//! 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;

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

pub 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());
    };
}

pub const default_max_load_percentage = 80;

/// 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.
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();

        comptime {
            std.hash_map.verifyContext(Context, K, K, u64, false);
        }

        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 = @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,

        /// 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,

        // 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 {
            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,
        };

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

        /// Initialize a hash map with a given capacity and a buffer. The
        /// buffer must fit within the size defined by `layoutForCapacity`.
        pub fn init(new_capacity: Size, buf: []u8) Self {
            const layout = layoutForCapacity(new_capacity);

            // Ensure our base pointer is aligned to the max alignment
            const base = std.mem.alignForward(usize, @intFromPtr(buf.ptr), max_align);
            assert(base >= layout.total_size);

            // Get all our main pointers
            const metadata_ptr: [*]Metadata = @ptrFromInt(base + @sizeOf(Header));
            const keys_ptr: [*]K = @ptrFromInt(base + layout.keys_start);
            const values_ptr: [*]V = @ptrFromInt(base + layout.vals_start);

            // Build our map
            var map: Self = .{ .metadata = metadata_ptr };
            const hdr = map.header();
            hdr.capacity = new_capacity;
            if (@sizeOf([*]K) != 0) hdr.keys = keys_ptr;
            if (@sizeOf([*]V) != 0) hdr.values = values_ptr;
            map.initMetadatas();
            map.available = @truncate((new_capacity * max_load_percentage) / 100);

            return map;
        }

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

        pub fn ensureTotalCapacity2(self: *Self, new_size: Size) Allocator.Error!void {
            if (new_size > self.size) try self.growIfNeeded2(new_size - self.size);
        }

        pub fn ensureUnusedCapacity2(self: *Self, additional_size: Size) Allocator.Error!void {
            return ensureTotalCapacity2(self, self.count() + additional_size);
        }

        pub fn clearRetainingCapacity(self: *Self) void {
            if (self.metadata) |_| {
                self.initMetadatas();
                self.size = 0;
                self.available = @as(u32, @truncate((self.capacity() * max_load_percentage) / 100));
            }
        }

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

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

        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 putNoClobber2(self: *Self, key: K, value: V) Allocator.Error!void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call putNoClobberContext instead.");
            return self.putNoClobberContext2(key, value, undefined);
        }
        pub fn putNoClobberContext2(self: *Self, key: K, value: V, ctx: Context) Allocator.Error!void {
            assert(!self.containsContext(key, ctx));
            try self.growIfNeeded2(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;
            }

            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 fetchPut2(self: *Self, key: K, value: V) Allocator.Error!?KV {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call fetchPutContext instead.");
            return self.fetchPutContext2(key, value, undefined);
        }
        pub fn fetchPutContext2(self: *Self, key: K, value: V, ctx: Context) Allocator.Error!?KV {
            const gop = try self.getOrPutContext2(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.
        /// 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 {
            comptime std.hash_map.verifyContext(@TypeOf(ctx), @TypeOf(key), K, Hash, false);

            if (self.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);
            // 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);
            // 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 put2(self: *Self, key: K, value: V) Allocator.Error!void {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call putContext instead.");
            return self.putContext2(key, value, undefined);
        }
        pub fn putContext2(self: *Self, key: K, value: V, ctx: Context) Allocator.Error!void {
            const result = try self.getOrPutContext2(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 getOrPut2(self: *Self, key: K) Allocator.Error!GetOrPutResult {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getOrPutContext instead.");
            return self.getOrPutContext2(key, undefined);
        }
        pub fn getOrPutContext2(self: *Self, key: K, ctx: Context) Allocator.Error!GetOrPutResult {
            const gop = try self.getOrPutContextAdapted2(key, ctx);
            if (!gop.found_existing) {
                gop.key_ptr.* = key;
            }
            return gop;
        }
        pub fn getOrPutAdapted2(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.getOrPutContextAdapted2(key, key_ctx);
        }
        pub fn getOrPutContextAdapted2(self: *Self, key: anytype, key_ctx: anytype) Allocator.Error!GetOrPutResult {
            self.growIfNeeded2(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 {
            comptime std.hash_map.verifyContext(@TypeOf(ctx), @TypeOf(key), K, Hash, false);

            // 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;
            }
            // 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 getOrPutValue2(self: *Self, key: K, value: V) Allocator.Error!Entry {
            if (@sizeOf(Context) != 0)
                @compileError("Cannot infer context " ++ @typeName(Context) ++ ", call getOrPutValueContext instead.");
            return self.getOrPutValueContext2(key, value, undefined);
        }
        pub fn getOrPutValueContext2(self: *Self, key: K, value: V, ctx: Context) Allocator.Error!Entry {
            const res = try self.getOrPutAdapted2(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.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.
        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);
        }

        // 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: *const 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 growIfNeeded2(self: *Self, new_count: Size) Allocator.Error!void {
            if (new_count > self.available) return error.OutOfMemory;
        }

        /// The memory layout for the underlying buffer for a given capacity.
        pub const Layout = struct {
            /// The total size of the buffer required. The buffer is expected
            /// to be aligned to `max_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,
        };

        /// 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(std.math.isPowerOfTwo(new_capacity));
            const meta_size = @sizeOf(Header) + new_capacity * @sizeOf(Metadata);
            comptime assert(@alignOf(Metadata) == 1);

            const keys_start = std.mem.alignForward(usize, meta_size, 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);

            const total_size = std.mem.alignForward(usize, vals_end, max_align);
            return .{
                .total_size = total_size,
                .keys_start = keys_start,
                .vals_start = vals_start,
            };
        }
    };
}

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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);

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

    const count = 5;
    var i: u32 = 0;
    var total: u32 = 0;
    while (i < count) : (i += 1) {
        try map.put2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    var i: i32 = 0;
    while (i < 100) : (i += 1) {
        try map.ensureUnusedCapacity2(1);
        map.putAssumeCapacity(i, i);
        _ = map.remove(i);
    }
}

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

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

    map.clearRetainingCapacity();

    try map.put2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    try map.put2(0, 0);
    try expectEqual(map.count(), 1);
    try expectEqual(map.capacity(), Map.minimal_capacity);

    try testing.expectError(error.OutOfMemory, map.ensureTotalCapacity2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    var i: u32 = 0;
    while (i < 16) : (i += 1) {
        try map.put2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    var i: u32 = 0;
    while (i < 16) : (i += 1) {
        try map.putNoClobber2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    var i: u32 = 0;
    while (i < 16) : (i += 1) {
        try map.put2(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.put2(15, 15);
    try map.put2(13, 13);
    try map.put2(14, 14);
    try map.put2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

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

    const size = 32;
    const iterations = 100;

    var i: u32 = 0;
    while (i < size) : (i += 1) {
        try keys.append(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.put2(key, key);
        }
        try expectEqual(map.count(), size);

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

test "HashMap remove one million elements in random order" {
    const Map = AutoHashMapUnmanaged(u32, u32);
    const n = 1000 * 1000;
    const cap = Map.capacityForSize(n);

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

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

    var i: u32 = 0;
    while (i < n) : (i += 1) {
        keys.append(i) catch unreachable;
    }

    var prng = std.Random.DefaultPrng.init(0);
    const random = prng.random();
    random.shuffle(u32, keys.items);

    for (keys.items) |key| {
        map.put2(key, key) catch unreachable;
    }

    random.shuffle(u32, keys.items);
    i = 0;
    while (i < n) : (i += 1) {
        const key = keys.items[i];
        _ = map.remove(key);
    }
}

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

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

    var i: u32 = 0;
    while (i < 16) : (i += 1) {
        try map.put2(i, i);
    }

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

    i = 0;
    while (i < 16) : (i += 1) {
        try map.put2(i, i * 16 + 1);
    }

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

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

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

    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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    const limit = map.available;

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

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

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

    var i: u32 = 0;
    while (i < 10) : (i += 1) {
        try map.put2(i * 2, 2);
    }

    i = 0;
    while (i < 20) : (i += 1) {
        _ = try map.getOrPutValue2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    try testing.expect((try map.fetchPut2(1, 11)) == null);
    try testing.expect((try map.fetchPut2(2, 22)) == null);
    try testing.expect((try map.fetchPut2(3, 33)) == null);
    try testing.expect((try map.fetchPut2(4, 44)) == null);

    try map.putNoClobber2(5, 55);
    try testing.expect((try map.fetchPut2(5, 66)).?.value == 55);
    try testing.expect((try map.fetchPut2(5, 55)).?.value == 66);

    const gop1 = try map.getOrPut2(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.getOrPut2(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.getOrPutValue2(5, 5);
    try testing.expect(gop3.value_ptr.* == 77);

    const gop4 = try map.getOrPutValue2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    try map.ensureUnusedCapacity2(32);
    try testing.expectError(error.OutOfMemory, map.ensureUnusedCapacity2(cap + 1));
}

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

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

    var i: i32 = undefined;
    i = 0;
    while (i < 10) : (i += 1) {
        try map.put2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    try map.put2(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 buf = try alloc.alloc(u8, Map.layoutForCapacity(cap).total_size);
    defer alloc.free(buf);
    var map = Map.init(cap, buf);

    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);
}