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191 righe
4.9 KiB
191 righe
4.9 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#include "table/merger.h"
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#include "leveldb/comparator.h"
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#include "leveldb/iterator.h"
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#include "table/iterator_wrapper.h"
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namespace leveldb {
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namespace {
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class MergingIterator : public Iterator {
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public:
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MergingIterator(const Comparator* comparator, Iterator** children, int n)
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: comparator_(comparator),
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children_(new IteratorWrapper[n]),
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n_(n),
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current_(nullptr),
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direction_(kForward) {
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for (int i = 0; i < n; i++) {
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children_[i].Set(children[i]);
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}
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}
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~MergingIterator() override { delete[] children_; }
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bool Valid() const override { return (current_ != nullptr); }
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void SeekToFirst() override {
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for (int i = 0; i < n_; i++) {
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children_[i].SeekToFirst();
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}
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FindSmallest();
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direction_ = kForward;
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}
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void SeekToLast() override {
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for (int i = 0; i < n_; i++) {
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children_[i].SeekToLast();
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}
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FindLargest();
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direction_ = kReverse;
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}
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void Seek(const Slice& target) override {
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for (int i = 0; i < n_; i++) {
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children_[i].Seek(target);
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}
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FindSmallest();
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direction_ = kForward;
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}
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void Next() override {
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assert(Valid());
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// Ensure that all children are positioned after key().
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// If we are moving in the forward direction, it is already
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// true for all of the non-current_ children since current_ is
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// the smallest child and key() == current_->key(). Otherwise,
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// we explicitly position the non-current_ children.
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if (direction_ != kForward) {
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for (int i = 0; i < n_; i++) {
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IteratorWrapper* child = &children_[i];
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if (child != current_) {
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child->Seek(key());
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if (child->Valid() &&
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comparator_->Compare(key(), child->key()) == 0) {
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child->Next();
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}
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}
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}
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direction_ = kForward;
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}
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current_->Next();
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FindSmallest();
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}
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void Prev() override {
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assert(Valid());
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// Ensure that all children are positioned before key().
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// If we are moving in the reverse direction, it is already
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// true for all of the non-current_ children since current_ is
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// the largest child and key() == current_->key(). Otherwise,
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// we explicitly position the non-current_ children.
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if (direction_ != kReverse) {
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for (int i = 0; i < n_; i++) {
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IteratorWrapper* child = &children_[i];
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if (child != current_) {
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child->Seek(key());
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if (child->Valid()) {
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// Child is at first entry >= key(). Step back one to be < key()
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child->Prev();
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} else {
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// Child has no entries >= key(). Position at last entry.
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child->SeekToLast();
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}
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}
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}
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direction_ = kReverse;
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}
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current_->Prev();
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FindLargest();
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}
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Slice key() const override {
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assert(Valid());
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return current_->key();
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}
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Slice value() const override {
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assert(Valid());
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return current_->value();
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}
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Status status() const override {
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Status status;
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for (int i = 0; i < n_; i++) {
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status = children_[i].status();
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if (!status.ok()) {
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break;
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}
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}
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return status;
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}
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private:
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// Which direction is the iterator moving?
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enum Direction { kForward, kReverse };
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void FindSmallest();
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void FindLargest();
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// We might want to use a heap in case there are lots of children.
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// For now we use a simple array since we expect a very small number
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// of children in leveldb.
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const Comparator* comparator_;
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IteratorWrapper* children_;
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int n_;
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IteratorWrapper* current_;
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Direction direction_;
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};
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void MergingIterator::FindSmallest() {
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IteratorWrapper* smallest = nullptr;
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for (int i = 0; i < n_; i++) {
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IteratorWrapper* child = &children_[i];
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if (child->Valid()) {
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if (smallest == nullptr) {
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smallest = child;
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} else if (comparator_->Compare(child->key(), smallest->key()) < 0) {
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smallest = child;
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}
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}
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}
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current_ = smallest;
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}
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void MergingIterator::FindLargest() {
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IteratorWrapper* largest = nullptr;
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for (int i = n_ - 1; i >= 0; i--) {
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IteratorWrapper* child = &children_[i];
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if (child->Valid()) {
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if (largest == nullptr) {
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largest = child;
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} else if (comparator_->Compare(child->key(), largest->key()) > 0) {
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largest = child;
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}
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}
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}
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current_ = largest;
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}
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} // namespace
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Iterator* NewMergingIterator(const Comparator* comparator, Iterator** children,
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int n) {
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assert(n >= 0);
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if (n == 0) {
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return NewEmptyIterator();
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} else if (n == 1) {
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return children[0];
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} else {
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return new MergingIterator(comparator, children, n);
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}
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}
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} // namespace leveldb
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