src/tint/utils/hashmap_base.h - tint - Git at Google

 // Copyright 2022 The Tint Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 #ifndef SRC_TINT_UTILS_HASHMAP_BASE_H_
 #define SRC_TINT_UTILS_HASHMAP_BASE_H_

 #include <algorithm>
 #include <functional>
 #include <optional>
 #include <tuple>
 #include <utility>

 #include "src/tint/debug.h"
 #include "src/tint/utils/hash.h"
 #include "src/tint/utils/vector.h"

 namespace tint::utils {

 /// Action taken by a map mutation
 enum class MapAction {
     /// A new entry was added to the map
     kAdded,
     /// A existing entry in the map was replaced
     kReplaced,
     /// No action was taken as the map already contained an entry with the given key
     kKeptExisting,
 };

 /// KeyValue is a key-value pair.
 template <typename KEY, typename VALUE>
 struct KeyValue {
     /// The key type
     using Key = KEY;
     /// The value type
     using Value = VALUE;

     /// The key
     Key key;

     /// The value
     Value value;

     /// Equality operator
     /// @param other the RHS of the operator
     /// @returns true if both the key and value of this KeyValue are equal to the key and value
     /// of @p other
     template <typename K, typename V>
     bool operator==(const KeyValue<K, V>& other) const {
         return key == other.key && value == other.value;
     }

     /// Inequality operator
     /// @param other the RHS of the operator
     /// @returns true if either the key and value of this KeyValue are not equal to the key and
     /// value of @p other
     template <typename K, typename V>
     bool operator!=(const KeyValue<K, V>& other) const {
         return *this != other;
     }
 };

 /// Writes the KeyValue to the std::ostream.
 /// @param out the std::ostream to write to
 /// @param key_value the KeyValue to write
 /// @returns out so calls can be chained
 template <typename KEY, typename VALUE>
 std::ostream& operator<<(std::ostream& out, const KeyValue<KEY, VALUE>& key_value) {
     return out << "[" << key_value.key << ": " << key_value.value << "]";
 }

 /// A base class for Hashmap and Hashset that uses a robin-hood hashing algorithm.
 /// @see the fantastic tutorial: https://programming.guide/robin-hood-hashing.html
 template <typename KEY,
           typename VALUE,
           size_t N,
           typename HASH = Hasher<KEY>,
           typename EQUAL = std::equal_to<KEY>>
 class HashmapBase {
     static constexpr bool ValueIsVoid = std::is_same_v<VALUE, void>;

   public:
     /// The key type
     using Key = KEY;
     /// The value type
     using Value = VALUE;
     /// The entry type for the map.
     /// This is:
     /// - Key when Value is void (used by Hashset)
     /// - KeyValue<Key, Value> when Value is void (used by Hashmap)
     using Entry = std::conditional_t<ValueIsVoid, Key, KeyValue<Key, Value>>;

     /// STL-friendly alias to Entry. Used by gmock.
     using value_type = Entry;

   private:
     /// @returns the key from an entry
     static const Key& KeyOf(const Entry& entry) {
         if constexpr (ValueIsVoid) {
             return entry;
         } else {
             return entry.key;
         }
     }

     /// @returns a pointer to the value from an entry.
     static Value* ValueOf(Entry& entry) {
         if constexpr (ValueIsVoid) {
             return nullptr;  // Hashset only has keys
         } else {
             return &entry.value;
         }
     }

     /// A slot is a single entry in the underlying vector.
     /// A slot can either be empty or filled with a value. If the slot is empty, #hash and #distance
     /// will be zero.
     struct Slot {
         bool Equals(size_t key_hash, const Key& key) const {
             return key_hash == hash && EQUAL()(key, KeyOf(*entry));
         }

         /// The slot value. If this does not contain a value, then the slot is vacant.
         std::optional<Entry> entry;
         /// The precomputed hash of value.
         size_t hash = 0;
         size_t distance = 0;
     };

     /// The target length of the underlying vector length in relation to the number of entries in
     /// the map, expressed as a percentage. For example a value of `150` would mean there would be
     /// at least 50% more slots than the number of map entries.
     static constexpr size_t kRehashFactor = 150;

     /// @returns the target slot vector size to hold `n` map entries.
     static constexpr size_t NumSlots(size_t count) { return (count * kRehashFactor) / 100; }

     /// The fixed-size slot vector length, based on N and kRehashFactor.
     static constexpr size_t kNumFixedSlots = NumSlots(N);

     /// The minimum number of slots for the map.
     static constexpr size_t kMinSlots = std::max<size_t>(kNumFixedSlots, 4);

   public:
     /// Iterator for entries in the map.
     /// Iterators are invalidated if the map is modified.
     class Iterator {
       public:
         /// @returns the value pointed to by this iterator
         const Entry* operator->() const { return &current->entry.value(); }

         /// @returns a reference to the value at the iterator
         const Entry& operator*() const { return current->entry.value(); }

         /// Increments the iterator
         /// @returns this iterator
         Iterator& operator++() {
             if (current == end) {
                 return *this;
             }
             current++;
             SkipToNextValue();
             return *this;
         }

         /// Equality operator
         /// @param other the other iterator to compare this iterator to
         /// @returns true if this iterator is equal to other
         bool operator==(const Iterator& other) const { return current == other.current; }

         /// Inequality operator
         /// @param other the other iterator to compare this iterator to
         /// @returns true if this iterator is not equal to other
         bool operator!=(const Iterator& other) const { return current != other.current; }

       private:
         /// Friend class
         friend class HashmapBase;

         Iterator(const Slot* c, const Slot* e) : current(c), end(e) { SkipToNextValue(); }

         /// Moves the iterator forward, stopping at the next slot that is not empty.
         void SkipToNextValue() {
             while (current != end && !current->entry.has_value()) {
                 current++;
             }
         }

         const Slot* current;  /// The slot the iterator is pointing to
         const Slot* end;      /// One past the last slot in the map
     };

     /// Constructor
     HashmapBase() { slots_.Resize(kMinSlots); }

     /// Copy constructor
     /// @param other the other HashmapBase to copy
     HashmapBase(const HashmapBase& other) = default;

     /// Move constructor
     /// @param other the other HashmapBase to move
     HashmapBase(HashmapBase&& other) = default;

     /// Destructor
     ~HashmapBase() { Clear(); }

     /// Copy-assignment operator
     /// @param other the other HashmapBase to copy
     /// @returns this so calls can be chained
     HashmapBase& operator=(const HashmapBase& other) = default;

     /// Move-assignment operator
     /// @param other the other HashmapBase to move
     /// @returns this so calls can be chained
     HashmapBase& operator=(HashmapBase&& other) = default;

     /// Removes all entries from the map.
     void Clear() {
         slots_.Clear();  // Destructs all entries
         slots_.Resize(kMinSlots);
         count_ = 0;
         generation_++;
     }

     /// Removes an entry from the map.
     /// @param key the entry key.
     /// @returns true if an entry was removed.
     bool Remove(const Key& key) {
         const auto [found, start] = IndexOf(key);
         if (!found) {
             return false;
         }

         // Shuffle the entries backwards until we either find a free slot, or a slot that has zero
         // distance.
         Slot* prev = nullptr;
         Scan(start, [&](size_t, size_t index) {
             auto& slot = slots_[index];
             if (prev) {
                 // note: `distance == 0` also includes empty slots.
                 if (slot.distance == 0) {
                     // Clear the previous slot, and stop shuffling.
                     *prev = {};
                     return Action::kStop;
                 } else {
                     // Shuffle the slot backwards.
                     prev->entry = std::move(slot.entry);
                     prev->hash = slot.hash;
                     prev->distance = slot.distance - 1;
                 }
             }
             prev = &slot;
             return Action::kContinue;
         });

         // Entry was removed.
         count_--;
         generation_++;

         return true;
     }

     /// Checks whether an entry exists in the map
     /// @param key the key to search for.
     /// @returns true if the map contains an entry with the given value.
     bool Contains(const Key& key) const {
         const auto [found, _] = IndexOf(key);
         return found;
     }

     /// Pre-allocates memory so that the map can hold at least `capacity` entries.
     /// @param capacity the new capacity of the map.
     void Reserve(size_t capacity) {
         // Calculate the number of slots required to hold `capacity` entries.
         const size_t num_slots = std::max(NumSlots(capacity), kMinSlots);
         if (slots_.Length() >= num_slots) {
             // Already have enough slots.
             return;
         }

         // Move all the values out of the map and into a vector.
         Vector<Entry, N> entries;
         entries.Reserve(count_);
         for (auto& slot : slots_) {
             if (slot.entry.has_value()) {
                 entries.Push(std::move(slot.entry.value()));
             }
         }

         // Clear the map, grow the number of slots.
         Clear();
         slots_.Resize(num_slots);

         // As the number of slots has grown, the slot indices will have changed from before, so
         // re-add all the entries back into the map.
         for (auto& entry : entries) {
             if constexpr (ValueIsVoid) {
                 struct NoValue {};
                 Put<PutMode::kAdd>(std::move(entry), NoValue{});
             } else {
                 Put<PutMode::kAdd>(std::move(entry.key), std::move(entry.value));
             }
         }
     }

     /// @returns the number of entries in the map.
     size_t Count() const { return count_; }

     /// @returns true if the map contains no entries.
     bool IsEmpty() const { return count_ == 0; }

     /// @returns a monotonic counter which is incremented whenever the map is mutated.
     size_t Generation() const { return generation_; }

     /// @returns an iterator to the start of the map.
     Iterator begin() const { return Iterator{slots_.begin(), slots_.end()}; }

     /// @returns an iterator to the end of the map.
     Iterator end() const { return Iterator{slots_.end(), slots_.end()}; }

     /// A debug function for checking that the map is in good health.
     /// Asserts if the map is corrupted.
     void ValidateIntegrity() const {
         size_t num_alive = 0;
         for (size_t slot_idx = 0; slot_idx < slots_.Length(); slot_idx++) {
             const auto& slot = slots_[slot_idx];
             if (slot.entry.has_value()) {
                 num_alive++;
                 auto const [index, hash] = Hash(KeyOf(*slot.entry));
                 TINT_ASSERT(Utils, hash == slot.hash);
                 TINT_ASSERT(Utils, slot_idx == Wrap(index + slot.distance));
             }
         }
         TINT_ASSERT(Utils, num_alive == count_);
     }

   protected:
     /// The behaviour of Put() when an entry already exists with the given key.
     enum class PutMode {
         /// Do not replace existing entries with the new value.
         kAdd,
         /// Replace existing entries with the new value.
         kReplace,
     };

     /// Result of Put()
     struct PutResult {
         /// Whether the insert replaced or added a new entry to the map.
         MapAction action = MapAction::kAdded;
         /// A pointer to the inserted entry value.
         Value* value = nullptr;

         /// @returns true if the entry was added to the map, or an existing entry was replaced.
         operator bool() const { return action != MapAction::kKeptExisting; }
     };

     /// The common implementation for Add() and Replace()
     /// @param key the key of the entry to add to the map.
     /// @param value the value of the entry to add to the map.
     /// @returns A PutResult describing the result of the insertion
     template <PutMode MODE, typename K, typename V>
     PutResult Put(K&& key, V&& value) {
         // Ensure the map can fit a new entry
         if (ShouldRehash(count_ + 1)) {
             Reserve((count_ + 1) * 2);
         }

         const auto hash = Hash(key);

         auto make_entry = [&]() {
             if constexpr (ValueIsVoid) {
                 return std::forward<K>(key);
             } else {
                 return Entry{std::forward<K>(key), std::forward<V>(value)};
             }
         };

         PutResult result{};
         Scan(hash.scan_start, [&](size_t distance, size_t index) {
             auto& slot = slots_[index];
             if (!slot.entry.has_value()) {
                 // Found an empty slot.
                 // Place value directly into the slot, and we're done.
                 slot.entry.emplace(make_entry());
                 slot.hash = hash.code;
                 slot.distance = distance;
                 count_++;
                 generation_++;
                 result = PutResult{MapAction::kAdded, ValueOf(*slot.entry)};
                 return Action::kStop;
             }

             // Slot has an entry

             if (slot.Equals(hash.code, key)) {
                 // Slot is equal to value. Replace or preserve?
                 if constexpr (MODE == PutMode::kReplace) {
                     slot.entry = make_entry();
                     generation_++;
                     result = PutResult{MapAction::kReplaced, ValueOf(*slot.entry)};
                 } else {
                     result = PutResult{MapAction::kKeptExisting, ValueOf(*slot.entry)};
                 }
                 return Action::kStop;
             }

             if (slot.distance < distance) {
                 // Existing slot has a closer distance than the value we're attempting to insert.
                 // Steal from the rich!
                 // Move the current slot to a temporary (evicted), and put the value into the slot.
                 Slot evicted{make_entry(), hash.code, distance};
                 std::swap(evicted, slot);

                 // Find a new home for the evicted slot.
                 evicted.distance++;  // We've already swapped at index.
                 InsertShuffle(Wrap(index + 1), std::move(evicted));

                 count_++;
                 generation_++;
                 result = PutResult{MapAction::kAdded, ValueOf(*slot.entry)};

                 return Action::kStop;
             }
             return Action::kContinue;
         });

         return result;
     }

     /// Return type of the Scan() callback.
     enum class Action {
         /// Continue scanning for a slot
         kContinue,
         /// Immediately stop scanning for a slot
         kStop,
     };

     /// Sequentially visits each of the slots starting with the slot with the index @p start,
     /// calling the callback function @p f for each slot until @p f returns Action::kStop.
     /// @param start the index of the first slot to start scanning from.
     /// @param f the callback function which:
     /// * must be a function with the signature `Action(size_t distance, size_t index)`.
     /// * must return Action::kStop within one whole cycle of the slots.
     template <typename F>
     void Scan(size_t start, F&& f) const {
         size_t distance = 0;
         for (size_t index = start; index < slots_.Length(); index++) {
             if (f(distance, index) == Action::kStop) {
                 return;
             }
             distance++;
         }
         for (size_t index = 0; index < start; index++) {
             if (f(distance, index) == Action::kStop) {
                 return;
             }
             distance++;
         }
         tint::diag::List diags;
         TINT_ICE(Utils, diags) << "HashmapBase::Scan() looped entire map without finding a slot";
     }

     /// HashResult is the return value of Hash()
     struct HashResult {
         /// The target (zero-distance) slot index for the key.
         size_t scan_start;
         /// The calculated hash code of the key.
         size_t code;
     };

     /// @param key the key to hash
     /// @returns a tuple holding the target slot index for the given value, and the hash of the
     /// value, respectively.
     HashResult Hash(const Key& key) const {
         size_t hash = HASH()(key);
         size_t index = Wrap(hash);
         return {index, hash};
     }

     /// Looks for the key in the map.
     /// @param key the key to search for.
     /// @returns a tuple holding a boolean representing whether the key was found in the map, and
     /// if found, the index of the slot that holds the key.
     std::tuple<bool, size_t> IndexOf(const Key& key) const {
         const auto hash = Hash(key);

         bool found = false;
         size_t idx = 0;

         Scan(hash.scan_start, [&](size_t distance, size_t index) {
             auto& slot = slots_[index];
             if (!slot.entry.has_value()) {
                 return Action::kStop;
             }
             if (slot.Equals(hash.code, key)) {
                 found = true;
                 idx = index;
                 return Action::kStop;
             }
             if (slot.distance < distance) {
                 // If the slot distance is less than the current probe distance, then the slot must
                 // be for entry that has an index that comes after key. In this situation, we know
                 // that the map does not contain the key, as it would have been found before this
                 // slot. The "Lookup" section of https://programming.guide/robin-hood-hashing.html
                 // suggests that the condition should inverted, but this is wrong.
                 return Action::kStop;
             }
             return Action::kContinue;
         });

         return {found, idx};
     }

     /// Shuffles slots for an insertion that has been placed one slot before `start`.
     /// @param start the index of the first slot to start shuffling.
     /// @param evicted the slot content that was evicted for the insertion.
     void InsertShuffle(size_t start, Slot&& evicted) {
         Scan(start, [&](size_t, size_t index) {
             auto& slot = slots_[index];

             if (!slot.entry.has_value()) {
                 // Empty slot found for evicted.
                 slot = std::move(evicted);
                 return Action::kStop;  //  We're done.
             }

             if (slot.distance < evicted.distance) {
                 // Occupied slot has shorter distance to evicted.
                 // Swap slot and evicted.
                 std::swap(slot, evicted);
             }

             // evicted moves further from the target slot...
             evicted.distance++;

             return Action::kContinue;
         });
     }

     /// @param count the number of new entries in the map
     /// @returns true if the map should grow the slot vector, and rehash the items.
     bool ShouldRehash(size_t count) const { return NumSlots(count) > slots_.Length(); }

     /// @param index an input value
     /// @returns the input value modulo the number of slots.
     size_t Wrap(size_t index) const { return index % slots_.Length(); }

     /// The vector of slots. The vector length is equal to its capacity.
     Vector<Slot, kNumFixedSlots> slots_;

     /// The number of entries in the map.
     size_t count_ = 0;

     /// Counter that's incremented with each modification to the map.
     size_t generation_ = 0;
 };

 }  // namespace tint::utils

 #endif  // SRC_TINT_UTILS_HASHMAP_BASE_H_
	// Copyright 2022 The Tint Authors.
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	#ifndef SRC_TINT_UTILS_HASHMAP_BASE_H_
	#define SRC_TINT_UTILS_HASHMAP_BASE_H_

	#include <algorithm>
	#include <functional>
	#include <optional>
	#include <tuple>
	#include <utility>

	#include "src/tint/debug.h"
	#include "src/tint/utils/hash.h"
	#include "src/tint/utils/vector.h"

	namespace tint::utils {

	/// Action taken by a map mutation
	enum class MapAction {
	/// A new entry was added to the map
	kAdded,
	/// A existing entry in the map was replaced
	kReplaced,
	/// No action was taken as the map already contained an entry with the given key
	kKeptExisting,
	};

	/// KeyValue is a key-value pair.
	template <typename KEY, typename VALUE>
	struct KeyValue {
	/// The key type
	using Key = KEY;
	/// The value type
	using Value = VALUE;

	/// The key
	Key key;

	/// The value
	Value value;

	/// Equality operator
	/// @param other the RHS of the operator
	/// @returns true if both the key and value of this KeyValue are equal to the key and value
	/// of @p other
	template <typename K, typename V>
	bool operator==(const KeyValue<K, V>& other) const {
	return key == other.key && value == other.value;
	}

	/// Inequality operator
	/// @param other the RHS of the operator
	/// @returns true if either the key and value of this KeyValue are not equal to the key and
	/// value of @p other
	template <typename K, typename V>
	bool operator!=(const KeyValue<K, V>& other) const {
	return *this != other;
	}
	};

	/// Writes the KeyValue to the std::ostream.
	/// @param out the std::ostream to write to
	/// @param key_value the KeyValue to write
	/// @returns out so calls can be chained
	template <typename KEY, typename VALUE>
	std::ostream& operator<<(std::ostream& out, const KeyValue<KEY, VALUE>& key_value) {
	return out << "[" << key_value.key << ": " << key_value.value << "]";
	}

	/// A base class for Hashmap and Hashset that uses a robin-hood hashing algorithm.
	/// @see the fantastic tutorial: https://programming.guide/robin-hood-hashing.html
	template <typename KEY,
	typename VALUE,
	size_t N,
	typename HASH = Hasher<KEY>,
	typename EQUAL = std::equal_to<KEY>>
	class HashmapBase {
	static constexpr bool ValueIsVoid = std::is_same_v<VALUE, void>;

	public:
	/// The key type
	using Key = KEY;
	/// The value type
	using Value = VALUE;
	/// The entry type for the map.
	/// This is:
	/// - Key when Value is void (used by Hashset)
	/// - KeyValue<Key, Value> when Value is void (used by Hashmap)
	using Entry = std::conditional_t<ValueIsVoid, Key, KeyValue<Key, Value>>;

	/// STL-friendly alias to Entry. Used by gmock.
	using value_type = Entry;

	private:
	/// @returns the key from an entry
	static const Key& KeyOf(const Entry& entry) {
	if constexpr (ValueIsVoid) {
	return entry;
	} else {
	return entry.key;
	}
	}

	/// @returns a pointer to the value from an entry.
	static Value* ValueOf(Entry& entry) {
	if constexpr (ValueIsVoid) {
	return nullptr; // Hashset only has keys
	} else {
	return &entry.value;
	}
	}

	/// A slot is a single entry in the underlying vector.
	/// A slot can either be empty or filled with a value. If the slot is empty, #hash and #distance
	/// will be zero.
	struct Slot {
	bool Equals(size_t key_hash, const Key& key) const {
	return key_hash == hash && EQUAL()(key, KeyOf(*entry));
	}

	/// The slot value. If this does not contain a value, then the slot is vacant.
	std::optional<Entry> entry;
	/// The precomputed hash of value.
	size_t hash = 0;
	size_t distance = 0;
	};

	/// The target length of the underlying vector length in relation to the number of entries in
	/// the map, expressed as a percentage. For example a value of `150` would mean there would be
	/// at least 50% more slots than the number of map entries.
	static constexpr size_t kRehashFactor = 150;

	/// @returns the target slot vector size to hold `n` map entries.
	static constexpr size_t NumSlots(size_t count) { return (count * kRehashFactor) / 100; }

	/// The fixed-size slot vector length, based on N and kRehashFactor.
	static constexpr size_t kNumFixedSlots = NumSlots(N);

	/// The minimum number of slots for the map.
	static constexpr size_t kMinSlots = std::max<size_t>(kNumFixedSlots, 4);

	public:
	/// Iterator for entries in the map.
	/// Iterators are invalidated if the map is modified.
	class Iterator {
	public:
	/// @returns the value pointed to by this iterator
	const Entry* operator->() const { return &current->entry.value(); }

	/// @returns a reference to the value at the iterator
	const Entry& operator*() const { return current->entry.value(); }

	/// Increments the iterator
	/// @returns this iterator
	Iterator& operator++() {
	if (current == end) {
	return *this;
	}
	current++;
	SkipToNextValue();
	return *this;
	}

	/// Equality operator
	/// @param other the other iterator to compare this iterator to
	/// @returns true if this iterator is equal to other
	bool operator==(const Iterator& other) const { return current == other.current; }

	/// Inequality operator
	/// @param other the other iterator to compare this iterator to
	/// @returns true if this iterator is not equal to other
	bool operator!=(const Iterator& other) const { return current != other.current; }

	private:
	/// Friend class
	friend class HashmapBase;

	Iterator(const Slot* c, const Slot* e) : current(c), end(e) { SkipToNextValue(); }

	/// Moves the iterator forward, stopping at the next slot that is not empty.
	void SkipToNextValue() {
	while (current != end && !current->entry.has_value()) {
	current++;
	}
	}

	const Slot* current; /// The slot the iterator is pointing to
	const Slot* end; /// One past the last slot in the map
	};

	/// Constructor
	HashmapBase() { slots_.Resize(kMinSlots); }

	/// Copy constructor
	/// @param other the other HashmapBase to copy
	HashmapBase(const HashmapBase& other) = default;

	/// Move constructor
	/// @param other the other HashmapBase to move
	HashmapBase(HashmapBase&& other) = default;

	/// Destructor
	~HashmapBase() { Clear(); }

	/// Copy-assignment operator
	/// @param other the other HashmapBase to copy
	/// @returns this so calls can be chained
	HashmapBase& operator=(const HashmapBase& other) = default;

	/// Move-assignment operator
	/// @param other the other HashmapBase to move
	/// @returns this so calls can be chained
	HashmapBase& operator=(HashmapBase&& other) = default;

	/// Removes all entries from the map.
	void Clear() {
	slots_.Clear(); // Destructs all entries
	slots_.Resize(kMinSlots);
	count_ = 0;
	generation_++;
	}

	/// Removes an entry from the map.
	/// @param key the entry key.
	/// @returns true if an entry was removed.
	bool Remove(const Key& key) {
	const auto [found, start] = IndexOf(key);
	if (!found) {
	return false;
	}

	// Shuffle the entries backwards until we either find a free slot, or a slot that has zero
	// distance.
	Slot* prev = nullptr;
	Scan(start, [&](size_t, size_t index) {
	auto& slot = slots_[index];
	if (prev) {
	// note: `distance == 0` also includes empty slots.
	if (slot.distance == 0) {
	// Clear the previous slot, and stop shuffling.
	*prev = {};
	return Action::kStop;
	} else {
	// Shuffle the slot backwards.
	prev->entry = std::move(slot.entry);
	prev->hash = slot.hash;
	prev->distance = slot.distance - 1;
	}
	}
	prev = &slot;
	return Action::kContinue;
	});

	// Entry was removed.
	count_--;
	generation_++;

	return true;
	}

	/// Checks whether an entry exists in the map
	/// @param key the key to search for.
	/// @returns true if the map contains an entry with the given value.
	bool Contains(const Key& key) const {
	const auto [found, _] = IndexOf(key);
	return found;
	}

	/// Pre-allocates memory so that the map can hold at least `capacity` entries.
	/// @param capacity the new capacity of the map.
	void Reserve(size_t capacity) {
	// Calculate the number of slots required to hold `capacity` entries.
	const size_t num_slots = std::max(NumSlots(capacity), kMinSlots);
	if (slots_.Length() >= num_slots) {
	// Already have enough slots.
	return;
	}

	// Move all the values out of the map and into a vector.
	Vector<Entry, N> entries;
	entries.Reserve(count_);
	for (auto& slot : slots_) {
	if (slot.entry.has_value()) {
	entries.Push(std::move(slot.entry.value()));
	}
	}

	// Clear the map, grow the number of slots.
	Clear();
	slots_.Resize(num_slots);

	// As the number of slots has grown, the slot indices will have changed from before, so
	// re-add all the entries back into the map.
	for (auto& entry : entries) {
	if constexpr (ValueIsVoid) {
	struct NoValue {};
	Put<PutMode::kAdd>(std::move(entry), NoValue{});
	} else {
	Put<PutMode::kAdd>(std::move(entry.key), std::move(entry.value));
	}
	}
	}

	/// @returns the number of entries in the map.
	size_t Count() const { return count_; }

	/// @returns true if the map contains no entries.
	bool IsEmpty() const { return count_ == 0; }

	/// @returns a monotonic counter which is incremented whenever the map is mutated.
	size_t Generation() const { return generation_; }

	/// @returns an iterator to the start of the map.
	Iterator begin() const { return Iterator{slots_.begin(), slots_.end()}; }

	/// @returns an iterator to the end of the map.
	Iterator end() const { return Iterator{slots_.end(), slots_.end()}; }

	/// A debug function for checking that the map is in good health.
	/// Asserts if the map is corrupted.
	void ValidateIntegrity() const {
	size_t num_alive = 0;
	for (size_t slot_idx = 0; slot_idx < slots_.Length(); slot_idx++) {
	const auto& slot = slots_[slot_idx];
	if (slot.entry.has_value()) {
	num_alive++;
	auto const [index, hash] = Hash(KeyOf(*slot.entry));
	TINT_ASSERT(Utils, hash == slot.hash);
	TINT_ASSERT(Utils, slot_idx == Wrap(index + slot.distance));
	}
	}
	TINT_ASSERT(Utils, num_alive == count_);
	}

	protected:
	/// The behaviour of Put() when an entry already exists with the given key.
	enum class PutMode {
	/// Do not replace existing entries with the new value.
	kAdd,
	/// Replace existing entries with the new value.
	kReplace,
	};

	/// Result of Put()
	struct PutResult {
	/// Whether the insert replaced or added a new entry to the map.
	MapAction action = MapAction::kAdded;
	/// A pointer to the inserted entry value.
	Value* value = nullptr;

	/// @returns true if the entry was added to the map, or an existing entry was replaced.
	operator bool() const { return action != MapAction::kKeptExisting; }
	};

	/// The common implementation for Add() and Replace()
	/// @param key the key of the entry to add to the map.
	/// @param value the value of the entry to add to the map.
	/// @returns A PutResult describing the result of the insertion
	template <PutMode MODE, typename K, typename V>
	PutResult Put(K&& key, V&& value) {
	// Ensure the map can fit a new entry
	if (ShouldRehash(count_ + 1)) {
	Reserve((count_ + 1) * 2);
	}

	const auto hash = Hash(key);

	auto make_entry = [&]() {
	if constexpr (ValueIsVoid) {
	return std::forward<K>(key);
	} else {
	return Entry{std::forward<K>(key), std::forward<V>(value)};
	}
	};

	PutResult result{};
	Scan(hash.scan_start, [&](size_t distance, size_t index) {
	auto& slot = slots_[index];
	if (!slot.entry.has_value()) {
	// Found an empty slot.
	// Place value directly into the slot, and we're done.
	slot.entry.emplace(make_entry());
	slot.hash = hash.code;
	slot.distance = distance;
	count_++;
	generation_++;
	result = PutResult{MapAction::kAdded, ValueOf(*slot.entry)};
	return Action::kStop;
	}

	// Slot has an entry

	if (slot.Equals(hash.code, key)) {
	// Slot is equal to value. Replace or preserve?
	if constexpr (MODE == PutMode::kReplace) {
	slot.entry = make_entry();
	generation_++;
	result = PutResult{MapAction::kReplaced, ValueOf(*slot.entry)};
	} else {
	result = PutResult{MapAction::kKeptExisting, ValueOf(*slot.entry)};
	}
	return Action::kStop;
	}

	if (slot.distance < distance) {
	// Existing slot has a closer distance than the value we're attempting to insert.
	// Steal from the rich!
	// Move the current slot to a temporary (evicted), and put the value into the slot.
	Slot evicted{make_entry(), hash.code, distance};
	std::swap(evicted, slot);

	// Find a new home for the evicted slot.
	evicted.distance++; // We've already swapped at index.
	InsertShuffle(Wrap(index + 1), std::move(evicted));

	count_++;
	generation_++;
	result = PutResult{MapAction::kAdded, ValueOf(*slot.entry)};

	return Action::kStop;
	}
	return Action::kContinue;
	});

	return result;
	}

	/// Return type of the Scan() callback.
	enum class Action {
	/// Continue scanning for a slot
	kContinue,
	/// Immediately stop scanning for a slot
	kStop,
	};

	/// Sequentially visits each of the slots starting with the slot with the index @p start,
	/// calling the callback function @p f for each slot until @p f returns Action::kStop.
	/// @param start the index of the first slot to start scanning from.
	/// @param f the callback function which:
	/// * must be a function with the signature `Action(size_t distance, size_t index)`.
	/// * must return Action::kStop within one whole cycle of the slots.
	template <typename F>
	void Scan(size_t start, F&& f) const {
	size_t distance = 0;
	for (size_t index = start; index < slots_.Length(); index++) {
	if (f(distance, index) == Action::kStop) {
	return;
	}
	distance++;
	}
	for (size_t index = 0; index < start; index++) {
	if (f(distance, index) == Action::kStop) {
	return;
	}
	distance++;
	}
	tint::diag::List diags;
	TINT_ICE(Utils, diags) << "HashmapBase::Scan() looped entire map without finding a slot";
	}

	/// HashResult is the return value of Hash()
	struct HashResult {
	/// The target (zero-distance) slot index for the key.
	size_t scan_start;
	/// The calculated hash code of the key.
	size_t code;
	};

	/// @param key the key to hash
	/// @returns a tuple holding the target slot index for the given value, and the hash of the
	/// value, respectively.
	HashResult Hash(const Key& key) const {
	size_t hash = HASH()(key);
	size_t index = Wrap(hash);
	return {index, hash};
	}

	/// Looks for the key in the map.
	/// @param key the key to search for.
	/// @returns a tuple holding a boolean representing whether the key was found in the map, and
	/// if found, the index of the slot that holds the key.
	std::tuple<bool, size_t> IndexOf(const Key& key) const {
	const auto hash = Hash(key);

	bool found = false;
	size_t idx = 0;

	Scan(hash.scan_start, [&](size_t distance, size_t index) {
	auto& slot = slots_[index];
	if (!slot.entry.has_value()) {
	return Action::kStop;
	}
	if (slot.Equals(hash.code, key)) {
	found = true;
	idx = index;
	return Action::kStop;
	}
	if (slot.distance < distance) {
	// If the slot distance is less than the current probe distance, then the slot must
	// be for entry that has an index that comes after key. In this situation, we know
	// that the map does not contain the key, as it would have been found before this
	// slot. The "Lookup" section of https://programming.guide/robin-hood-hashing.html
	// suggests that the condition should inverted, but this is wrong.
	return Action::kStop;
	}
	return Action::kContinue;
	});

	return {found, idx};
	}

	/// Shuffles slots for an insertion that has been placed one slot before `start`.
	/// @param start the index of the first slot to start shuffling.
	/// @param evicted the slot content that was evicted for the insertion.
	void InsertShuffle(size_t start, Slot&& evicted) {
	Scan(start, [&](size_t, size_t index) {
	auto& slot = slots_[index];

	if (!slot.entry.has_value()) {
	// Empty slot found for evicted.
	slot = std::move(evicted);
	return Action::kStop; // We're done.
	}

	if (slot.distance < evicted.distance) {
	// Occupied slot has shorter distance to evicted.
	// Swap slot and evicted.
	std::swap(slot, evicted);
	}

	// evicted moves further from the target slot...
	evicted.distance++;

	return Action::kContinue;
	});
	}

	/// @param count the number of new entries in the map
	/// @returns true if the map should grow the slot vector, and rehash the items.
	bool ShouldRehash(size_t count) const { return NumSlots(count) > slots_.Length(); }

	/// @param index an input value
	/// @returns the input value modulo the number of slots.
	size_t Wrap(size_t index) const { return index % slots_.Length(); }

	/// The vector of slots. The vector length is equal to its capacity.
	Vector<Slot, kNumFixedSlots> slots_;

	/// The number of entries in the map.
	size_t count_ = 0;

	/// Counter that's incremented with each modification to the map.
	size_t generation_ = 0;
	};

	} // namespace tint::utils

	#endif // SRC_TINT_UTILS_HASHMAP_BASE_H_