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292 rindas
8.8 KiB
292 rindas
8.8 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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//
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// Decodes the blocks generated by block_builder.cc.
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#include "table/block.h"
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#include <algorithm>
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#include <cstdint>
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#include <vector>
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#include "leveldb/comparator.h"
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#include "table/format.h"
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#include "util/coding.h"
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#include "util/logging.h"
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namespace leveldb {
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inline uint32_t Block::NumRestarts() const {
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assert(size_ >= sizeof(uint32_t));
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return DecodeFixed32(data_ + size_ - sizeof(uint32_t));
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}
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Block::Block(const BlockContents& contents)
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: data_(contents.data.data()),
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size_(contents.data.size()),
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owned_(contents.heap_allocated) {
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if (size_ < sizeof(uint32_t)) {
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size_ = 0; // Error marker
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} else {
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size_t max_restarts_allowed = (size_ - sizeof(uint32_t)) / sizeof(uint32_t);
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if (NumRestarts() > max_restarts_allowed) {
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// The size is too small for NumRestarts()
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size_ = 0;
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} else {
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restart_offset_ = size_ - (1 + NumRestarts()) * sizeof(uint32_t);
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}
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}
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}
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Block::~Block() {
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if (owned_) {
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delete[] data_;
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}
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}
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// Helper routine: decode the next block entry starting at "p",
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// storing the number of shared key bytes, non_shared key bytes,
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// and the length of the value in "*shared", "*non_shared", and
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// "*value_length", respectively. Will not dereference past "limit".
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//
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// If any errors are detected, returns nullptr. Otherwise, returns a
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// pointer to the key delta (just past the three decoded values).
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static inline const char* DecodeEntry(const char* p, const char* limit,
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uint32_t* shared, uint32_t* non_shared,
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uint32_t* value_length) {
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if (limit - p < 3) return nullptr;
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*shared = reinterpret_cast<const uint8_t*>(p)[0];
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*non_shared = reinterpret_cast<const uint8_t*>(p)[1];
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*value_length = reinterpret_cast<const uint8_t*>(p)[2];
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if ((*shared | *non_shared | *value_length) < 128) {
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// Fast path: all three values are encoded in one byte each
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p += 3;
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} else {
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if ((p = GetVarint32Ptr(p, limit, shared)) == nullptr) return nullptr;
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if ((p = GetVarint32Ptr(p, limit, non_shared)) == nullptr) return nullptr;
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if ((p = GetVarint32Ptr(p, limit, value_length)) == nullptr) return nullptr;
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}
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if (static_cast<uint32_t>(limit - p) < (*non_shared + *value_length)) {
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return nullptr;
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}
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return p;
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}
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class Block::Iter : public Iterator {
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private:
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const Comparator* const comparator_;
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const char* const data_; // underlying block contents
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uint32_t const restarts_; // Offset of restart array (list of fixed32)
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uint32_t const num_restarts_; // Number of uint32_t entries in restart array
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// current_ is offset in data_ of current entry. >= restarts_ if !Valid
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uint32_t current_;
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uint32_t restart_index_; // Index of restart block in which current_ falls
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std::string key_;
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Slice value_;
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Status status_;
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inline int Compare(const Slice& a, const Slice& b) const {
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return comparator_->Compare(a, b);
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}
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// Return the offset in data_ just past the end of the current entry.
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inline uint32_t NextEntryOffset() const {
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return (value_.data() + value_.size()) - data_;
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}
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uint32_t GetRestartPoint(uint32_t index) {
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assert(index < num_restarts_);
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return DecodeFixed32(data_ + restarts_ + index * sizeof(uint32_t));
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}
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void SeekToRestartPoint(uint32_t index) {
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key_.clear();
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restart_index_ = index;
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// current_ will be fixed by ParseNextKey();
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// ParseNextKey() starts at the end of value_, so set value_ accordingly
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uint32_t offset = GetRestartPoint(index);
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value_ = Slice(data_ + offset, 0);
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}
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public:
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Iter(const Comparator* comparator, const char* data, uint32_t restarts,
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uint32_t num_restarts)
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: comparator_(comparator),
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data_(data),
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restarts_(restarts),
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num_restarts_(num_restarts),
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current_(restarts_),
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restart_index_(num_restarts_) {
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assert(num_restarts_ > 0);
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}
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bool Valid() const override { return current_ < restarts_; }
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Status status() const override { return status_; }
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Slice key() const override {
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assert(Valid());
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return key_;
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}
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Slice value() const override {
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assert(Valid());
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return value_;
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}
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void Next() override {
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assert(Valid());
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ParseNextKey();
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}
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void Prev() override {
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assert(Valid());
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// Scan backwards to a restart point before current_
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const uint32_t original = current_;
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while (GetRestartPoint(restart_index_) >= original) {
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if (restart_index_ == 0) {
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// No more entries
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current_ = restarts_;
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restart_index_ = num_restarts_;
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return;
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}
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restart_index_--;
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}
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SeekToRestartPoint(restart_index_);
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do {
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// Loop until end of current entry hits the start of original entry
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} while (ParseNextKey() && NextEntryOffset() < original);
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}
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void Seek(const Slice& target) override {
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// Binary search in restart array to find the last restart point
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// with a key < target
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uint32_t left = 0;
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uint32_t right = num_restarts_ - 1;
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int current_key_compare = 0;
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if (Valid()) {
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// If we're already scanning, use the current position as a starting
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// point. This is beneficial if the key we're seeking to is ahead of the
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// current position.
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current_key_compare = Compare(key_, target);
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if (current_key_compare < 0) {
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// key_ is smaller than target
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left = restart_index_;
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} else if (current_key_compare > 0) {
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right = restart_index_;
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} else {
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// We're seeking to the key we're already at.
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return;
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}
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}
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while (left < right) {
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uint32_t mid = (left + right + 1) / 2;
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uint32_t region_offset = GetRestartPoint(mid);
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uint32_t shared, non_shared, value_length;
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const char* key_ptr =
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DecodeEntry(data_ + region_offset, data_ + restarts_, &shared,
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&non_shared, &value_length);
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if (key_ptr == nullptr || (shared != 0)) {
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CorruptionError();
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return;
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}
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Slice mid_key(key_ptr, non_shared);
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if (Compare(mid_key, target) < 0) {
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// Key at "mid" is smaller than "target". Therefore all
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// blocks before "mid" are uninteresting.
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left = mid;
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} else {
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// Key at "mid" is >= "target". Therefore all blocks at or
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// after "mid" are uninteresting.
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right = mid - 1;
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}
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}
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// We might be able to use our current position within the restart block.
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// This is true if we determined the key we desire is in the current block
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// and is after than the current key.
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assert(current_key_compare == 0 || Valid());
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bool skip_seek = left == restart_index_ && current_key_compare < 0;
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if (!skip_seek) {
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SeekToRestartPoint(left);
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}
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// Linear search (within restart block) for first key >= target
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while (true) {
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if (!ParseNextKey()) {
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return;
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}
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if (Compare(key_, target) >= 0) {
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return;
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}
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}
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}
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void SeekToFirst() override {
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SeekToRestartPoint(0);
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ParseNextKey();
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}
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void SeekToLast() override {
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SeekToRestartPoint(num_restarts_ - 1);
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while (ParseNextKey() && NextEntryOffset() < restarts_) {
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// Keep skipping
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}
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}
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private:
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void CorruptionError() {
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current_ = restarts_;
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restart_index_ = num_restarts_;
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status_ = Status::Corruption("bad entry in block");
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key_.clear();
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value_.clear();
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}
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bool ParseNextKey() {
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current_ = NextEntryOffset();
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const char* p = data_ + current_;
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const char* limit = data_ + restarts_; // Restarts come right after data
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if (p >= limit) {
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// No more entries to return. Mark as invalid.
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current_ = restarts_;
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restart_index_ = num_restarts_;
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return false;
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}
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// Decode next entry
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uint32_t shared, non_shared, value_length;
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p = DecodeEntry(p, limit, &shared, &non_shared, &value_length);
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if (p == nullptr || key_.size() < shared) {
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CorruptionError();
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return false;
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} else {
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key_.resize(shared);
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key_.append(p, non_shared);
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value_ = Slice(p + non_shared, value_length);
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while (restart_index_ + 1 < num_restarts_ &&
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GetRestartPoint(restart_index_ + 1) < current_) {
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++restart_index_;
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}
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return true;
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}
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}
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};
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Iterator* Block::NewIterator(const Comparator* comparator) {
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if (size_ < sizeof(uint32_t)) {
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return NewErrorIterator(Status::Corruption("bad block contents"));
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}
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const uint32_t num_restarts = NumRestarts();
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if (num_restarts == 0) {
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return NewEmptyIterator();
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} else {
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return new Iter(comparator, data_, restart_offset_, num_restarts);
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}
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}
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} // namespace leveldb
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