}
}
~MergingIterator() override { delete[] children_; }
bool Valid() const override { return (current_ != nullptr); }
void SeekToFirst() override {
for (int i = 0; i < n_; i++) {
children_[i].SeekToFirst();
}
FindSmallest();
direction_ = kForward;
}
void SeekToLast() override {
for (int i = 0; i < n_; i++) {
children_[i].SeekToLast();
}
FindLargest();
direction_ = kReverse;
}
void Seek(const Slice& target) override {
for (int i = 0; i < n_; i++) {
children_[i].Seek(target);
}
FindSmallest();
direction_ = kForward;
}
void Next() override {
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();
}
void Prev() override {
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();
}
Slice key() const override {
assert(Valid());
return current_->key();
}
Slice value() const override {
assert(Valid());
return current_->value();
}
Status status() const override {
Status status;
for (int i = 0; i < n_; i++) {
status = children_[i].status();
if (!status.ok()) {
break;
}
}
return status;
}
private:
// Which direction is the iterator moving?
enum Direction { kForward, kReverse };
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_;
Direction direction_;
};
void MergingIterator::FindSmallest() {
IteratorWrapper* smallest = nullptr;
for (int i = 0; i < n_; i++) {
IteratorWrapper* child = &children_[i];
if (child->Valid()) {
if (smallest == nullptr) {
smallest = child;
} else if (comparator_->Compare(child->key(), smallest->key()) < 0) {
smallest = child;
}
}
}
current_ = smallest;
}
void MergingIterator::FindLargest() {
IteratorWrapper* largest = nullptr;
for (int i = n_ - 1; i >= 0; i--) {
IteratorWrapper* child = &children_[i];
if (child->Valid()) {
if (largest == nullptr) {
largest = child;
} else if (comparator_->Compare(child->key(), largest->key()) > 0) {
largest = child;
}
}
}
current_ = largest;
}
} // namespace
Iterator* NewMergingIterator(const Comparator* comparator, Iterator** children,
int n) {
assert(n >= 0);
if (n == 0) {
return NewEmptyIterator();
} else if (n == 1) {
return children[0];
} else {
return new MergingIterator(comparator, children, n);
}
}
} // namespace leveldb
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