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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#include "table/block_based/block_prefix_index.h"
#include <vector>
#include "memory/arena.h"
#include "rocksdb/comparator.h"
#include "rocksdb/slice.h"
#include "rocksdb/slice_transform.h"
#include "util/coding.h"
#include "util/hash.h"
namespace ROCKSDB_NAMESPACE {
inline uint32_t Hash(const Slice& s) {
return ROCKSDB_NAMESPACE::Hash(s.data(), s.size(), 0);
}
inline uint32_t PrefixToBucket(const Slice& prefix, uint32_t num_buckets) {
return Hash(prefix) % num_buckets;
}
// The prefix block index is simply a bucket array, with each entry pointing to
// the blocks that span the prefixes hashed to this bucket.
//
// To reduce memory footprint, if there is only one block per bucket, the entry
// stores the block id directly. If there are more than one blocks per bucket,
// because of hash collision or a single prefix spanning multiple blocks,
// the entry points to an array of block ids. The block array is an array of
// uint32_t's. The first uint32_t indicates the total number of blocks, followed
// by the block ids.
//
// To differentiate the two cases, the high order bit of the entry indicates
// whether it is a 'pointer' into a separate block array.
// 0x7FFFFFFF is reserved for empty bucket.
const uint32_t kNoneBlock = 0x7FFFFFFF;
const uint32_t kBlockArrayMask = 0x80000000;
inline bool IsNone(uint32_t block_id) { return block_id == kNoneBlock; }
inline bool IsBlockId(uint32_t block_id) {
return (block_id & kBlockArrayMask) == 0;
}
inline uint32_t DecodeIndex(uint32_t block_id) {
uint32_t index = block_id ^ kBlockArrayMask;
assert(index < kBlockArrayMask);
return index;
}
inline uint32_t EncodeIndex(uint32_t index) {
assert(index < kBlockArrayMask);
return index | kBlockArrayMask;
}
// temporary storage for prefix information during index building
struct PrefixRecord {
Slice prefix;
uint32_t start_block;
uint32_t end_block;
uint32_t num_blocks;
PrefixRecord* next;
};
class BlockPrefixIndex::Builder {
public:
void Add(const Slice& key_prefix, uint32_t start_block, uint32_t num_blocks) {
PrefixRecord* record = reinterpret_cast<PrefixRecord*>(
arena_.AllocateAligned(sizeof(PrefixRecord)));
record->prefix = key_prefix;
record->start_block = start_block;
record->end_block = start_block + num_blocks - 1;
record->num_blocks = num_blocks;
prefixes_.push_back(record);
}
BlockPrefixIndex* Finish(const SliceTransform* prefix_extractor) {
// For now, use roughly 1:1 prefix to bucket ratio.
uint32_t num_buckets = static_cast<uint32_t>(prefixes_.size()) + 1;
// Collect prefix records that hash to the same bucket, into a single
// linklist.
std::vector<PrefixRecord*> prefixes_per_bucket(num_buckets, nullptr);
std::vector<uint32_t> num_blocks_per_bucket(num_buckets, 0);
for (PrefixRecord* current : prefixes_) {
uint32_t bucket = PrefixToBucket(current->prefix, num_buckets);
// merge the prefix block span if the first block of this prefix is
// connected to the last block of the previous prefix.
PrefixRecord* prev = prefixes_per_bucket[bucket];
if (prev) {
assert(current->start_block >= prev->end_block);
auto distance = current->start_block - prev->end_block;
if (distance <= 1) {
prev->end_block = current->end_block;
prev->num_blocks = prev->end_block - prev->start_block + 1;
num_blocks_per_bucket[bucket] += (current->num_blocks + distance - 1);
continue;
}
}
current->next = prev;
prefixes_per_bucket[bucket] = current;
num_blocks_per_bucket[bucket] += current->num_blocks;
}
// Calculate the block array buffer size
uint32_t total_block_array_entries = 0;
for (uint32_t i = 0; i < num_buckets; i++) {
uint32_t num_blocks = num_blocks_per_bucket[i];
if (num_blocks > 1) {
total_block_array_entries += (num_blocks + 1);
}
}
// Populate the final prefix block index
uint32_t* block_array_buffer = new uint32_t[total_block_array_entries];
uint32_t* buckets = new uint32_t[num_buckets];
uint32_t offset = 0;
for (uint32_t i = 0; i < num_buckets; i++) {
uint32_t num_blocks = num_blocks_per_bucket[i];
if (num_blocks == 0) {
assert(prefixes_per_bucket[i] == nullptr);
buckets[i] = kNoneBlock;
} else if (num_blocks == 1) {
assert(prefixes_per_bucket[i] != nullptr);
assert(prefixes_per_bucket[i]->next == nullptr);
buckets[i] = prefixes_per_bucket[i]->start_block;
} else {
assert(total_block_array_entries > 0);
assert(prefixes_per_bucket[i] != nullptr);
buckets[i] = EncodeIndex(offset);
block_array_buffer[offset] = num_blocks;
uint32_t* last_block = &block_array_buffer[offset + num_blocks];
auto current = prefixes_per_bucket[i];
// populate block ids from largest to smallest
while (current != nullptr) {
for (uint32_t iter = 0; iter < current->num_blocks; iter++) {
*last_block = current->end_block - iter;
last_block--;
}
current = current->next;
}
assert(last_block == &block_array_buffer[offset]);
offset += (num_blocks + 1);
}
}
assert(offset == total_block_array_entries);
return new BlockPrefixIndex(prefix_extractor, num_buckets, buckets,
total_block_array_entries, block_array_buffer);
}
private:
std::vector<PrefixRecord*> prefixes_;
Arena arena_;
};
Status BlockPrefixIndex::Create(const SliceTransform* prefix_extractor,
const Slice& prefixes, const Slice& prefix_meta,
BlockPrefixIndex** prefix_index) {
uint64_t pos = 0;
auto meta_pos = prefix_meta;
Status s;
Builder builder;
while (!meta_pos.empty()) {
uint32_t prefix_size = 0;
uint32_t entry_index = 0;
uint32_t num_blocks = 0;
if (!GetVarint32(&meta_pos, &prefix_size) ||
!GetVarint32(&meta_pos, &entry_index) ||
!GetVarint32(&meta_pos, &num_blocks)) {
s = Status::Corruption(
"Corrupted prefix meta block: unable to read from it.");
break;
}
if (pos + prefix_size > prefixes.size()) {
s = Status::Corruption(
"Corrupted prefix meta block: size inconsistency.");
break;
}
Slice prefix(prefixes.data() + pos, prefix_size);
builder.Add(prefix, entry_index, num_blocks);
pos += prefix_size;
}
if (s.ok() && pos != prefixes.size()) {
s = Status::Corruption("Corrupted prefix meta block");
}
if (s.ok()) {
*prefix_index = builder.Finish(prefix_extractor);
}
return s;
}
uint32_t BlockPrefixIndex::GetBlocks(const Slice& key, uint32_t** blocks) {
Slice prefix = internal_prefix_extractor_.Transform(key);
uint32_t bucket = PrefixToBucket(prefix, num_buckets_);
uint32_t block_id = buckets_[bucket];
if (IsNone(block_id)) {
return 0;
} else if (IsBlockId(block_id)) {
*blocks = &buckets_[bucket];
return 1;
} else {
uint32_t index = DecodeIndex(block_id);
assert(index < num_block_array_buffer_entries_);
*blocks = &block_array_buffer_[index + 1];
uint32_t num_blocks = block_array_buffer_[index];
assert(num_blocks > 1);
assert(index + num_blocks < num_block_array_buffer_entries_);
return num_blocks;
}
}
} // namespace ROCKSDB_NAMESPACE
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