path: root/src/terminal/hash_map.zig

//! This file contains a fork of the Zig stdlib HashMap implementation tuned
//! for use with our terminal page representation.
//!
//! The main goal we need to achieve that wasn't possible with the stdlib
//! HashMap is to utilize offsets rather than full pointers so that we can
//! copy around the entire backing memory and keep the hash map working.
//!
//! Additionally, for serialization/deserialization purposes, we need to be
//! able to create a HashMap instance and manually set the offsets up. The
//! stdlib HashMap does not export Metadata so this isn't possible.
//!
//! Also, I want to be able to understand possible capacity for a given K,V
//! type and fixed memory amount. The stdlib HashMap doesn't publish its
//! internal allocation size calculation.
//!
//! Finally, I removed many of the APIs that we'll never require for our
//! usage just so that this file is smaller, easier to understand, and has
//! less opportunity for bugs.
//!
//! Besides these shortcomings, the stdlib HashMap has some great qualities
//! that we want to keep, namely the fact that it is backed by a single large
//! allocation rather than pointers to separate allocations. This is important
//! because our terminal page representation is backed by a single large
//! allocation so we can give the HashMap a slice of memory to operate in.
//!
//! I haven't carefully benchmarked this implementation against other hash
//! map implementations. It's possible using some of the newer variants out
//! there would be better. However, I trust the built-in version is pretty good
//! and its more important to get the terminal page representation working
//! first then we can measure and improve this later if we find it to be a
//! bottleneck.

const std = @import("std");
const builtin = @import("builtin");
const assert = std.debug.assert;
const autoHash = std.hash.autoHash;
const math = std.math;
const mem = std.mem;
const Allocator = mem.Allocator;
const Wyhash = std.hash.Wyhash;

const Offset = @import("size.zig").Offset;
const OffsetBuf = @import("size.zig").OffsetBuf;
const getOffset = @import("size.zig").getOffset;

pub fn AutoOffsetHashMap(comptime K: type, comptime V: type) type {
    return OffsetHashMap(K, V, AutoContext(K));
}

fn AutoHashMapUnmanaged(comptime K: type, comptime V: type) type {
    return HashMapUnmanaged(K, V, AutoContext(K));
}

fn AutoContext(comptime K: type) type {
    return struct {
        pub const hash = std.hash_map.getAutoHashFn(K, @This());
        pub const eql = std.hash_map.getAutoEqlFn(K, @This());
    };
}

/// A HashMap type that uses offsets rather than pointers, making it
/// possible to efficiently move around the backing memory without
/// invalidating the HashMap.
pub fn OffsetHashMap(
    comptime K: type,
    comptime V: type,
    comptime Context: type,
) type {
    return struct {
        const Self = @This();

        /// This is the pointer-based map that we're wrapping.
        pub const Unmanaged = HashMapUnmanaged(K, V, Context);
        pub const Layout = Unmanaged.Layout;

        /// This is the alignment that the base pointer must have.
        pub const base_align = Unmanaged.base_align;

        metadata: Offset(Unmanaged.Metadata) = .{},

        /// Returns the total size of the backing memory required for a
        /// HashMap with the given capacity. The base ptr must also be
        /// aligned to base_align.
        pub fn layout(cap: Unmanaged.Size) Layout {
            return Unmanaged.layoutForCapacity(cap);
        }

        /// Initialize a new HashMap with the given capacity and backing
        /// memory. The backing memory must be aligned to base_align.
        pub fn init(buf: OffsetBuf, l: Layout) Self {
            assert(base_align.check(@intFromPtr(buf.start())));

            const m = Unmanaged.init(buf, l);
            return .{ .metadata = getOffset(
                Unmanaged.Metadata,
                buf,
                @ptrCast(m.metadata.?),
            ) };
        }

        /// Returns the pointer-based map from a base pointer.
        pub fn map(self: Self, base: anytype) Unmanaged {
            return .{ .metadata = self.metadata.ptr(base) };
        }
    };
}

/// Fork of stdlib.HashMap as of Zig 0.12 modified to to use offsets
/// for the key/values pointer. The metadata is still a pointer to limit
/// the amount of arithmetic required to access it. See the file comment
/// for full details.
fn HashMapUnmanaged(
    comptime K: type,
    comptime V: type,
    comptime Context: type,
) type {
    return struct {
        const Self = @This();

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

        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 base_align: mem.Alignment = .fromByteUnits(@max(
            header_align,
            key_align,
            val_align,
        ));

        // 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,

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

        // 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 {
            /// The keys/values offset are relative to the metadata
            values: Offset(V),
            keys: Offset(K),
            capacity: Size,
            size: 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.header().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,
        };

        /// Initialize a hash map with a given capacity and a buffer. The
        /// buffer must fit within the size defined by `layoutForCapacity`.
        pub fn init(buf: OffsetBuf, layout: Layout) Self {
            assert(base_align.check(@intFromPtr(buf.start())));

            // Get all our main pointers
            const metadata_buf = buf.rebase(@sizeOf(Header));
            const metadata_ptr: [*]Metadata = @ptrCast(metadata_buf.start());

            // Build our map
            var map: Self = .{ .metadata = metadata_ptr };
            const hdr = map.header();
            hdr.capacity = layout.capacity;
            hdr.size = 0;
            if (@sizeOf([*]K) != 0) hdr.keys = metadata_buf.member(K, layout.keys_start);
            if (@sizeOf([*]V) != 0) hdr.values = metadata_buf.member(V, layout.vals_start);
            map.initMetadatas();

            return map;
        }

        pub fn ensureTotalCapacity(self: *Self, new_size: Size) Allocator.Error!void {
            if (new_size > self.header().size) {
                try self.growIfNeeded(new_size - self.header().size);
            }
        }

        pub fn ensureUnusedCapacity(self: *Self, additional_size: Size) Allocator.Error!void {
            return ensureTotalCapacity(self, self.count() + additional_size);
        }

        pub fn clearRetainingCapacity(self: *Self) void {
            if (self.metadata) |_| {
                self.initMetadatas();
                self.header().size = 0;
            }
        }

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

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

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

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

        pub fn capacity(self: *const 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: *const 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: *const 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, key: K, value: V) Allocator.Error!void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call putNoClobberContext instead.");
            return self.putNoClobberContext(key, value, undefined);
        }
        pub fn putNoClobberContext(self: *Self, key: K, value: V, ctx: Context) Allocator.Error!void {
            assert(!self.containsContext(key, ctx));
            try self.growIfNeeded(1);

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

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

            const fingerprint = Metadata.takeFingerprint(hash);
            metadata[0].fill(fingerprint);
            self.keys()[idx] = key;
            self.values()[idx] = value;
            self.header().size += 1;
        }

        /// Inserts a new `Entry` into the hash map, returning the previous one, if any.
        pub fn fetchPut(self: *Self, key: K, value: V) Allocator.Error!?KV {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call fetchPutContext instead.");
            return self.fetchPutContext(key, value, undefined);
        }
        pub fn fetchPutContext(self: *Self, key: K, value: V, ctx: Context) Allocator.Error!?KV {
            const gop = try self.getOrPutContext(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.header().size -= 1;
                return result;
            }

            return null;
        }

        /// Find the index containing the data for the given key.
        /// Whether this function returns null is almost always
        /// branched on after this function returns, and this function
        /// returns null/not null from separate code paths.  We
        /// want the optimizer to remove that branch and instead directly
        /// fuse the basic blocks after the branch to the basic blocks
        /// from this function.  To encourage that, this function is
        /// marked as inline.
        inline fn getIndex(self: Self, key: anytype, ctx: anytype) ?usize {
            if (self.header().size == 0) {
                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 = ctx.hash(key);
            if (@TypeOf(hash) != Hash) {
                @compileError("Context " ++ @typeName(@TypeOf(ctx)) ++ " has a generic hash function that returns the wrong type! " ++ @typeName(Hash) ++ " was expected, but found " ++ @typeName(@TypeOf(hash)));
            }
            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 you get a compile error on this line, it means that your generic eql
                    // function is invalid for these parameters.
                    const eql = ctx.eql(key, test_key.*);
                    // verifyContext can't verify the return type of generic eql functions,
                    // so we need to double-check it here.
                    if (@TypeOf(eql) != bool) {
                        @compileError("Context " ++ @typeName(@TypeOf(ctx)) ++ " has a generic eql function that returns the wrong type! bool was expected, but found " ++ @typeName(@TypeOf(eql)));
                    }
                    if (eql) {
                        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, key: K, value: V) Allocator.Error!void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call putContext instead.");
            return self.putContext(key, value, undefined);
        }
        pub fn putContext(self: *Self, key: K, value: V, ctx: Context) Allocator.Error!void {
            const result = try self.getOrPutContext(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, key: K) Allocator.Error!GetOrPutResult {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getOrPutContext instead.");
            return self.getOrPutContext(key, undefined);
        }
        pub fn getOrPutContext(self: *Self, key: K, ctx: Context) Allocator.Error!GetOrPutResult {
            const gop = try self.getOrPutContextAdapted(key, ctx);
            if (!gop.found_existing) {
                gop.key_ptr.* = key;
            }
            return gop;
        }
        pub fn getOrPutAdapted(self: *Self, key: anytype, key_ctx: anytype) Allocator.Error!GetOrPutResult {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getOrPutContextAdapted instead.");
            return self.getOrPutContextAdapted(key, key_ctx);
        }
        pub fn getOrPutContextAdapted(self: *Self, key: anytype, key_ctx: anytype) Allocator.Error!GetOrPutResult {
            self.growIfNeeded(1) 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 = ctx.hash(key);
            // verifyContext can't verify the return type of generic hash functions,
            // so we need to double-check it here.
            if (@TypeOf(hash) != Hash) {
                @compileError("Context " ++ @typeName(@TypeOf(ctx)) ++ " has a generic hash function that returns the wrong type! " ++ @typeName(Hash) ++ " was expected, but found " ++ @typeName(@TypeOf(hash)));
            }
            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.
                    const eql = ctx.eql(key, test_key.*);
                    // verifyContext can't verify the return type of generic eql functions,
                    // so we need to double-check it here.
                    if (@TypeOf(eql) != bool) {
                        @compileError("Context " ++ @typeName(@TypeOf(ctx)) ++ " has a generic eql function that returns the wrong type! bool was expected, but found " ++ @typeName(@TypeOf(eql)));
                    }
                    if (eql) {
                        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;
            }

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

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

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

        fn growIfNeeded(self: *Self, new_count: Size) Allocator.Error!void {
            const available = self.capacity() - self.header().size;
            if (new_count > available) return error.OutOfMemory;
        }

        /// The memory layout for the underlying buffer for a given capacity.
        const Layout = struct {
            /// The total size of the buffer required. The buffer is expected
            /// to be aligned to `base_align`.
            total_size: usize,

            /// The offset to the start of the keys data.
            keys_start: usize,

            /// The offset to the start of the values data.
            vals_start: usize,

            /// The capacity that was used to calculate this layout.
            capacity: Size,
        };

        /// Returns the memory layout for the buffer for a given capacity.
        /// The actual size may be able to fit more than the given capacity
        /// because capacity is rounded up to the next power of two. This is
        /// a design requirement for this hash map implementation.
        pub fn layoutForCapacity(new_capacity: Size) Layout {
            assert(new_capacity == 0 or std.math.isPowerOfTwo(new_capacity));

            // Pack our metadata, keys, and values.
            const meta_start = @sizeOf(Header);
            const meta_end = @sizeOf(Header) + new_capacity * @sizeOf(Metadata);
            const keys_start = std.mem.alignForward(usize, meta_end, key_align);
            const keys_end = keys_start + new_capacity * @sizeOf(K);
            const vals_start = std.mem.alignForward(usize, keys_end, val_align);
            const vals_end = vals_start + new_capacity * @sizeOf(V);

            // Our total memory size required is the end of our values
            // aligned to the base required alignment.
            const total_size = std.mem.alignForward(
                usize,
                vals_end,
                base_align.toByteUnits(),
            );

            // The offsets we actually store in the map are from the
            // metadata pointer so that we can use self.metadata as
            // the base.
            const keys_offset = keys_start - meta_start;
            const vals_offset = vals_start - meta_start;

            return .{
                .total_size = total_size,
                .keys_start = keys_offset,
                .vals_start = vals_offset,
                .capacity = new_capacity,
            };
        }
    };
}

const testing = std.testing;
const expect = std.testing.expect;
const expectEqual = std.testing.expectEqual;

test "HashMap basic usage" {
    const Map = AutoHashMapUnmanaged(u32, u32);

    const alloc = testing.allocator;
    const cap = 16;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);

    var map = Map.init(.init(buf), layout);

    const count = 5;
    var i: u32 = 0;
    var total: u32 = 0;
    while (i < count) : (i += 1) {
        try map.put(i, i);
        total += i;
    }

    var sum: u32 = 0;
    var it = map.iterator();
    while (it.next()) |kv| {
        sum += kv.key_ptr.*;
    }
    try expectEqual(total, sum);

    i = 0;
    sum = 0;
    while (i < count) : (i += 1) {
        try expectEqual(i, map.get(i).?);
        sum += map.get(i).?;
    }
    try expectEqual(total, sum);
}

test "HashMap ensureTotalCapacity" {
    const Map = AutoHashMapUnmanaged(i32, i32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    const initial_capacity = map.capacity();
    try testing.expect(initial_capacity >= 20);
    var i: i32 = 0;
    while (i < 20) : (i += 1) {
        try testing.expect(map.fetchPutAssumeCapacity(i, i + 10) == null);
    }
    // shouldn't resize from putAssumeCapacity
    try testing.expect(initial_capacity == map.capacity());
}

test "HashMap ensureUnusedCapacity with tombstones" {
    const Map = AutoHashMapUnmanaged(i32, i32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var i: i32 = 0;
    while (i < 100) : (i += 1) {
        try map.ensureUnusedCapacity(1);
        map.putAssumeCapacity(i, i);
        _ = map.remove(i);
    }
}

test "HashMap clearRetainingCapacity" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 16;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    map.clearRetainingCapacity();

    try map.put(1, 1);
    try expectEqual(map.get(1).?, 1);
    try expectEqual(map.count(), 1);

    map.clearRetainingCapacity();
    map.putAssumeCapacity(1, 1);
    try expectEqual(map.get(1).?, 1);
    try expectEqual(map.count(), 1);

    const actual_cap = map.capacity();
    try expect(actual_cap > 0);

    map.clearRetainingCapacity();
    map.clearRetainingCapacity();
    try expectEqual(map.count(), 0);
    try expectEqual(map.capacity(), actual_cap);
    try expect(!map.contains(1));
}

test "HashMap ensureTotalCapacity with existing elements" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = Map.minimal_capacity;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    try map.put(0, 0);
    try expectEqual(map.count(), 1);
    try expectEqual(map.capacity(), Map.minimal_capacity);

    try testing.expectError(error.OutOfMemory, map.ensureTotalCapacity(65));
    try expectEqual(map.count(), 1);
    try expectEqual(map.capacity(), Map.minimal_capacity);
}

test "HashMap remove" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var i: u32 = 0;
    while (i < 16) : (i += 1) {
        try map.put(i, i);
    }

    i = 0;
    while (i < 16) : (i += 1) {
        if (i % 3 == 0) {
            _ = map.remove(i);
        }
    }
    try expectEqual(map.count(), 10);
    var it = map.iterator();
    while (it.next()) |kv| {
        try expectEqual(kv.key_ptr.*, kv.value_ptr.*);
        try expect(kv.key_ptr.* % 3 != 0);
    }

    i = 0;
    while (i < 16) : (i += 1) {
        if (i % 3 == 0) {
            try expect(!map.contains(i));
        } else {
            try expectEqual(map.get(i).?, i);
        }
    }
}

test "HashMap reverse removes" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var i: u32 = 0;
    while (i < 16) : (i += 1) {
        try map.putNoClobber(i, i);
    }

    i = 16;
    while (i > 0) : (i -= 1) {
        _ = map.remove(i - 1);
        try expect(!map.contains(i - 1));
        var j: u32 = 0;
        while (j < i - 1) : (j += 1) {
            try expectEqual(map.get(j).?, j);
        }
    }

    try expectEqual(map.count(), 0);
}

test "HashMap multiple removes on same metadata" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var i: u32 = 0;
    while (i < 16) : (i += 1) {
        try map.put(i, i);
    }

    _ = map.remove(7);
    _ = map.remove(15);
    _ = map.remove(14);
    _ = map.remove(13);
    try expect(!map.contains(7));
    try expect(!map.contains(15));
    try expect(!map.contains(14));
    try expect(!map.contains(13));

    i = 0;
    while (i < 13) : (i += 1) {
        if (i == 7) {
            try expect(!map.contains(i));
        } else {
            try expectEqual(map.get(i).?, i);
        }
    }

    try map.put(15, 15);
    try map.put(13, 13);
    try map.put(14, 14);
    try map.put(7, 7);
    i = 0;
    while (i < 16) : (i += 1) {
        try expectEqual(map.get(i).?, i);
    }
}

test "HashMap put and remove loop in random order" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 64;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var keys: std.ArrayList(u32) = .empty;
    defer keys.deinit(alloc);

    const size = 32;
    const iterations = 100;

    var i: u32 = 0;
    while (i < size) : (i += 1) {
        try keys.append(alloc, i);
    }
    var prng = std.Random.DefaultPrng.init(0);
    const random = prng.random();

    while (i < iterations) : (i += 1) {
        random.shuffle(u32, keys.items);

        for (keys.items) |key| {
            try map.put(key, key);
        }
        try expectEqual(map.count(), size);

        for (keys.items) |key| {
            _ = map.remove(key);
        }
        try expectEqual(map.count(), 0);
    }
}

test "HashMap put" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var i: u32 = 0;
    while (i < 16) : (i += 1) {
        try map.put(i, i);
    }

    i = 0;
    while (i < 16) : (i += 1) {
        try expectEqual(map.get(i).?, i);
    }

    i = 0;
    while (i < 16) : (i += 1) {
        try map.put(i, i * 16 + 1);
    }

    i = 0;
    while (i < 16) : (i += 1) {
        try expectEqual(map.get(i).?, i * 16 + 1);
    }
}

test "HashMap put full load" {
    const Map = AutoHashMapUnmanaged(usize, usize);
    const cap = 16;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    for (0..cap) |i| try map.put(i, i);
    for (0..cap) |i| try expectEqual(map.get(i).?, i);

    try testing.expectError(error.OutOfMemory, map.put(cap, cap));
}

test "HashMap putAssumeCapacity" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var i: u32 = 0;
    while (i < 20) : (i += 1) {
        map.putAssumeCapacityNoClobber(i, i);
    }

    i = 0;
    var sum = i;
    while (i < 20) : (i += 1) {
        sum += map.getPtr(i).?.*;
    }
    try expectEqual(sum, 190);

    i = 0;
    while (i < 20) : (i += 1) {
        map.putAssumeCapacity(i, 1);
    }

    i = 0;
    sum = i;
    while (i < 20) : (i += 1) {
        sum += map.get(i).?;
    }
    try expectEqual(sum, 20);
}

test "HashMap repeat putAssumeCapacity/remove" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    const limit = cap;

    var i: u32 = 0;
    while (i < limit) : (i += 1) {
        map.putAssumeCapacityNoClobber(i, i);
    }

    // Repeatedly delete/insert an entry without resizing the map.
    // Put to different keys so entries don't land in the just-freed slot.
    i = 0;
    while (i < 10 * limit) : (i += 1) {
        try testing.expect(map.remove(i));
        if (i % 2 == 0) {
            map.putAssumeCapacityNoClobber(limit + i, i);
        } else {
            map.putAssumeCapacity(limit + i, i);
        }
    }

    i = 9 * limit;
    while (i < 10 * limit) : (i += 1) {
        try expectEqual(map.get(limit + i), i);
    }
    try expectEqual(map.count(), limit);
}

test "HashMap getOrPut" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var i: u32 = 0;
    while (i < 10) : (i += 1) {
        try map.put(i * 2, 2);
    }

    i = 0;
    while (i < 20) : (i += 1) {
        _ = try map.getOrPutValue(i, 1);
    }

    i = 0;
    var sum = i;
    while (i < 20) : (i += 1) {
        sum += map.get(i).?;
    }

    try expectEqual(sum, 30);
}

test "HashMap basic hash map usage" {
    const Map = AutoHashMapUnmanaged(i32, i32);
    const cap = 32;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    try testing.expect((try map.fetchPut(1, 11)) == null);
    try testing.expect((try map.fetchPut(2, 22)) == null);
    try testing.expect((try map.fetchPut(3, 33)) == null);
    try testing.expect((try map.fetchPut(4, 44)) == null);

    try map.putNoClobber(5, 55);
    try testing.expect((try map.fetchPut(5, 66)).?.value == 55);
    try testing.expect((try map.fetchPut(5, 55)).?.value == 66);

    const gop1 = try map.getOrPut(5);
    try testing.expect(gop1.found_existing == true);
    try testing.expect(gop1.value_ptr.* == 55);
    gop1.value_ptr.* = 77;
    try testing.expect(map.getEntry(5).?.value_ptr.* == 77);

    const gop2 = try map.getOrPut(99);
    try testing.expect(gop2.found_existing == false);
    gop2.value_ptr.* = 42;
    try testing.expect(map.getEntry(99).?.value_ptr.* == 42);

    const gop3 = try map.getOrPutValue(5, 5);
    try testing.expect(gop3.value_ptr.* == 77);

    const gop4 = try map.getOrPutValue(100, 41);
    try testing.expect(gop4.value_ptr.* == 41);

    try testing.expect(map.contains(2));
    try testing.expect(map.getEntry(2).?.value_ptr.* == 22);
    try testing.expect(map.get(2).? == 22);

    const rmv1 = map.fetchRemove(2);
    try testing.expect(rmv1.?.key == 2);
    try testing.expect(rmv1.?.value == 22);
    try testing.expect(map.fetchRemove(2) == null);
    try testing.expect(map.remove(2) == false);
    try testing.expect(map.getEntry(2) == null);
    try testing.expect(map.get(2) == null);

    try testing.expect(map.remove(3) == true);
}

test "HashMap ensureUnusedCapacity" {
    const Map = AutoHashMapUnmanaged(u64, u64);
    const cap = 64;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    try map.ensureUnusedCapacity(32);
    try testing.expectError(error.OutOfMemory, map.ensureUnusedCapacity(cap + 1));
}

test "HashMap removeByPtr" {
    const Map = AutoHashMapUnmanaged(i32, u64);
    const cap = 64;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    var i: i32 = undefined;
    i = 0;
    while (i < 10) : (i += 1) {
        try map.put(i, 0);
    }

    try testing.expect(map.count() == 10);

    i = 0;
    while (i < 10) : (i += 1) {
        const key_ptr = map.getKeyPtr(i);
        try testing.expect(key_ptr != null);

        if (key_ptr) |ptr| {
            map.removeByPtr(ptr);
        }
    }

    try testing.expect(map.count() == 0);
}

test "HashMap removeByPtr 0 sized key" {
    const Map = AutoHashMapUnmanaged(i32, u64);
    const cap = 64;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    try map.put(0, 0);

    try testing.expect(map.count() == 1);

    const key_ptr = map.getKeyPtr(0);
    try testing.expect(key_ptr != null);

    if (key_ptr) |ptr| {
        map.removeByPtr(ptr);
    }

    try testing.expect(map.count() == 0);
}

test "HashMap repeat fetchRemove" {
    const Map = AutoHashMapUnmanaged(u64, void);
    const cap = 64;

    const alloc = testing.allocator;
    const layout = Map.layoutForCapacity(cap);
    const buf = try alloc.alignedAlloc(u8, Map.base_align, layout.total_size);
    defer alloc.free(buf);
    var map = Map.init(.init(buf), layout);

    map.putAssumeCapacity(0, {});
    map.putAssumeCapacity(1, {});
    map.putAssumeCapacity(2, {});
    map.putAssumeCapacity(3, {});

    // fetchRemove() should make slots available.
    var i: usize = 0;
    while (i < 10) : (i += 1) {
        try testing.expect(map.fetchRemove(3) != null);
        map.putAssumeCapacity(3, {});
    }

    try testing.expect(map.get(0) != null);
    try testing.expect(map.get(1) != null);
    try testing.expect(map.get(2) != null);
    try testing.expect(map.get(3) != null);
}

test "OffsetHashMap basic usage" {
    const OffsetMap = AutoOffsetHashMap(u32, u32);
    const cap = 16;

    const alloc = testing.allocator;
    const layout = OffsetMap.layout(cap);
    const buf = try alloc.alignedAlloc(u8, OffsetMap.base_align, layout.total_size);
    defer alloc.free(buf);
    var offset_map = OffsetMap.init(.init(buf), layout);
    var map = offset_map.map(buf.ptr);

    const count = 5;
    var i: u32 = 0;
    var total: u32 = 0;
    while (i < count) : (i += 1) {
        try map.put(i, i);
        total += i;
    }

    var sum: u32 = 0;
    var it = map.iterator();
    while (it.next()) |kv| {
        sum += kv.key_ptr.*;
    }
    try expectEqual(total, sum);

    i = 0;
    sum = 0;
    while (i < count) : (i += 1) {
        try expectEqual(i, map.get(i).?);
        sum += map.get(i).?;
    }
    try expectEqual(total, sum);
}

test "OffsetHashMap remake map" {
    const OffsetMap = AutoOffsetHashMap(u32, u32);
    const cap = 16;

    const alloc = testing.allocator;
    const layout = OffsetMap.layout(cap);
    const buf = try alloc.alignedAlloc(u8, OffsetMap.base_align, layout.total_size);
    defer alloc.free(buf);
    var offset_map = OffsetMap.init(.init(buf), layout);

    {
        var map = offset_map.map(buf.ptr);
        try map.put(5, 5);
    }

    {
        var map = offset_map.map(buf.ptr);
        try expectEqual(5, map.get(5).?);
    }
}