Type Function HashMapUnmanaged [src]

Alias for std.hash_map.HashMapUnmanaged

A HashMap based on open addressing and linear probing. A lookup or modification typically incurs only 2 cache misses. No order is guaranteed and any modification invalidates live iterators. It achieves good performance with quite high load factors (by default, grow is triggered at 80% full) and only one byte of overhead per element. The struct itself is only 16 bytes for a small footprint. This comes at the price of handling size with u32, which should be reasonable enough for almost all uses. Deletions are achieved with tombstones. Default initialization of this struct is deprecated; use .empty instead.

Prototype

 pub fn HashMapUnmanaged( comptime K: type, comptime V: type, comptime Context: type, comptime max_load_percentage: u64, ) type

Parameters

K: typeV: typeContext: typemax_load_percentage: u64

Source

  pub fn HashMapUnmanaged(
    comptime K: type,
    comptime V: type,
    comptime Context: type,
    comptime max_load_percentage: u64,
) type {
    if (max_load_percentage <= 0 or max_load_percentage >= 100)
        @compileError("max_load_percentage must be between 0 and 100.");
    return struct {
        const Self = @This();

        // This is actually a midway pointer to the single buffer containing
        // a `Header` field, the `Metadata`s and `Entry`s.
        // At `-@sizeOf(Header)` is the Header field.
        // At `sizeOf(Metadata) * capacity + offset`, which is pointed to by
        // self.header().entries, is the array of entries.
        // This means that the hashmap only holds one live allocation, to
        // reduce memory fragmentation and struct size.
        /// Pointer to the metadata.
        metadata: ?[*]Metadata = null,

        /// Current number of elements in the hashmap.
        size: Size = 0,

        // Having a countdown to grow reduces the number of instructions to
        // execute when determining if the hashmap has enough capacity already.
        /// Number of available slots before a grow is needed to satisfy the
        /// `max_load_percentage`.
        available: Size = 0,

        /// Used to detect memory safety violations.
        pointer_stability: std.debug.SafetyLock = .{},

        // This is purely empirical and not a /very smart magic constant™/.
        /// Capacity of the first grow when bootstrapping the hashmap.
        const minimal_capacity = 8;

        /// A map containing no keys or values.
        pub const empty: Self = .{
            .metadata = null,
            .size = 0,
            .available = 0,
        };

        // This hashmap is specially designed for sizes that fit in a u32.
        pub const Size = u32;

        // u64 hashes guarantee us that the fingerprint bits will never be used
        // to compute the index of a slot, maximizing the use of entropy.
        pub const Hash = u64;

        pub const Entry = struct {
            key_ptr: *K,
            value_ptr: *V,
        };

        pub const KV = struct {
            key: K,
            value: V,
        };

        const Header = struct {
            values: [*]V,
            keys: [*]K,
            capacity: Size,
        };

        /// Metadata for a slot. It can be in three states: empty, used or
        /// tombstone. Tombstones indicate that an entry was previously used,
        /// they are a simple way to handle removal.
        /// To this state, we add 7 bits from the slot's key hash. These are
        /// used as a fast way to disambiguate between entries without
        /// having to use the equality function. If two fingerprints are
        /// different, we know that we don't have to compare the keys at all.
        /// The 7 bits are the highest ones from a 64 bit hash. This way, not
        /// only we use the `log2(capacity)` lowest bits from the hash to determine
        /// a slot index, but we use 7 more bits to quickly resolve collisions
        /// when multiple elements with different hashes end up wanting to be in the same slot.
        /// Not using the equality function means we don't have to read into
        /// the entries array, likely avoiding a cache miss and a potentially
        /// costly function call.
        const Metadata = packed struct {
            const FingerPrint = u7;

            const free: FingerPrint = 0;
            const tombstone: FingerPrint = 1;

            fingerprint: FingerPrint = free,
            used: u1 = 0,

            const slot_free = @as(u8, @bitCast(Metadata{ .fingerprint = free }));
            const slot_tombstone = @as(u8, @bitCast(Metadata{ .fingerprint = tombstone }));

            pub fn isUsed(self: Metadata) bool {
                return self.used == 1;
            }

            pub fn isTombstone(self: Metadata) bool {
                return @as(u8, @bitCast(self)) == slot_tombstone;
            }

            pub fn isFree(self: Metadata) bool {
                return @as(u8, @bitCast(self)) == slot_free;
            }

            pub fn takeFingerprint(hash: Hash) FingerPrint {
                const hash_bits = @typeInfo(Hash).int.bits;
                const fp_bits = @typeInfo(FingerPrint).int.bits;
                return @as(FingerPrint, @truncate(hash >> (hash_bits - fp_bits)));
            }

            pub fn fill(self: *Metadata, fp: FingerPrint) void {
                self.used = 1;
                self.fingerprint = fp;
            }

            pub fn remove(self: *Metadata) void {
                self.used = 0;
                self.fingerprint = tombstone;
            }
        };

        comptime {
            assert(@sizeOf(Metadata) == 1);
            assert(@alignOf(Metadata) == 1);
        }

        pub const Iterator = struct {
            hm: *const Self,
            index: Size = 0,

            pub fn next(it: *Iterator) ?Entry {
                assert(it.index <= it.hm.capacity());
                if (it.hm.size == 0) return null;

                const cap = it.hm.capacity();
                const end = it.hm.metadata.? + cap;
                var metadata = it.hm.metadata.? + it.index;

                while (metadata != end) : ({
                    metadata += 1;
                    it.index += 1;
                }) {
                    if (metadata[0].isUsed()) {
                        const key = &it.hm.keys()[it.index];
                        const value = &it.hm.values()[it.index];
                        it.index += 1;
                        return Entry{ .key_ptr = key, .value_ptr = value };
                    }
                }

                return null;
            }
        };

        pub const KeyIterator = FieldIterator(K);
        pub const ValueIterator = FieldIterator(V);

        fn FieldIterator(comptime T: type) type {
            return struct {
                len: usize,
                metadata: [*]const Metadata,
                items: [*]T,

                pub fn next(self: *@This()) ?*T {
                    while (self.len > 0) {
                        self.len -= 1;
                        const used = self.metadata[0].isUsed();
                        const item = &self.items[0];
                        self.metadata += 1;
                        self.items += 1;
                        if (used) {
                            return item;
                        }
                    }
                    return null;
                }
            };
        }

        pub const GetOrPutResult = struct {
            key_ptr: *K,
            value_ptr: *V,
            found_existing: bool,
        };

        pub const Managed = HashMap(K, V, Context, max_load_percentage);

        pub fn promote(self: Self, allocator: Allocator) Managed {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call promoteContext instead.");
            return promoteContext(self, allocator, undefined);
        }

        pub fn promoteContext(self: Self, allocator: Allocator, ctx: Context) Managed {
            return .{
                .unmanaged = self,
                .allocator = allocator,
                .ctx = ctx,
            };
        }

        /// Puts the hash map into a state where any method call that would
        /// cause an existing key or value pointer to become invalidated will
        /// instead trigger an assertion.
        ///
        /// An additional call to `lockPointers` in such state also triggers an
        /// assertion.
        ///
        /// `unlockPointers` returns the hash map to the previous state.
        pub fn lockPointers(self: *Self) void {
            self.pointer_stability.lock();
        }

        /// Undoes a call to `lockPointers`.
        pub fn unlockPointers(self: *Self) void {
            self.pointer_stability.unlock();
        }

        fn isUnderMaxLoadPercentage(size: Size, cap: Size) bool {
            return size * 100 < max_load_percentage * cap;
        }

        pub fn deinit(self: *Self, allocator: Allocator) void {
            self.pointer_stability.assertUnlocked();
            self.deallocate(allocator);
            self.* = undefined;
        }

        fn capacityForSize(size: Size) Size {
            var new_cap: u32 = @intCast((@as(u64, size) * 100) / max_load_percentage + 1);
            new_cap = math.ceilPowerOfTwo(u32, new_cap) catch unreachable;
            return new_cap;
        }

        pub fn ensureTotalCapacity(self: *Self, allocator: Allocator, new_size: Size) Allocator.Error!void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call ensureTotalCapacityContext instead.");
            return ensureTotalCapacityContext(self, allocator, new_size, undefined);
        }
        pub fn ensureTotalCapacityContext(self: *Self, allocator: Allocator, new_size: Size, ctx: Context) Allocator.Error!void {
            self.pointer_stability.lock();
            defer self.pointer_stability.unlock();
            if (new_size > self.size)
                try self.growIfNeeded(allocator, new_size - self.size, ctx);
        }

        pub fn ensureUnusedCapacity(self: *Self, allocator: Allocator, additional_size: Size) Allocator.Error!void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call ensureUnusedCapacityContext instead.");
            return ensureUnusedCapacityContext(self, allocator, additional_size, undefined);
        }
        pub fn ensureUnusedCapacityContext(self: *Self, allocator: Allocator, additional_size: Size, ctx: Context) Allocator.Error!void {
            return ensureTotalCapacityContext(self, allocator, self.count() + additional_size, ctx);
        }

        pub fn clearRetainingCapacity(self: *Self) void {
            self.pointer_stability.lock();
            defer self.pointer_stability.unlock();
            if (self.metadata) |_| {
                self.initMetadatas();
                self.size = 0;
                self.available = @truncate((self.capacity() * max_load_percentage) / 100);
            }
        }

        pub fn clearAndFree(self: *Self, allocator: Allocator) void {
            self.pointer_stability.lock();
            defer self.pointer_stability.unlock();
            self.deallocate(allocator);
            self.size = 0;
            self.available = 0;
        }

        pub fn count(self: Self) Size {
            return self.size;
        }

        fn header(self: Self) *Header {
            return @ptrCast(@as([*]Header, @ptrCast(@alignCast(self.metadata.?))) - 1);
        }

        fn keys(self: Self) [*]K {
            return self.header().keys;
        }

        fn values(self: Self) [*]V {
            return self.header().values;
        }

        pub fn capacity(self: Self) Size {
            if (self.metadata == null) return 0;

            return self.header().capacity;
        }

        pub fn iterator(self: *const Self) Iterator {
            return .{ .hm = self };
        }

        pub fn keyIterator(self: Self) KeyIterator {
            if (self.metadata) |metadata| {
                return .{
                    .len = self.capacity(),
                    .metadata = metadata,
                    .items = self.keys(),
                };
            } else {
                return .{
                    .len = 0,
                    .metadata = undefined,
                    .items = undefined,
                };
            }
        }

        pub fn valueIterator(self: Self) ValueIterator {
            if (self.metadata) |metadata| {
                return .{
                    .len = self.capacity(),
                    .metadata = metadata,
                    .items = self.values(),
                };
            } else {
                return .{
                    .len = 0,
                    .metadata = undefined,
                    .items = undefined,
                };
            }
        }

        /// Insert an entry in the map. Assumes it is not already present.
        pub fn putNoClobber(self: *Self, allocator: Allocator, key: K, value: V) Allocator.Error!void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call putNoClobberContext instead.");
            return self.putNoClobberContext(allocator, key, value, undefined);
        }
        pub fn putNoClobberContext(self: *Self, allocator: Allocator, key: K, value: V, ctx: Context) Allocator.Error!void {
            {
                self.pointer_stability.lock();
                defer self.pointer_stability.unlock();
                try self.growIfNeeded(allocator, 1, ctx);
            }
            self.putAssumeCapacityNoClobberContext(key, value, ctx);
        }

        /// Asserts there is enough capacity to store the new key-value pair.
        /// Clobbers any existing data. To detect if a put would clobber
        /// existing data, see `getOrPutAssumeCapacity`.
        pub fn putAssumeCapacity(self: *Self, key: K, value: V) void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call putAssumeCapacityContext instead.");
            return self.putAssumeCapacityContext(key, value, undefined);
        }
        pub fn putAssumeCapacityContext(self: *Self, key: K, value: V, ctx: Context) void {
            const gop = self.getOrPutAssumeCapacityContext(key, ctx);
            gop.value_ptr.* = value;
        }

        /// Insert an entry in the map. Assumes it is not already present,
        /// and that no allocation is needed.
        pub fn putAssumeCapacityNoClobber(self: *Self, key: K, value: V) void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call putAssumeCapacityNoClobberContext instead.");
            return self.putAssumeCapacityNoClobberContext(key, value, undefined);
        }
        pub fn putAssumeCapacityNoClobberContext(self: *Self, key: K, value: V, ctx: Context) void {
            assert(!self.containsContext(key, ctx));

            const hash: Hash = ctx.hash(key);
            const mask = self.capacity() - 1;
            var idx: usize = @truncate(hash & mask);

            var metadata = self.metadata.? + idx;
            while (metadata[0].isUsed()) {
                idx = (idx + 1) & mask;
                metadata = self.metadata.? + idx;
            }

            assert(self.available > 0);
            self.available -= 1;

            const fingerprint = Metadata.takeFingerprint(hash);
            metadata[0].fill(fingerprint);
            self.keys()[idx] = key;
            self.values()[idx] = value;

            self.size += 1;
        }

        /// Inserts a new `Entry` into the hash map, returning the previous one, if any.
        pub fn fetchPut(self: *Self, allocator: Allocator, key: K, value: V) Allocator.Error!?KV {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call fetchPutContext instead.");
            return self.fetchPutContext(allocator, key, value, undefined);
        }
        pub fn fetchPutContext(self: *Self, allocator: Allocator, key: K, value: V, ctx: Context) Allocator.Error!?KV {
            const gop = try self.getOrPutContext(allocator, key, ctx);
            var result: ?KV = null;
            if (gop.found_existing) {
                result = KV{
                    .key = gop.key_ptr.*,
                    .value = gop.value_ptr.*,
                };
            }
            gop.value_ptr.* = value;
            return result;
        }

        /// Inserts a new `Entry` into the hash map, returning the previous one, if any.
        /// If insertion happens, asserts there is enough capacity without allocating.
        pub fn fetchPutAssumeCapacity(self: *Self, key: K, value: V) ?KV {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call fetchPutAssumeCapacityContext instead.");
            return self.fetchPutAssumeCapacityContext(key, value, undefined);
        }
        pub fn fetchPutAssumeCapacityContext(self: *Self, key: K, value: V, ctx: Context) ?KV {
            const gop = self.getOrPutAssumeCapacityContext(key, ctx);
            var result: ?KV = null;
            if (gop.found_existing) {
                result = KV{
                    .key = gop.key_ptr.*,
                    .value = gop.value_ptr.*,
                };
            }
            gop.value_ptr.* = value;
            return result;
        }

        /// If there is an `Entry` with a matching key, it is deleted from
        /// the hash map, and then returned from this function.
        pub fn fetchRemove(self: *Self, key: K) ?KV {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call fetchRemoveContext instead.");
            return self.fetchRemoveContext(key, undefined);
        }
        pub fn fetchRemoveContext(self: *Self, key: K, ctx: Context) ?KV {
            return self.fetchRemoveAdapted(key, ctx);
        }
        pub fn fetchRemoveAdapted(self: *Self, key: anytype, ctx: anytype) ?KV {
            if (self.getIndex(key, ctx)) |idx| {
                const old_key = &self.keys()[idx];
                const old_val = &self.values()[idx];
                const result = KV{
                    .key = old_key.*,
                    .value = old_val.*,
                };
                self.metadata.?[idx].remove();
                old_key.* = undefined;
                old_val.* = undefined;
                self.size -= 1;
                self.available += 1;
                return result;
            }

            return null;
        }

        /// Find the index containing the data for the given key.
        fn getIndex(self: Self, key: anytype, ctx: anytype) ?usize {
            if (self.size == 0) {
                // We use cold instead of unlikely to force a jump to this case,
                // no matter the weight of the opposing side.
                @branchHint(.cold);
                return null;
            }

            // If you get a compile error on this line, it means that your generic hash
            // function is invalid for these parameters.
            const hash: Hash = ctx.hash(key);

            const mask = self.capacity() - 1;
            const fingerprint = Metadata.takeFingerprint(hash);
            // Don't loop indefinitely when there are no empty slots.
            var limit = self.capacity();
            var idx = @as(usize, @truncate(hash & mask));

            var metadata = self.metadata.? + idx;
            while (!metadata[0].isFree() and limit != 0) {
                if (metadata[0].isUsed() and metadata[0].fingerprint == fingerprint) {
                    const test_key = &self.keys()[idx];

                    if (ctx.eql(key, test_key.*)) {
                        return idx;
                    }
                }

                limit -= 1;
                idx = (idx + 1) & mask;
                metadata = self.metadata.? + idx;
            }

            return null;
        }

        pub fn getEntry(self: Self, key: K) ?Entry {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getEntryContext instead.");
            return self.getEntryContext(key, undefined);
        }
        pub fn getEntryContext(self: Self, key: K, ctx: Context) ?Entry {
            return self.getEntryAdapted(key, ctx);
        }
        pub fn getEntryAdapted(self: Self, key: anytype, ctx: anytype) ?Entry {
            if (self.getIndex(key, ctx)) |idx| {
                return Entry{
                    .key_ptr = &self.keys()[idx],
                    .value_ptr = &self.values()[idx],
                };
            }
            return null;
        }

        /// Insert an entry if the associated key is not already present, otherwise update preexisting value.
        pub fn put(self: *Self, allocator: Allocator, key: K, value: V) Allocator.Error!void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call putContext instead.");
            return self.putContext(allocator, key, value, undefined);
        }
        pub fn putContext(self: *Self, allocator: Allocator, key: K, value: V, ctx: Context) Allocator.Error!void {
            const result = try self.getOrPutContext(allocator, key, ctx);
            result.value_ptr.* = value;
        }

        /// Get an optional pointer to the actual key associated with adapted key, if present.
        pub fn getKeyPtr(self: Self, key: K) ?*K {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getKeyPtrContext instead.");
            return self.getKeyPtrContext(key, undefined);
        }
        pub fn getKeyPtrContext(self: Self, key: K, ctx: Context) ?*K {
            return self.getKeyPtrAdapted(key, ctx);
        }
        pub fn getKeyPtrAdapted(self: Self, key: anytype, ctx: anytype) ?*K {
            if (self.getIndex(key, ctx)) |idx| {
                return &self.keys()[idx];
            }
            return null;
        }

        /// Get a copy of the actual key associated with adapted key, if present.
        pub fn getKey(self: Self, key: K) ?K {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getKeyContext instead.");
            return self.getKeyContext(key, undefined);
        }
        pub fn getKeyContext(self: Self, key: K, ctx: Context) ?K {
            return self.getKeyAdapted(key, ctx);
        }
        pub fn getKeyAdapted(self: Self, key: anytype, ctx: anytype) ?K {
            if (self.getIndex(key, ctx)) |idx| {
                return self.keys()[idx];
            }
            return null;
        }

        /// Get an optional pointer to the value associated with key, if present.
        pub fn getPtr(self: Self, key: K) ?*V {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getPtrContext instead.");
            return self.getPtrContext(key, undefined);
        }
        pub fn getPtrContext(self: Self, key: K, ctx: Context) ?*V {
            return self.getPtrAdapted(key, ctx);
        }
        pub fn getPtrAdapted(self: Self, key: anytype, ctx: anytype) ?*V {
            if (self.getIndex(key, ctx)) |idx| {
                return &self.values()[idx];
            }
            return null;
        }

        /// Get a copy of the value associated with key, if present.
        pub fn get(self: Self, key: K) ?V {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getContext instead.");
            return self.getContext(key, undefined);
        }
        pub fn getContext(self: Self, key: K, ctx: Context) ?V {
            return self.getAdapted(key, ctx);
        }
        pub fn getAdapted(self: Self, key: anytype, ctx: anytype) ?V {
            if (self.getIndex(key, ctx)) |idx| {
                return self.values()[idx];
            }
            return null;
        }

        pub fn getOrPut(self: *Self, allocator: Allocator, key: K) Allocator.Error!GetOrPutResult {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getOrPutContext instead.");
            return self.getOrPutContext(allocator, key, undefined);
        }
        pub fn getOrPutContext(self: *Self, allocator: Allocator, key: K, ctx: Context) Allocator.Error!GetOrPutResult {
            const gop = try self.getOrPutContextAdapted(allocator, key, ctx, ctx);
            if (!gop.found_existing) {
                gop.key_ptr.* = key;
            }
            return gop;
        }
        pub fn getOrPutAdapted(self: *Self, allocator: Allocator, key: anytype, key_ctx: anytype) Allocator.Error!GetOrPutResult {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getOrPutContextAdapted instead.");
            return self.getOrPutContextAdapted(allocator, key, key_ctx, undefined);
        }
        pub fn getOrPutContextAdapted(self: *Self, allocator: Allocator, key: anytype, key_ctx: anytype, ctx: Context) Allocator.Error!GetOrPutResult {
            {
                self.pointer_stability.lock();
                defer self.pointer_stability.unlock();
                self.growIfNeeded(allocator, 1, ctx) catch |err| {
                    // If allocation fails, try to do the lookup anyway.
                    // If we find an existing item, we can return it.
                    // Otherwise return the error, we could not add another.
                    const index = self.getIndex(key, key_ctx) orelse return err;
                    return GetOrPutResult{
                        .key_ptr = &self.keys()[index],
                        .value_ptr = &self.values()[index],
                        .found_existing = true,
                    };
                };
            }
            return self.getOrPutAssumeCapacityAdapted(key, key_ctx);
        }

        pub fn getOrPutAssumeCapacity(self: *Self, key: K) GetOrPutResult {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getOrPutAssumeCapacityContext instead.");
            return self.getOrPutAssumeCapacityContext(key, undefined);
        }
        pub fn getOrPutAssumeCapacityContext(self: *Self, key: K, ctx: Context) GetOrPutResult {
            const result = self.getOrPutAssumeCapacityAdapted(key, ctx);
            if (!result.found_existing) {
                result.key_ptr.* = key;
            }
            return result;
        }
        pub fn getOrPutAssumeCapacityAdapted(self: *Self, key: anytype, ctx: anytype) GetOrPutResult {

            // If you get a compile error on this line, it means that your generic hash
            // function is invalid for these parameters.
            const hash: Hash = ctx.hash(key);

            const mask = self.capacity() - 1;
            const fingerprint = Metadata.takeFingerprint(hash);
            var limit = self.capacity();
            var idx = @as(usize, @truncate(hash & mask));

            var first_tombstone_idx: usize = self.capacity(); // invalid index
            var metadata = self.metadata.? + idx;
            while (!metadata[0].isFree() and limit != 0) {
                if (metadata[0].isUsed() and metadata[0].fingerprint == fingerprint) {
                    const test_key = &self.keys()[idx];
                    // If you get a compile error on this line, it means that your generic eql
                    // function is invalid for these parameters.

                    if (ctx.eql(key, test_key.*)) {
                        return GetOrPutResult{
                            .key_ptr = test_key,
                            .value_ptr = &self.values()[idx],
                            .found_existing = true,
                        };
                    }
                } else if (first_tombstone_idx == self.capacity() and metadata[0].isTombstone()) {
                    first_tombstone_idx = idx;
                }

                limit -= 1;
                idx = (idx + 1) & mask;
                metadata = self.metadata.? + idx;
            }

            if (first_tombstone_idx < self.capacity()) {
                // Cheap try to lower probing lengths after deletions. Recycle a tombstone.
                idx = first_tombstone_idx;
                metadata = self.metadata.? + idx;
            }
            // We're using a slot previously free or a tombstone.
            self.available -= 1;

            metadata[0].fill(fingerprint);
            const new_key = &self.keys()[idx];
            const new_value = &self.values()[idx];
            new_key.* = undefined;
            new_value.* = undefined;
            self.size += 1;

            return GetOrPutResult{
                .key_ptr = new_key,
                .value_ptr = new_value,
                .found_existing = false,
            };
        }

        pub fn getOrPutValue(self: *Self, allocator: Allocator, key: K, value: V) Allocator.Error!Entry {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getOrPutValueContext instead.");
            return self.getOrPutValueContext(allocator, key, value, undefined);
        }
        pub fn getOrPutValueContext(self: *Self, allocator: Allocator, key: K, value: V, ctx: Context) Allocator.Error!Entry {
            const res = try self.getOrPutAdapted(allocator, key, ctx);
            if (!res.found_existing) {
                res.key_ptr.* = key;
                res.value_ptr.* = value;
            }
            return Entry{ .key_ptr = res.key_ptr, .value_ptr = res.value_ptr };
        }

        /// Return true if there is a value associated with key in the map.
        pub fn contains(self: Self, key: K) bool {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call containsContext instead.");
            return self.containsContext(key, undefined);
        }
        pub fn containsContext(self: Self, key: K, ctx: Context) bool {
            return self.containsAdapted(key, ctx);
        }
        pub fn containsAdapted(self: Self, key: anytype, ctx: anytype) bool {
            return self.getIndex(key, ctx) != null;
        }

        fn removeByIndex(self: *Self, idx: usize) void {
            self.metadata.?[idx].remove();
            self.keys()[idx] = undefined;
            self.values()[idx] = undefined;
            self.size -= 1;
            self.available += 1;
        }

        /// If there is an `Entry` with a matching key, it is deleted from
        /// the hash map, and this function returns true.  Otherwise this
        /// function returns false.
        ///
        /// TODO: answer the question in these doc comments, does this
        /// increase the unused capacity by one?
        pub fn remove(self: *Self, key: K) bool {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call removeContext instead.");
            return self.removeContext(key, undefined);
        }

        /// TODO: answer the question in these doc comments, does this
        /// increase the unused capacity by one?
        pub fn removeContext(self: *Self, key: K, ctx: Context) bool {
            return self.removeAdapted(key, ctx);
        }

        /// TODO: answer the question in these doc comments, does this
        /// increase the unused capacity by one?
        pub fn removeAdapted(self: *Self, key: anytype, ctx: anytype) bool {
            if (self.getIndex(key, ctx)) |idx| {
                self.removeByIndex(idx);
                return true;
            }

            return false;
        }

        /// Delete the entry with key pointed to by key_ptr from the hash map.
        /// key_ptr is assumed to be a valid pointer to a key that is present
        /// in the hash map.
        ///
        /// TODO: answer the question in these doc comments, does this
        /// increase the unused capacity by one?
        pub fn removeByPtr(self: *Self, key_ptr: *K) void {
            // TODO: replace with pointer subtraction once supported by zig
            // if @sizeOf(K) == 0 then there is at most one item in the hash
            // map, which is assumed to exist as key_ptr must be valid.  This
            // item must be at index 0.
            const idx = if (@sizeOf(K) > 0)
                (@intFromPtr(key_ptr) - @intFromPtr(self.keys())) / @sizeOf(K)
            else
                0;

            self.removeByIndex(idx);
        }

        fn initMetadatas(self: *Self) void {
            @memset(@as([*]u8, @ptrCast(self.metadata.?))[0 .. @sizeOf(Metadata) * self.capacity()], 0);
        }

        // This counts the number of occupied slots (not counting tombstones), which is
        // what has to stay under the max_load_percentage of capacity.
        fn load(self: Self) Size {
            const max_load = (self.capacity() * max_load_percentage) / 100;
            assert(max_load >= self.available);
            return @as(Size, @truncate(max_load - self.available));
        }

        fn growIfNeeded(self: *Self, allocator: Allocator, new_count: Size, ctx: Context) Allocator.Error!void {
            if (new_count > self.available) {
                try self.grow(allocator, capacityForSize(self.load() + new_count), ctx);
            }
        }

        pub fn clone(self: Self, allocator: Allocator) Allocator.Error!Self {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call cloneContext instead.");
            return self.cloneContext(allocator, @as(Context, undefined));
        }
        pub fn cloneContext(self: Self, allocator: Allocator, new_ctx: anytype) Allocator.Error!HashMapUnmanaged(K, V, @TypeOf(new_ctx), max_load_percentage) {
            var other: HashMapUnmanaged(K, V, @TypeOf(new_ctx), max_load_percentage) = .empty;
            if (self.size == 0)
                return other;

            const new_cap = capacityForSize(self.size);
            try other.allocate(allocator, new_cap);
            other.initMetadatas();
            other.available = @truncate((new_cap * max_load_percentage) / 100);

            var i: Size = 0;
            var metadata = self.metadata.?;
            const keys_ptr = self.keys();
            const values_ptr = self.values();
            while (i < self.capacity()) : (i += 1) {
                if (metadata[i].isUsed()) {
                    other.putAssumeCapacityNoClobberContext(keys_ptr[i], values_ptr[i], new_ctx);
                    if (other.size == self.size)
                        break;
                }
            }

            return other;
        }

        /// Set the map to an empty state, making deinitialization a no-op, and
        /// returning a copy of the original.
        pub fn move(self: *Self) Self {
            self.pointer_stability.assertUnlocked();
            const result = self.*;
            self.* = .empty;
            return result;
        }

        /// Rehash the map, in-place.
        ///
        /// Over time, due to the current tombstone-based implementation, a
        /// HashMap could become fragmented due to the buildup of tombstone
        /// entries that causes a performance degradation due to excessive
        /// probing. The kind of pattern that might cause this is a long-lived
        /// HashMap with repeated inserts and deletes.
        ///
        /// After this function is called, there will be no tombstones in
        /// the HashMap, each of the entries is rehashed and any existing
        /// key/value pointers into the HashMap are invalidated.
        pub fn rehash(self: *Self, ctx: anytype) void {
            const mask = self.capacity() - 1;

            var metadata = self.metadata.?;
            var keys_ptr = self.keys();
            var values_ptr = self.values();
            var curr: Size = 0;

            // While we are re-hashing every slot, we will use the
            // fingerprint to mark used buckets as being used and either free
            // (needing to be rehashed) or tombstone (already rehashed).

            while (curr < self.capacity()) : (curr += 1) {
                metadata[curr].fingerprint = Metadata.free;
            }

            // Now iterate over all the buckets, rehashing them

            curr = 0;
            while (curr < self.capacity()) {
                if (!metadata[curr].isUsed()) {
                    assert(metadata[curr].isFree());
                    curr += 1;
                    continue;
                }

                const hash = ctx.hash(keys_ptr[curr]);
                const fingerprint = Metadata.takeFingerprint(hash);
                var idx = @as(usize, @truncate(hash & mask));

                // For each bucket, rehash to an index:
                // 1) before the cursor, probed into a free slot, or
                // 2) equal to the cursor, no need to move, or
                // 3) ahead of the cursor, probing over already rehashed

                while ((idx < curr and metadata[idx].isUsed()) or
                    (idx > curr and metadata[idx].fingerprint == Metadata.tombstone))
                {
                    idx = (idx + 1) & mask;
                }

                if (idx < curr) {
                    assert(metadata[idx].isFree());
                    metadata[idx].fill(fingerprint);
                    keys_ptr[idx] = keys_ptr[curr];
                    values_ptr[idx] = values_ptr[curr];

                    metadata[curr].used = 0;
                    assert(metadata[curr].isFree());
                    keys_ptr[curr] = undefined;
                    values_ptr[curr] = undefined;

                    curr += 1;
                } else if (idx == curr) {
                    metadata[idx].fingerprint = fingerprint;
                    curr += 1;
                } else {
                    assert(metadata[idx].fingerprint != Metadata.tombstone);
                    metadata[idx].fingerprint = Metadata.tombstone;
                    if (metadata[idx].isUsed()) {
                        std.mem.swap(K, &keys_ptr[curr], &keys_ptr[idx]);
                        std.mem.swap(V, &values_ptr[curr], &values_ptr[idx]);
                    } else {
                        metadata[idx].used = 1;
                        keys_ptr[idx] = keys_ptr[curr];
                        values_ptr[idx] = values_ptr[curr];

                        metadata[curr].fingerprint = Metadata.free;
                        metadata[curr].used = 0;
                        keys_ptr[curr] = undefined;
                        values_ptr[curr] = undefined;

                        curr += 1;
                    }
                }
            }
        }

        fn grow(self: *Self, allocator: Allocator, new_capacity: Size, ctx: Context) Allocator.Error!void {
            @branchHint(.cold);
            const new_cap = @max(new_capacity, minimal_capacity);
            assert(new_cap > self.capacity());
            assert(std.math.isPowerOfTwo(new_cap));

            var map: Self = .{};
            try map.allocate(allocator, new_cap);
            errdefer comptime unreachable;
            map.pointer_stability.lock();
            map.initMetadatas();
            map.available = @truncate((new_cap * max_load_percentage) / 100);

            if (self.size != 0) {
                const old_capacity = self.capacity();
                for (
                    self.metadata.?[0..old_capacity],
                    self.keys()[0..old_capacity],
                    self.values()[0..old_capacity],
                ) |m, k, v| {
                    if (!m.isUsed()) continue;
                    map.putAssumeCapacityNoClobberContext(k, v, ctx);
                    if (map.size == self.size) break;
                }
            }

            self.size = 0;
            self.pointer_stability = .{};
            std.mem.swap(Self, self, &map);
            map.deinit(allocator);
        }

        fn allocate(self: *Self, allocator: Allocator, new_capacity: Size) Allocator.Error!void {
            const header_align = @alignOf(Header);
            const key_align = if (@sizeOf(K) == 0) 1 else @alignOf(K);
            const val_align = if (@sizeOf(V) == 0) 1 else @alignOf(V);
            const max_align = comptime @max(header_align, key_align, val_align);

            const new_cap: usize = new_capacity;
            const meta_size = @sizeOf(Header) + new_cap * @sizeOf(Metadata);
            comptime assert(@alignOf(Metadata) == 1);

            const keys_start = std.mem.alignForward(usize, meta_size, key_align);
            const keys_end = keys_start + new_cap * @sizeOf(K);

            const vals_start = std.mem.alignForward(usize, keys_end, val_align);
            const vals_end = vals_start + new_cap * @sizeOf(V);

            const total_size = std.mem.alignForward(usize, vals_end, max_align);

            const slice = try allocator.alignedAlloc(u8, max_align, total_size);
            const ptr: [*]u8 = @ptrCast(slice.ptr);

            const metadata = ptr + @sizeOf(Header);

            const hdr = @as(*Header, @ptrCast(@alignCast(ptr)));
            if (@sizeOf([*]V) != 0) {
                hdr.values = @ptrCast(@alignCast((ptr + vals_start)));
            }
            if (@sizeOf([*]K) != 0) {
                hdr.keys = @ptrCast(@alignCast((ptr + keys_start)));
            }
            hdr.capacity = new_capacity;
            self.metadata = @ptrCast(@alignCast(metadata));
        }

        fn deallocate(self: *Self, allocator: Allocator) void {
            if (self.metadata == null) return;

            const header_align = @alignOf(Header);
            const key_align = if (@sizeOf(K) == 0) 1 else @alignOf(K);
            const val_align = if (@sizeOf(V) == 0) 1 else @alignOf(V);
            const max_align = comptime @max(header_align, key_align, val_align);

            const cap: usize = self.capacity();
            const meta_size = @sizeOf(Header) + cap * @sizeOf(Metadata);
            comptime assert(@alignOf(Metadata) == 1);

            const keys_start = std.mem.alignForward(usize, meta_size, key_align);
            const keys_end = keys_start + cap * @sizeOf(K);

            const vals_start = std.mem.alignForward(usize, keys_end, val_align);
            const vals_end = vals_start + cap * @sizeOf(V);

            const total_size = std.mem.alignForward(usize, vals_end, max_align);

            const slice = @as([*]align(max_align) u8, @alignCast(@ptrCast(self.header())))[0..total_size];
            allocator.free(slice);

            self.metadata = null;
            self.available = 0;
        }

        /// This function is used in the debugger pretty formatters in tools/ to fetch the
        /// header type to facilitate fancy debug printing for this type.
        fn dbHelper(self: *Self, hdr: *Header, entry: *Entry) void {
            _ = self;
            _ = hdr;
            _ = entry;
        }

        comptime {
            if (!builtin.strip_debug_info) _ = switch (builtin.zig_backend) {
                .stage2_llvm => &dbHelper,
                .stage2_x86_64 => KV,
                else => {},
            };
        }
    };
}  