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fix(base): UnorderedMap inserts no longer duplicate a key past a tombstone #23

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Merged
Force67 merged 1 commit into from
Sep 27, 2026
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fix(base): UnorderedMap inserts no longer duplicate a key past a tombstone #23

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131 changes: 59 additions & 72 deletions base/containers/unordered_map.h
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Original file line number Diff line number Diff line change
Expand Up @@ -173,6 +173,37 @@ class UnorderedMap {
return ~mem_size(0);
}

// Returns the slot holding |key| (second = false), or the slot a new |key|
// goes into (second = true). The probe must reach an empty slot before
// reusing a tombstone, since |key| may sit further along the chain.
Pair<mem_size, bool> FindOrPrepareInsert(const K& key) {
if (bucket_count_ == 0 ||
(size_ + tombstone_count_ + 1) > static_cast<mem_size>(bucket_count_ * kMaxLoadFactor)) {
GrowAndRehash();
}

constexpr mem_size kNone = ~mem_size(0);
mem_size tombstone = kNone;
mem_size idx = BucketFor(key);
while (slots_[idx].state != kEmpty) {
if (slots_[idx].state == kDeleted) {
if (tombstone == kNone)
tombstone = idx;
} else if (equal_(*SlotKey(slots_[idx]), key)) {
return {idx, false};
}
idx = (idx + 1) & (bucket_count_ - 1);
}
return {tombstone != kNone ? tombstone : idx, true};
}

void MarkOccupied(Slot& s) {
if (s.state == kDeleted)
--tombstone_count_;
s.state = kOccupied;
++size_;
}

public:
UnorderedMap() = default;

Expand Down Expand Up @@ -264,25 +295,14 @@ class UnorderedMap {

// Element access
V& operator[](const K& key) {
if (bucket_count_ == 0 ||
(size_ + tombstone_count_ + 1) > static_cast<mem_size>(bucket_count_ * kMaxLoadFactor)) {
GrowAndRehash();
}

mem_size idx = BucketFor(key);
while (true) {
if (slots_[idx].state == kEmpty || slots_[idx].state == kDeleted) {
::new (&slots_[idx].key_storage[0]) K(key);
::new (&slots_[idx].val_storage[0]) V();
slots_[idx].state = kOccupied;
++size_;
return *SlotVal(slots_[idx]);
}
if (slots_[idx].state == kOccupied && equal_(*SlotKey(slots_[idx]), key)) {
return *SlotVal(slots_[idx]);
}
idx = (idx + 1) & (bucket_count_ - 1);
const Pair<mem_size, bool> slot = FindOrPrepareInsert(key);
Slot& s = slots_[slot.first];
if (slot.second) {
::new (&s.key_storage[0]) K(key);
::new (&s.val_storage[0]) V();
MarkOccupied(s);
}
return *SlotVal(s);
}

V* find(const K& key) {
Expand Down Expand Up @@ -335,47 +355,25 @@ class UnorderedMap {

// Insert or assign
Pair<V*, bool> insert(const K& key, const V& value) {
if (bucket_count_ == 0 ||
(size_ + tombstone_count_ + 1) > static_cast<mem_size>(bucket_count_ * kMaxLoadFactor)) {
GrowAndRehash();
}

mem_size idx = BucketFor(key);
while (true) {
if (slots_[idx].state == kEmpty || slots_[idx].state == kDeleted) {
::new (&slots_[idx].key_storage[0]) K(key);
::new (&slots_[idx].val_storage[0]) V(value);
slots_[idx].state = kOccupied;
++size_;
return {SlotVal(slots_[idx]), true};
}
if (slots_[idx].state == kOccupied && equal_(*SlotKey(slots_[idx]), key)) {
return {SlotVal(slots_[idx]), false}; // already exists
}
idx = (idx + 1) & (bucket_count_ - 1);
const Pair<mem_size, bool> slot = FindOrPrepareInsert(key);
Slot& s = slots_[slot.first];
if (slot.second) {
::new (&s.key_storage[0]) K(key);
::new (&s.val_storage[0]) V(value);
MarkOccupied(s);
}
return {SlotVal(s), slot.second};
}

Pair<V*, bool> insert(const K& key, V&& value) {
if (bucket_count_ == 0 ||
(size_ + tombstone_count_ + 1) > static_cast<mem_size>(bucket_count_ * kMaxLoadFactor)) {
GrowAndRehash();
}

mem_size idx = BucketFor(key);
while (true) {
if (slots_[idx].state == kEmpty || slots_[idx].state == kDeleted) {
::new (&slots_[idx].key_storage[0]) K(key);
::new (&slots_[idx].val_storage[0]) V(base::move(value));
slots_[idx].state = kOccupied;
++size_;
return {SlotVal(slots_[idx]), true};
}
if (slots_[idx].state == kOccupied && equal_(*SlotKey(slots_[idx]), key)) {
return {SlotVal(slots_[idx]), false};
}
idx = (idx + 1) & (bucket_count_ - 1);
const Pair<mem_size, bool> slot = FindOrPrepareInsert(key);
Slot& s = slots_[slot.first];
if (slot.second) {
::new (&s.key_storage[0]) K(key);
::new (&s.val_storage[0]) V(base::move(value));
MarkOccupied(s);
}
return {SlotVal(s), slot.second};
}

// emplace under the std spelling; identical semantics (an existing key is
Expand All @@ -387,25 +385,14 @@ class UnorderedMap {

template <typename... TArgs>
Pair<V*, bool> emplace(const K& key, TArgs&&... args) {
if (bucket_count_ == 0 ||
(size_ + tombstone_count_ + 1) > static_cast<mem_size>(bucket_count_ * kMaxLoadFactor)) {
GrowAndRehash();
}

mem_size idx = BucketFor(key);
while (true) {
if (slots_[idx].state == kEmpty || slots_[idx].state == kDeleted) {
::new (&slots_[idx].key_storage[0]) K(key);
::new (&slots_[idx].val_storage[0]) V(base::forward<TArgs>(args)...);
slots_[idx].state = kOccupied;
++size_;
return {SlotVal(slots_[idx]), true};
}
if (slots_[idx].state == kOccupied && equal_(*SlotKey(slots_[idx]), key)) {
return {SlotVal(slots_[idx]), false};
}
idx = (idx + 1) & (bucket_count_ - 1);
const Pair<mem_size, bool> slot = FindOrPrepareInsert(key);
Slot& s = slots_[slot.first];
if (slot.second) {
::new (&s.key_storage[0]) K(key);
::new (&s.val_storage[0]) V(base::forward<TArgs>(args)...);
MarkOccupied(s);
}
return {SlotVal(s), slot.second};
}

void clear() {
Expand Down
43 changes: 43 additions & 0 deletions base/containers/unordered_map_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -98,6 +98,49 @@ TEST(UnorderedMapTest, LookupSurvivesTombstones) {
EXPECT_EQ(map.size(), 32u);
}

// Every key lands in bucket 0, so key 2 sits one slot past key 1.
struct CollidingHash {
mem_size operator()(i32) const { return 0; }
};

TEST(UnorderedMapTest, SubscriptFindsAKeyPastATombstone) {
base::UnorderedMap<i32, i32, CollidingHash> map;
map[1] = 10;
map[2] = 20;
map.erase(1);

map[2] = 21;

EXPECT_EQ(map.size(), 1u);
EXPECT_EQ(*map.find(2), 21);
map.erase(2);
EXPECT_EQ(map.find(2), nullptr);
}

TEST(UnorderedMapTest, InsertAndEmplaceFindAKeyPastATombstone) {
base::UnorderedMap<i32, i32, CollidingHash> map;
map.insert(1, 10);
map.insert(2, 20);
map.erase(1);

EXPECT_FALSE(map.insert(2, 99).second);
EXPECT_FALSE(map.emplace(2, 99).second);

EXPECT_EQ(map.size(), 1u);
EXPECT_EQ(*map.find(2), 20);
}

TEST(UnorderedMapTest, InsertReusesATombstone) {
base::UnorderedMap<i32, i32> map;
map.insert(1, 10);
const mem_size capacity = map.remaining_capacity();
map.erase(1);

map.insert(1, 11);

EXPECT_EQ(map.remaining_capacity(), capacity);
}

TEST(UnorderedMapTest, GrowsAndRehashes) {
base::UnorderedMap<i32, i32> map;
for (i32 i = 0; i < 1000; ++i) map.insert(i, i);
Expand Down