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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).
//
// 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.
//
// InlineSkipList is derived from SkipList (skiplist.h), but it optimizes
// the memory layout by requiring that the key storage be allocated through
// the skip list instance. For the common case of SkipList<const char*,
// Cmp> this saves 1 pointer per skip list node and gives better cache
// locality, at the expense of wasted padding from using AllocateAligned
// instead of Allocate for the keys. The unused padding will be from
// 0 to sizeof(void*)-1 bytes, and the space savings are sizeof(void*)
// bytes, so despite the padding the space used is always less than
// SkipList<const char*, ..>.
//
// Thread safety -------------
//
// Writes via Insert require external synchronization, most likely a mutex.
// InsertConcurrently can be safely called concurrently with reads and
// with other concurrent inserts. Reads require a guarantee that the
// InlineSkipList will not be destroyed while the read is in progress.
// Apart from that, reads progress without any internal locking or
// synchronization.
//
// Invariants:
//
// (1) Allocated nodes are never deleted until the InlineSkipList is
// destroyed. This is trivially guaranteed by the code since we never
// delete any skip list nodes.
//
// (2) The contents of a Node except for the next/prev pointers are
// immutable after the Node has been linked into the InlineSkipList.
// Only Insert() modifies the list, and it is careful to initialize a
// node and use release-stores to publish the nodes in one or more lists.
//
// ... prev vs. next pointer ordering ...
//
#pragma once
#include <assert.h>
#include <stdlib.h>
#include <algorithm>
#include <atomic>
#include <type_traits>
#include "port/likely.h"
#include "port/port.h"
#include "rocksdb/slice.h"
#include "util/allocator.h"
#include "util/coding.h"
#include "util/random.h"
namespace rocksdb {
template <class Comparator>
class InlineSkipList {
private:
struct Node;
struct Splice;
public:
using DecodedKey = \
typename std::remove_reference<Comparator>::type::DecodedType;
static const uint16_t kMaxPossibleHeight = 32;
// Create a new InlineSkipList object that will use "cmp" for comparing
// keys, and will allocate memory using "*allocator". Objects allocated
// in the allocator must remain allocated for the lifetime of the
// skiplist object.
explicit InlineSkipList(Comparator cmp, Allocator* allocator,
int32_t max_height = 12,
int32_t branching_factor = 4);
// Allocates a key and a skip-list node, returning a pointer to the key
// portion of the node. This method is thread-safe if the allocator
// is thread-safe.
char* AllocateKey(size_t key_size);
// Allocate a splice using allocator.
Splice* AllocateSplice();
// Inserts a key allocated by AllocateKey, after the actual key value
// has been filled in.
//
// REQUIRES: nothing that compares equal to key is currently in the list.
// REQUIRES: no concurrent calls to any of inserts.
bool Insert(const char* key);
// Inserts a key allocated by AllocateKey with a hint of last insert
// position in the skip-list. If hint points to nullptr, a new hint will be
// populated, which can be used in subsequent calls.
//
// It can be used to optimize the workload where there are multiple groups
// of keys, and each key is likely to insert to a location close to the last
// inserted key in the same group. One example is sequential inserts.
//
// REQUIRES: nothing that compares equal to key is currently in the list.
// REQUIRES: no concurrent calls to any of inserts.
bool InsertWithHint(const char* key, void** hint);
// Like Insert, but external synchronization is not required.
bool InsertConcurrently(const char* key);
// Inserts a node into the skip list. key must have been allocated by
// AllocateKey and then filled in by the caller. If UseCAS is true,
// then external synchronization is not required, otherwise this method
// may not be called concurrently with any other insertions.
//
// Regardless of whether UseCAS is true, the splice must be owned
// exclusively by the current thread. If allow_partial_splice_fix is
// true, then the cost of insertion is amortized O(log D), where D is
// the distance from the splice to the inserted key (measured as the
// number of intervening nodes). Note that this bound is very good for
// sequential insertions! If allow_partial_splice_fix is false then
// the existing splice will be ignored unless the current key is being
// inserted immediately after the splice. allow_partial_splice_fix ==
// false has worse running time for the non-sequential case O(log N),
// but a better constant factor.
template <bool UseCAS>
bool Insert(const char* key, Splice* splice, bool allow_partial_splice_fix);
// Returns true iff an entry that compares equal to key is in the list.
bool Contains(const char* key) const;
// Return estimated number of entries smaller than `key`.
uint64_t EstimateCount(const char* key) const;
// Validate correctness of the skip-list.
void TEST_Validate() const;
// Iteration over the contents of a skip list
class Iterator {
public:
// Initialize an iterator over the specified list.
// The returned iterator is not valid.
explicit Iterator(const InlineSkipList* list);
// Change the underlying skiplist used for this iterator
// This enables us not changing the iterator without deallocating
// an old one and then allocating a new one
void SetList(const InlineSkipList* list);
// Returns true iff the iterator is positioned at a valid node.
bool Valid() const;
// Returns the key at the current position.
// REQUIRES: Valid()
const char* key() const;
// Advances to the next position.
// REQUIRES: Valid()
void Next();
// Advances to the previous position.
// REQUIRES: Valid()
void Prev();
// Advance to the first entry with a key >= target
void Seek(const char* target);
// Retreat to the last entry with a key <= target
void SeekForPrev(const char* target);
// Position at the first entry in list.
// Final state of iterator is Valid() iff list is not empty.
void SeekToFirst();
// Position at the last entry in list.
// Final state of iterator is Valid() iff list is not empty.
void SeekToLast();
private:
const InlineSkipList* list_;
Node* node_;
// Intentionally copyable
};
private:
const uint16_t kMaxHeight_;
const uint16_t kBranching_;
const uint32_t kScaledInverseBranching_;
Allocator* const allocator_; // Allocator used for allocations of nodes
// Immutable after construction
Comparator const compare_;
Node* const head_;
// Modified only by Insert(). Read racily by readers, but stale
// values are ok.
std::atomic<int> max_height_; // Height of the entire list
// seq_splice_ is a Splice used for insertions in the non-concurrent
// case. It caches the prev and next found during the most recent
// non-concurrent insertion.
Splice* seq_splice_;
inline int GetMaxHeight() const {
return max_height_.load(std::memory_order_relaxed);
}
int RandomHeight();
Node* AllocateNode(size_t key_size, int height);
bool Equal(const char* a, const char* b) const {
return (compare_(a, b) == 0);
}
bool LessThan(const char* a, const char* b) const {
return (compare_(a, b) < 0);
}
// Return true if key is greater than the data stored in "n". Null n
// is considered infinite. n should not be head_.
bool KeyIsAfterNode(const char* key, Node* n) const;
bool KeyIsAfterNode(const DecodedKey& key, Node* n) const;
// Returns the earliest node with a key >= key.
// Return nullptr if there is no such node.
Node* FindGreaterOrEqual(const char* key) const;
// Return the latest node with a key < key.
// Return head_ if there is no such node.
// Fills prev[level] with pointer to previous node at "level" for every
// level in [0..max_height_-1], if prev is non-null.
Node* FindLessThan(const char* key, Node** prev = nullptr) const;
// Return the latest node with a key < key on bottom_level. Start searching
// from root node on the level below top_level.
// Fills prev[level] with pointer to previous node at "level" for every
// level in [bottom_level..top_level-1], if prev is non-null.
Node* FindLessThan(const char* key, Node** prev, Node* root, int top_level,
int bottom_level) const;
// Return the last node in the list.
// Return head_ if list is empty.
Node* FindLast() const;
// Traverses a single level of the list, setting *out_prev to the last
// node before the key and *out_next to the first node after. Assumes
// that the key is not present in the skip list. On entry, before should
// point to a node that is before the key, and after should point to
// a node that is after the key. after should be nullptr if a good after
// node isn't conveniently available.
template<bool prefetch_before>
void FindSpliceForLevel(const DecodedKey& key, Node* before, Node* after, int level,
Node** out_prev, Node** out_next);
// Recomputes Splice levels from highest_level (inclusive) down to
// lowest_level (inclusive).
void RecomputeSpliceLevels(const DecodedKey& key, Splice* splice,
int recompute_level);
// No copying allowed
InlineSkipList(const InlineSkipList&);
InlineSkipList& operator=(const InlineSkipList&);
};
// Implementation details follow
template <class Comparator>
struct InlineSkipList<Comparator>::Splice {
// The invariant of a Splice is that prev_[i+1].key <= prev_[i].key <
// next_[i].key <= next_[i+1].key for all i. That means that if a
// key is bracketed by prev_[i] and next_[i] then it is bracketed by
// all higher levels. It is _not_ required that prev_[i]->Next(i) ==
// next_[i] (it probably did at some point in the past, but intervening
// or concurrent operations might have inserted nodes in between).
int height_ = 0;
Node** prev_;
Node** next_;
};
// The Node data type is more of a pointer into custom-managed memory than
// a traditional C++ struct. The key is stored in the bytes immediately
// after the struct, and the next_ pointers for nodes with height > 1 are
// stored immediately _before_ the struct. This avoids the need to include
// any pointer or sizing data, which reduces per-node memory overheads.
template <class Comparator>
struct InlineSkipList<Comparator>::Node {
// Stores the height of the node in the memory location normally used for
// next_[0]. This is used for passing data from AllocateKey to Insert.
void StashHeight(const int height) {
assert(sizeof(int) <= sizeof(next_[0]));
memcpy(static_cast<void*>(&next_[0]), &height, sizeof(int));
}
// Retrieves the value passed to StashHeight. Undefined after a call
// to SetNext or NoBarrier_SetNext.
int UnstashHeight() const {
int rv;
memcpy(&rv, &next_[0], sizeof(int));
return rv;
}
const char* Key() const { return reinterpret_cast<const char*>(&next_[1]); }
// Accessors/mutators for links. Wrapped in methods so we can add
// the appropriate barriers as necessary, and perform the necessary
// addressing trickery for storing links below the Node in memory.
Node* Next(int n) {
assert(n >= 0);
// Use an 'acquire load' so that we observe a fully initialized
// version of the returned Node.
return ((&next_[0] - n)->load(std::memory_order_acquire));
}
void SetNext(int n, Node* x) {
assert(n >= 0);
// Use a 'release store' so that anybody who reads through this
// pointer observes a fully initialized version of the inserted node.
(&next_[0] - n)->store(x, std::memory_order_release);
}
bool CASNext(int n, Node* expected, Node* x) {
assert(n >= 0);
return (&next_[0] - n)->compare_exchange_strong(expected, x);
}
// No-barrier variants that can be safely used in a few locations.
Node* NoBarrier_Next(int n) {
assert(n >= 0);
return (&next_[0] - n)->load(std::memory_order_relaxed);
}
void NoBarrier_SetNext(int n, Node* x) {
assert(n >= 0);
(&next_[0] - n)->store(x, std::memory_order_relaxed);
}
// Insert node after prev on specific level.
void InsertAfter(Node* prev, int level) {
// NoBarrier_SetNext() suffices since we will add a barrier when
// we publish a pointer to "this" in prev.
NoBarrier_SetNext(level, prev->NoBarrier_Next(level));
prev->SetNext(level, this);
}
private:
// next_[0] is the lowest level link (level 0). Higher levels are
// stored _earlier_, so level 1 is at next_[-1].
std::atomic<Node*> next_[1];
};
template <class Comparator>
inline InlineSkipList<Comparator>::Iterator::Iterator(
const InlineSkipList* list) {
SetList(list);
}
template <class Comparator>
inline void InlineSkipList<Comparator>::Iterator::SetList(
const InlineSkipList* list) {
list_ = list;
node_ = nullptr;
}
template <class Comparator>
inline bool InlineSkipList<Comparator>::Iterator::Valid() const {
return node_ != nullptr;
}
template <class Comparator>
inline const char* InlineSkipList<Comparator>::Iterator::key() const {
assert(Valid());
return node_->Key();
}
template <class Comparator>
inline void InlineSkipList<Comparator>::Iterator::Next() {
assert(Valid());
node_ = node_->Next(0);
}
template <class Comparator>
inline void InlineSkipList<Comparator>::Iterator::Prev() {
// Instead of using explicit "prev" links, we just search for the
// last node that falls before key.
assert(Valid());
node_ = list_->FindLessThan(node_->Key());
if (node_ == list_->head_) {
node_ = nullptr;
}
}
template <class Comparator>
inline void InlineSkipList<Comparator>::Iterator::Seek(const char* target) {
node_ = list_->FindGreaterOrEqual(target);
}
template <class Comparator>
inline void InlineSkipList<Comparator>::Iterator::SeekForPrev(
const char* target) {
Seek(target);
if (!Valid()) {
SeekToLast();
}
while (Valid() && list_->LessThan(target, key())) {
Prev();
}
}
template <class Comparator>
inline void InlineSkipList<Comparator>::Iterator::SeekToFirst() {
node_ = list_->head_->Next(0);
}
template <class Comparator>
inline void InlineSkipList<Comparator>::Iterator::SeekToLast() {
node_ = list_->FindLast();
if (node_ == list_->head_) {
node_ = nullptr;
}
}
template <class Comparator>
int InlineSkipList<Comparator>::RandomHeight() {
auto rnd = Random::GetTLSInstance();
// Increase height with probability 1 in kBranching
int height = 1;
while (height < kMaxHeight_ && height < kMaxPossibleHeight &&
rnd->Next() < kScaledInverseBranching_) {
height++;
}
assert(height > 0);
assert(height <= kMaxHeight_);
assert(height <= kMaxPossibleHeight);
return height;
}
template <class Comparator>
bool InlineSkipList<Comparator>::KeyIsAfterNode(const char* key,
Node* n) const {
// nullptr n is considered infinite
assert(n != head_);
return (n != nullptr) && (compare_(n->Key(), key) < 0);
}
template <class Comparator>
bool InlineSkipList<Comparator>::KeyIsAfterNode(const DecodedKey& key,
Node* n) const {
// nullptr n is considered infinite
assert(n != head_);
return (n != nullptr) && (compare_(n->Key(), key) < 0);
}
template <class Comparator>
typename InlineSkipList<Comparator>::Node*
InlineSkipList<Comparator>::FindGreaterOrEqual(const char* key) const {
// Note: It looks like we could reduce duplication by implementing
// this function as FindLessThan(key)->Next(0), but we wouldn't be able
// to exit early on equality and the result wouldn't even be correct.
// A concurrent insert might occur after FindLessThan(key) but before
// we get a chance to call Next(0).
Node* x = head_;
int level = GetMaxHeight() - 1;
Node* last_bigger = nullptr;
const DecodedKey key_decoded = compare_.decode_key(key);
while (true) {
Node* next = x->Next(level);
if (next != nullptr) {
PREFETCH(next->Next(level), 0, 1);
}
// Make sure the lists are sorted
assert(x == head_ || next == nullptr || KeyIsAfterNode(next->Key(), x));
// Make sure we haven't overshot during our search
assert(x == head_ || KeyIsAfterNode(key_decoded, x));
int cmp = (next == nullptr || next == last_bigger)
? 1
: compare_(next->Key(), key_decoded);
if (cmp == 0 || (cmp > 0 && level == 0)) {
return next;
} else if (cmp < 0) {
// Keep searching in this list
x = next;
} else {
// Switch to next list, reuse compare_() result
last_bigger = next;
level--;
}
}
}
template <class Comparator>
typename InlineSkipList<Comparator>::Node*
InlineSkipList<Comparator>::FindLessThan(const char* key, Node** prev) const {
return FindLessThan(key, prev, head_, GetMaxHeight(), 0);
}
template <class Comparator>
typename InlineSkipList<Comparator>::Node*
InlineSkipList<Comparator>::FindLessThan(const char* key, Node** prev,
Node* root, int top_level,
int bottom_level) const {
assert(top_level > bottom_level);
int level = top_level - 1;
Node* x = root;
// KeyIsAfter(key, last_not_after) is definitely false
Node* last_not_after = nullptr;
const DecodedKey key_decoded = compare_.decode_key(key);
while (true) {
assert(x != nullptr);
Node* next = x->Next(level);
if (next != nullptr) {
PREFETCH(next->Next(level), 0, 1);
}
assert(x == head_ || next == nullptr || KeyIsAfterNode(next->Key(), x));
assert(x == head_ || KeyIsAfterNode(key_decoded, x));
if (next != last_not_after && KeyIsAfterNode(key_decoded, next)) {
// Keep searching in this list
assert(next != nullptr);
x = next;
} else {
if (prev != nullptr) {
prev[level] = x;
}
if (level == bottom_level) {
return x;
} else {
// Switch to next list, reuse KeyIsAfterNode() result
last_not_after = next;
level--;
}
}
}
}
template <class Comparator>
typename InlineSkipList<Comparator>::Node*
InlineSkipList<Comparator>::FindLast() const {
Node* x = head_;
int level = GetMaxHeight() - 1;
while (true) {
Node* next = x->Next(level);
if (next == nullptr) {
if (level == 0) {
return x;
} else {
// Switch to next list
level--;
}
} else {
x = next;
}
}
}
template <class Comparator>
uint64_t InlineSkipList<Comparator>::EstimateCount(const char* key) const {
uint64_t count = 0;
Node* x = head_;
int level = GetMaxHeight() - 1;
const DecodedKey key_decoded = compare_.decode_key(key);
while (true) {
assert(x == head_ || compare_(x->Key(), key_decoded) < 0);
Node* next = x->Next(level);
if (next != nullptr) {
PREFETCH(next->Next(level), 0, 1);
}
if (next == nullptr || compare_(next->Key(), key_decoded) >= 0) {
if (level == 0) {
return count;
} else {
// Switch to next list
count *= kBranching_;
level--;
}
} else {
x = next;
count++;
}
}
}
template <class Comparator>
InlineSkipList<Comparator>::InlineSkipList(const Comparator cmp,
Allocator* allocator,
int32_t max_height,
int32_t branching_factor)
: kMaxHeight_(static_cast<uint16_t>(max_height)),
kBranching_(static_cast<uint16_t>(branching_factor)),
kScaledInverseBranching_((Random::kMaxNext + 1) / kBranching_),
allocator_(allocator),
compare_(cmp),
head_(AllocateNode(0, max_height)),
max_height_(1),
seq_splice_(AllocateSplice()) {
assert(max_height > 0 && kMaxHeight_ == static_cast<uint32_t>(max_height));
assert(branching_factor > 1 &&
kBranching_ == static_cast<uint32_t>(branching_factor));
assert(kScaledInverseBranching_ > 0);
for (int i = 0; i < kMaxHeight_; ++i) {
head_->SetNext(i, nullptr);
}
}
template <class Comparator>
char* InlineSkipList<Comparator>::AllocateKey(size_t key_size) {
return const_cast<char*>(AllocateNode(key_size, RandomHeight())->Key());
}
template <class Comparator>
typename InlineSkipList<Comparator>::Node*
InlineSkipList<Comparator>::AllocateNode(size_t key_size, int height) {
auto prefix = sizeof(std::atomic<Node*>) * (height - 1);
// prefix is space for the height - 1 pointers that we store before
// the Node instance (next_[-(height - 1) .. -1]). Node starts at
// raw + prefix, and holds the bottom-mode (level 0) skip list pointer
// next_[0]. key_size is the bytes for the key, which comes just after
// the Node.
char* raw = allocator_->AllocateAligned(prefix + sizeof(Node) + key_size);
Node* x = reinterpret_cast<Node*>(raw + prefix);
// Once we've linked the node into the skip list we don't actually need
// to know its height, because we can implicitly use the fact that we
// traversed into a node at level h to known that h is a valid level
// for that node. We need to convey the height to the Insert step,
// however, so that it can perform the proper links. Since we're not
// using the pointers at the moment, StashHeight temporarily borrow
// storage from next_[0] for that purpose.
x->StashHeight(height);
return x;
}
template <class Comparator>
typename InlineSkipList<Comparator>::Splice*
InlineSkipList<Comparator>::AllocateSplice() {
// size of prev_ and next_
size_t array_size = sizeof(Node*) * (kMaxHeight_ + 1);
char* raw = allocator_->AllocateAligned(sizeof(Splice) + array_size * 2);
Splice* splice = reinterpret_cast<Splice*>(raw);
splice->height_ = 0;
splice->prev_ = reinterpret_cast<Node**>(raw + sizeof(Splice));
splice->next_ = reinterpret_cast<Node**>(raw + sizeof(Splice) + array_size);
return splice;
}
template <class Comparator>
bool InlineSkipList<Comparator>::Insert(const char* key) {
return Insert<false>(key, seq_splice_, false);
}
template <class Comparator>
bool InlineSkipList<Comparator>::InsertConcurrently(const char* key) {
Node* prev[kMaxPossibleHeight];
Node* next[kMaxPossibleHeight];
Splice splice;
splice.prev_ = prev;
splice.next_ = next;
return Insert<true>(key, &splice, false);
}
template <class Comparator>
bool InlineSkipList<Comparator>::InsertWithHint(const char* key, void** hint) {
assert(hint != nullptr);
Splice* splice = reinterpret_cast<Splice*>(*hint);
if (splice == nullptr) {
splice = AllocateSplice();
*hint = reinterpret_cast<void*>(splice);
}
return Insert<false>(key, splice, true);
}
template <class Comparator>
template <bool prefetch_before>
void InlineSkipList<Comparator>::FindSpliceForLevel(const DecodedKey& key,
Node* before, Node* after,
int level, Node** out_prev,
Node** out_next) {
while (true) {
Node* next = before->Next(level);
if (next != nullptr) {
PREFETCH(next->Next(level), 0, 1);
}
if (prefetch_before == true) {
if (next != nullptr && level>0) {
PREFETCH(next->Next(level-1), 0, 1);
}
}
assert(before == head_ || next == nullptr ||
KeyIsAfterNode(next->Key(), before));
assert(before == head_ || KeyIsAfterNode(key, before));
if (next == after || !KeyIsAfterNode(key, next)) {
// found it
*out_prev = before;
*out_next = next;
return;
}
before = next;
}
}
template <class Comparator>
void InlineSkipList<Comparator>::RecomputeSpliceLevels(const DecodedKey& key,
Splice* splice,
int recompute_level) {
assert(recompute_level > 0);
assert(recompute_level <= splice->height_);
for (int i = recompute_level - 1; i >= 0; --i) {
FindSpliceForLevel<true>(key, splice->prev_[i + 1], splice->next_[i + 1], i,
&splice->prev_[i], &splice->next_[i]);
}
}
template <class Comparator>
template <bool UseCAS>
bool InlineSkipList<Comparator>::Insert(const char* key, Splice* splice,
bool allow_partial_splice_fix) {
Node* x = reinterpret_cast<Node*>(const_cast<char*>(key)) - 1;
const DecodedKey key_decoded = compare_.decode_key(key);
int height = x->UnstashHeight();
assert(height >= 1 && height <= kMaxHeight_);
int max_height = max_height_.load(std::memory_order_relaxed);
while (height > max_height) {
if (max_height_.compare_exchange_weak(max_height, height)) {
// successfully updated it
max_height = height;
break;
}
// else retry, possibly exiting the loop because somebody else
// increased it
}
assert(max_height <= kMaxPossibleHeight);
int recompute_height = 0;
if (splice->height_ < max_height) {
// Either splice has never been used or max_height has grown since
// last use. We could potentially fix it in the latter case, but
// that is tricky.
splice->prev_[max_height] = head_;
splice->next_[max_height] = nullptr;
splice->height_ = max_height;
recompute_height = max_height;
} else {
// Splice is a valid proper-height splice that brackets some
// key, but does it bracket this one? We need to validate it and
// recompute a portion of the splice (levels 0..recompute_height-1)
// that is a superset of all levels that don't bracket the new key.
// Several choices are reasonable, because we have to balance the work
// saved against the extra comparisons required to validate the Splice.
//
// One strategy is just to recompute all of orig_splice_height if the
// bottom level isn't bracketing. This pessimistically assumes that
// we will either get a perfect Splice hit (increasing sequential
// inserts) or have no locality.
//
// Another strategy is to walk up the Splice's levels until we find
// a level that brackets the key. This strategy lets the Splice
// hint help for other cases: it turns insertion from O(log N) into
// O(log D), where D is the number of nodes in between the key that
// produced the Splice and the current insert (insertion is aided
// whether the new key is before or after the splice). If you have
// a way of using a prefix of the key to map directly to the closest
// Splice out of O(sqrt(N)) Splices and we make it so that splices
// can also be used as hints during read, then we end up with Oshman's
// and Shavit's SkipTrie, which has O(log log N) lookup and insertion
// (compare to O(log N) for skip list).
//
// We control the pessimistic strategy with allow_partial_splice_fix.
// A good strategy is probably to be pessimistic for seq_splice_,
// optimistic if the caller actually went to the work of providing
// a Splice.
while (recompute_height < max_height) {
if (splice->prev_[recompute_height]->Next(recompute_height) !=
splice->next_[recompute_height]) {
// splice isn't tight at this level, there must have been some inserts
// to this
// location that didn't update the splice. We might only be a little
// stale, but if
// the splice is very stale it would be O(N) to fix it. We haven't used
// up any of
// our budget of comparisons, so always move up even if we are
// pessimistic about
// our chances of success.
++recompute_height;
} else if (splice->prev_[recompute_height] != head_ &&
!KeyIsAfterNode(key_decoded,
splice->prev_[recompute_height])) {
// key is from before splice
if (allow_partial_splice_fix) {
// skip all levels with the same node without more comparisons
Node* bad = splice->prev_[recompute_height];
while (splice->prev_[recompute_height] == bad) {
++recompute_height;
}
} else {
// we're pessimistic, recompute everything
recompute_height = max_height;
}
} else if (KeyIsAfterNode(key_decoded,
splice->next_[recompute_height])) {
// key is from after splice
if (allow_partial_splice_fix) {
Node* bad = splice->next_[recompute_height];
while (splice->next_[recompute_height] == bad) {
++recompute_height;
}
} else {
recompute_height = max_height;
}
} else {
// this level brackets the key, we won!
break;
}
}
}
assert(recompute_height <= max_height);
if (recompute_height > 0) {
RecomputeSpliceLevels(key_decoded, splice, recompute_height);
}
bool splice_is_valid = true;
if (UseCAS) {
for (int i = 0; i < height; ++i) {
while (true) {
// Checking for duplicate keys on the level 0 is sufficient
if (UNLIKELY(i == 0 && splice->next_[i] != nullptr &&
compare_(x->Key(), splice->next_[i]->Key()) >= 0)) {
// duplicate key
return false;
}
if (UNLIKELY(i == 0 && splice->prev_[i] != head_ &&
compare_(splice->prev_[i]->Key(), x->Key()) >= 0)) {
// duplicate key
return false;
}
assert(splice->next_[i] == nullptr ||
compare_(x->Key(), splice->next_[i]->Key()) < 0);
assert(splice->prev_[i] == head_ ||
compare_(splice->prev_[i]->Key(), x->Key()) < 0);
x->NoBarrier_SetNext(i, splice->next_[i]);
if (splice->prev_[i]->CASNext(i, splice->next_[i], x)) {
// success
break;
}
// CAS failed, we need to recompute prev and next. It is unlikely
// to be helpful to try to use a different level as we redo the
// search, because it should be unlikely that lots of nodes have
// been inserted between prev[i] and next[i]. No point in using
// next[i] as the after hint, because we know it is stale.
FindSpliceForLevel<false>(key_decoded, splice->prev_[i], nullptr, i,
&splice->prev_[i], &splice->next_[i]);
// Since we've narrowed the bracket for level i, we might have
// violated the Splice constraint between i and i-1. Make sure
// we recompute the whole thing next time.
if (i > 0) {
splice_is_valid = false;
}
}
}
} else {
for (int i = 0; i < height; ++i) {
if (i >= recompute_height &&
splice->prev_[i]->Next(i) != splice->next_[i]) {
FindSpliceForLevel<false>(key_decoded, splice->prev_[i], nullptr, i,
&splice->prev_[i], &splice->next_[i]);
}
// Checking for duplicate keys on the level 0 is sufficient
if (UNLIKELY(i == 0 && splice->next_[i] != nullptr &&
compare_(x->Key(), splice->next_[i]->Key()) >= 0)) {
// duplicate key
return false;
}
if (UNLIKELY(i == 0 && splice->prev_[i] != head_ &&
compare_(splice->prev_[i]->Key(), x->Key()) >= 0)) {
// duplicate key
return false;
}
assert(splice->next_[i] == nullptr ||
compare_(x->Key(), splice->next_[i]->Key()) < 0);
assert(splice->prev_[i] == head_ ||
compare_(splice->prev_[i]->Key(), x->Key()) < 0);
assert(splice->prev_[i]->Next(i) == splice->next_[i]);
x->NoBarrier_SetNext(i, splice->next_[i]);
splice->prev_[i]->SetNext(i, x);
}
}
if (splice_is_valid) {
for (int i = 0; i < height; ++i) {
splice->prev_[i] = x;
}
assert(splice->prev_[splice->height_] == head_);
assert(splice->next_[splice->height_] == nullptr);
for (int i = 0; i < splice->height_; ++i) {
assert(splice->next_[i] == nullptr ||
compare_(key, splice->next_[i]->Key()) < 0);
assert(splice->prev_[i] == head_ ||
compare_(splice->prev_[i]->Key(), key) <= 0);
assert(splice->prev_[i + 1] == splice->prev_[i] ||
splice->prev_[i + 1] == head_ ||
compare_(splice->prev_[i + 1]->Key(), splice->prev_[i]->Key()) <
0);
assert(splice->next_[i + 1] == splice->next_[i] ||
splice->next_[i + 1] == nullptr ||
compare_(splice->next_[i]->Key(), splice->next_[i + 1]->Key()) <
0);
}
} else {
splice->height_ = 0;
}
return true;
}
template <class Comparator>
bool InlineSkipList<Comparator>::Contains(const char* key) const {
Node* x = FindGreaterOrEqual(key);
if (x != nullptr && Equal(key, x->Key())) {
return true;
} else {
return false;
}
}
template <class Comparator>
void InlineSkipList<Comparator>::TEST_Validate() const {
// Interate over all levels at the same time, and verify nodes appear in
// the right order, and nodes appear in upper level also appear in lower
// levels.
Node* nodes[kMaxPossibleHeight];
int max_height = GetMaxHeight();
assert(max_height > 0);
for (int i = 0; i < max_height; i++) {
nodes[i] = head_;
}
while (nodes[0] != nullptr) {
Node* l0_next = nodes[0]->Next(0);
if (l0_next == nullptr) {
break;
}
assert(nodes[0] == head_ || compare_(nodes[0]->Key(), l0_next->Key()) < 0);
nodes[0] = l0_next;
int i = 1;
while (i < max_height) {
Node* next = nodes[i]->Next(i);
if (next == nullptr) {
break;
}
auto cmp = compare_(nodes[0]->Key(), next->Key());
assert(cmp <= 0);
if (cmp == 0) {
assert(next == nodes[0]);
nodes[i] = next;
} else {
break;
}
i++;
}
}
for (int i = 1; i < max_height; i++) {
assert(nodes[i] != nullptr && nodes[i]->Next(i) == nullptr);
}
}
} // namespace rocksdb
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