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