browser/src/slab.zig

const std = @import("std");
const assert = std.debug.assert;

const Allocator = std.mem.Allocator;
const Alignment = std.mem.Alignment;

const Slab = struct {
    alignment: Alignment,
    item_size: usize,
    max_slot_count: usize,

    bitset: std.bit_set.DynamicBitSetUnmanaged,
    chunks: std.ArrayListUnmanaged([]u8),

    pub fn init(
        allocator: Allocator,
        alignment: Alignment,
        item_size: usize,
        max_slot_count: usize,
    ) !Slab {
        return .{
            .alignment = alignment,
            .item_size = item_size,
            .bitset = try .initFull(allocator, 0),
            .chunks = .empty,
            .max_slot_count = max_slot_count,
        };
    }

    pub fn deinit(self: *Slab, allocator: Allocator) void {
        self.bitset.deinit(allocator);

        for (self.chunks.items) |chunk| {
            allocator.rawFree(chunk, self.alignment, @returnAddress());
        }

        self.chunks.deinit(allocator);
    }

    inline fn calculateChunkSize(self: *Slab, chunk_index: usize) usize {
        const safe_index: u6 = @intCast(@min(std.math.maxInt(u6), chunk_index));
        const exponential = @as(usize, 1) << safe_index;
        return @min(exponential, self.max_slot_count);
    }

    inline fn toBitsetIndex(self: *Slab, chunk_index: usize, slot_index: usize) usize {
        var offset: usize = 0;
        for (0..chunk_index) |i| {
            const chunk_size = self.calculateChunkSize(i);
            offset += chunk_size;
        }
        return offset + slot_index;
    }

    inline fn toChunkAndSlotIndices(self: *Slab, bitset_index: usize) struct { usize, usize } {
        var offset: usize = 0;
        var chunk_index: usize = 0;

        while (chunk_index < self.chunks.items.len) : (chunk_index += 1) {
            const chunk_size = self.calculateChunkSize(chunk_index);
            if (bitset_index < offset + chunk_size) {
                return .{ chunk_index, bitset_index - offset };
            }

            offset += chunk_size;
        }

        unreachable;
    }

    fn alloc(self: *Slab, allocator: Allocator) ![]u8 {
        if (self.bitset.findFirstSet()) |index| {
            const chunk_index, const slot_index = self.toChunkAndSlotIndices(index);

            // if we have a free slot
            self.bitset.unset(index);

            const chunk = self.chunks.items[chunk_index];
            const offset = slot_index * self.item_size;
            return chunk.ptr[offset..][0..self.item_size];
        } else {
            const old_capacity = self.bitset.bit_length;

            // if we have don't have a free slot
            try self.allocateChunk(allocator);

            const first_slot_index = old_capacity;
            self.bitset.unset(first_slot_index);

            const new_chunk = self.chunks.items[self.chunks.items.len - 1];
            return new_chunk.ptr[0..self.item_size];
        }
    }

    fn free(self: *Slab, ptr: [*]u8) void {
        const addr = @intFromPtr(ptr);

        for (self.chunks.items, 0..) |chunk, i| {
            const chunk_start = @intFromPtr(chunk.ptr);
            const chunk_end = chunk_start + chunk.len;

            if (addr >= chunk_start and addr < chunk_end) {
                const offset = addr - chunk_start;
                const slot_index = offset / self.item_size;

                const bitset_index = self.toBitsetIndex(i, slot_index);
                assert(!self.bitset.isSet(bitset_index));

                self.bitset.set(bitset_index);
                return;
            }
        }

        unreachable;
    }

    fn allocateChunk(self: *Slab, allocator: Allocator) !void {
        const next_chunk_size = self.calculateChunkSize(self.chunks.items.len);
        const chunk_len = self.item_size * next_chunk_size;

        const chunk_ptr = allocator.rawAlloc(
            chunk_len,
            self.alignment,
            @returnAddress(),
        ) orelse return error.FailedChildAllocation;

        const chunk = chunk_ptr[0..chunk_len];
        try self.chunks.append(allocator, chunk);

        const new_capacity = self.bitset.bit_length + next_chunk_size;
        try self.bitset.resize(allocator, new_capacity, true);
    }

    const Stats = struct {
        key: SlabKey,
        item_size: usize,
        chunk_count: usize,
        total_slots: usize,
        slots_in_use: usize,
        slots_free: usize,
        bytes_allocated: usize,
        bytes_in_use: usize,
        bytes_free: usize,
        utilization_ratio: f64,
    };

    fn getStats(self: *const Slab, key: SlabKey) Stats {
        const total_slots = self.bitset.bit_length;
        const free_slots = self.bitset.count();
        const used_slots = total_slots - free_slots;
        const bytes_allocated = total_slots * self.item_size;
        const bytes_in_use = used_slots * self.item_size;

        const utilization_ratio = if (bytes_allocated > 0)
            @as(f64, @floatFromInt(bytes_in_use)) / @as(f64, @floatFromInt(bytes_allocated))
        else
            0.0;

        return .{
            .key = key,
            .item_size = self.item_size,
            .chunk_count = self.chunks.items.len,
            .total_slots = total_slots,
            .slots_in_use = used_slots,
            .slots_free = free_slots,
            .bytes_allocated = bytes_allocated,
            .bytes_in_use = bytes_in_use,
            .bytes_free = free_slots * self.item_size,
            .utilization_ratio = utilization_ratio,
        };
    }
};

const SlabKey = struct {
    size: usize,
    alignment: Alignment,
};

pub const SlabAllocator = struct {
    const Self = @This();

    child_allocator: Allocator,
    max_slot_count: usize,

    slabs: std.ArrayHashMapUnmanaged(SlabKey, Slab, struct {
        const Context = @This();

        pub fn hash(_: Context, key: SlabKey) u32 {
            var hasher = std.hash.Wyhash.init(0);
            std.hash.autoHash(&hasher, key.size);
            std.hash.autoHash(&hasher, key.alignment);
            return @truncate(hasher.final());
        }

        pub fn eql(_: Context, a: SlabKey, b: SlabKey, _: usize) bool {
            return a.size == b.size and a.alignment == b.alignment;
        }
    }, false) = .empty,

    pub fn init(child_allocator: Allocator, max_slot_count: usize) Self {
        assert(std.math.isPowerOfTwo(max_slot_count));

        return .{
            .child_allocator = child_allocator,
            .slabs = .empty,
            .max_slot_count = max_slot_count,
        };
    }

    pub fn deinit(self: *Self) void {
        for (self.slabs.values()) |*slab| {
            slab.deinit(self.child_allocator);
        }

        self.slabs.deinit(self.child_allocator);
    }

    pub const ResetKind = enum {
        /// Free all chunks and release all memory.
        clear,
        /// Keep all chunks, reset trees to reuse memory.
        retain_capacity,
    };

    /// This clears all of the stored memory, freeing the currently used chunks.
    pub fn reset(self: *Self, kind: ResetKind) void {
        switch (kind) {
            .clear => {
                for (self.slabs.values()) |*slab| {
                    for (slab.chunks.items) |chunk| {
                        self.child_allocator.free(chunk);
                    }

                    slab.chunks.clearAndFree(self.child_allocator);
                    slab.bitset.deinit(self.child_allocator);
                }

                self.slabs.clearAndFree(self.child_allocator);
            },
            .retain_capacity => {
                for (self.slabs.values()) |*slab| {
                    slab.bitset.setAll();
                }
            },
        }
    }

    const Stats = struct {
        total_allocated_bytes: usize,
        bytes_in_use: usize,
        bytes_free: usize,
        slab_count: usize,
        total_chunks: usize,
        total_slots: usize,
        slots_in_use: usize,
        slots_free: usize,
        fragmentation_ratio: f64,
        utilization_ratio: f64,
        slabs: []const Slab.Stats,

        pub fn print(self: *const Stats, stream: *std.io.Writer) !void {
            try stream.print("\n", .{});
            try stream.print("\n=== Slab Allocator Statistics ===\n", .{});
            try stream.print("Overall Memory:\n", .{});
            try stream.print("  Total allocated: {} bytes ({d:.2} MB)\n", .{
                self.total_allocated_bytes,
                @as(f64, @floatFromInt(self.total_allocated_bytes)) / 1_048_576.0,
            });
            try stream.print("  In use:          {} bytes ({d:.2} MB)\n", .{
                self.bytes_in_use,
                @as(f64, @floatFromInt(self.bytes_in_use)) / 1_048_576.0,
            });
            try stream.print("  Free:            {} bytes ({d:.2} MB)\n", .{
                self.bytes_free,
                @as(f64, @floatFromInt(self.bytes_free)) / 1_048_576.0,
            });

            try stream.print("\nOverall Structure:\n", .{});
            try stream.print("  Slab Count:    {}\n", .{self.slab_count});
            try stream.print("  Total chunks:    {}\n", .{self.total_chunks});
            try stream.print("  Total slots:     {}\n", .{self.total_slots});
            try stream.print("  Slots in use:    {}\n", .{self.slots_in_use});
            try stream.print("  Slots free:      {}\n", .{self.slots_free});

            try stream.print("\nOverall Efficiency:\n", .{});
            try stream.print("  Utilization:     {d:.1}%\n", .{self.utilization_ratio * 100.0});
            try stream.print("  Fragmentation:   {d:.1}%\n", .{self.fragmentation_ratio * 100.0});

            if (self.slabs.len > 0) {
                try stream.print("\nPer-Slab Breakdown:\n", .{});
                try stream.print(
                    "  {s:>5} | {s:>4} | {s:>6} | {s:>6} | {s:>6} | {s:>10} | {s:>6}\n",
                    .{ "Size", "Algn", "Chunks", "Slots", "InUse", "Bytes", "Util%" },
                );
                try stream.print(
                    "  {s:-<5}-+-{s:-<4}-+-{s:-<6}-+-{s:-<6}-+-{s:-<6}-+-{s:-<10}-+-{s:-<6}\n",
                    .{ "", "", "", "", "", "", "" },
                );

                for (self.slabs) |slab| {
                    try stream.print("  {d:5} | {d:4} | {d:6} | {d:6} | {d:6} | {d:10} | {d:5.1}%\n", .{
                        slab.key.size,
                        @intFromEnum(slab.key.alignment),
                        slab.chunk_count,
                        slab.total_slots,
                        slab.slots_in_use,
                        slab.bytes_allocated,
                        slab.utilization_ratio * 100.0,
                    });
                }
            }
        }
    };

    pub fn getStats(self: *Self, a: std.mem.Allocator) !Stats {
        var slab_stats: std.ArrayList(Slab.Stats) = try .initCapacity(a, self.slabs.entries.len);
        errdefer slab_stats.deinit(a);

        var stats = Stats{
            .total_allocated_bytes = 0,
            .bytes_in_use = 0,
            .bytes_free = 0,
            .slab_count = self.slabs.count(),
            .total_chunks = 0,
            .total_slots = 0,
            .slots_in_use = 0,
            .slots_free = 0,
            .fragmentation_ratio = 0.0,
            .utilization_ratio = 0.0,
            .slabs = &.{},
        };

        var it = self.slabs.iterator();
        while (it.next()) |entry| {
            const key = entry.key_ptr.*;
            const slab = entry.value_ptr;
            const slab_stat = slab.getStats(key);

            slab_stats.appendAssumeCapacity(slab_stat);

            stats.total_allocated_bytes += slab_stat.bytes_allocated;
            stats.bytes_in_use += slab_stat.bytes_in_use;
            stats.bytes_free += slab_stat.bytes_free;
            stats.total_chunks += slab_stat.chunk_count;
            stats.total_slots += slab_stat.total_slots;
            stats.slots_in_use += slab_stat.slots_in_use;
            stats.slots_free += slab_stat.slots_free;
        }

        if (stats.total_allocated_bytes > 0) {
            stats.fragmentation_ratio = @as(f64, @floatFromInt(stats.bytes_free)) /
                @as(f64, @floatFromInt(stats.total_allocated_bytes));
            stats.utilization_ratio = @as(f64, @floatFromInt(stats.bytes_in_use)) /
                @as(f64, @floatFromInt(stats.total_allocated_bytes));
        }

        stats.slabs = try slab_stats.toOwnedSlice(a);
        return stats;
    }

    pub const vtable = Allocator.VTable{
        .alloc = alloc,
        .free = free,
        .remap = Allocator.noRemap,
        .resize = Allocator.noResize,
    };

    pub fn allocator(self: *Self) Allocator {
        return .{
            .ptr = self,
            .vtable = &vtable,
        };
    }

    fn alloc(ctx: *anyopaque, len: usize, alignment: Alignment, ret_addr: usize) ?[*]u8 {
        const self: *Self = @ptrCast(@alignCast(ctx));
        _ = ret_addr;

        const aligned_len = std.mem.alignForward(usize, len, alignment.toByteUnits());

        const list_gop = self.slabs.getOrPut(
            self.child_allocator,
            SlabKey{ .size = aligned_len, .alignment = alignment },
        ) catch return null;

        if (!list_gop.found_existing) {
            list_gop.value_ptr.* = Slab.init(
                self.child_allocator,
                alignment,
                aligned_len,
                self.max_slot_count,
            ) catch return null;
        }

        const list = list_gop.value_ptr;
        const buf = list.alloc(self.child_allocator) catch return null;
        return buf[0..len].ptr;
    }

    fn free(ctx: *anyopaque, memory: []u8, alignment: Alignment, ret_addr: usize) void {
        const self: *Self = @ptrCast(@alignCast(ctx));
        _ = ret_addr;

        const ptr = memory.ptr;
        const len = memory.len;
        const aligned_len = std.mem.alignForward(usize, len, alignment.toByteUnits());

        const list = self.slabs.getPtr(.{ .size = aligned_len, .alignment = alignment }).?;
        list.free(ptr);
    }
};

const testing = std.testing;

const TestSlabAllocator = SlabAllocator;

test "slab allocator - basic allocation and free" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate some memory
    const ptr1 = try allocator.alloc(u8, 100);
    try testing.expect(ptr1.len == 100);

    // Write to it to ensure it's valid
    @memset(ptr1, 42);
    try testing.expectEqual(@as(u8, 42), ptr1[50]);

    // Free it
    allocator.free(ptr1);
}

test "slab allocator - multiple allocations" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    const ptr1 = try allocator.alloc(u8, 64);
    const ptr2 = try allocator.alloc(u8, 128);
    const ptr3 = try allocator.alloc(u8, 256);

    // Ensure they don't overlap
    const addr1 = @intFromPtr(ptr1.ptr);
    const addr2 = @intFromPtr(ptr2.ptr);
    const addr3 = @intFromPtr(ptr3.ptr);

    try testing.expect(addr1 + 64 <= addr2 or addr2 + 128 <= addr1);
    try testing.expect(addr2 + 128 <= addr3 or addr3 + 256 <= addr2);

    allocator.free(ptr1);
    allocator.free(ptr2);
    allocator.free(ptr3);
}

test "slab allocator - no coalescing (different size classes)" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate two blocks of same size
    const ptr1 = try allocator.alloc(u8, 128);
    const ptr2 = try allocator.alloc(u8, 128);

    // Free them (no coalescing in slab allocator)
    allocator.free(ptr1);
    allocator.free(ptr2);

    // Can't allocate larger block from these freed 128-byte blocks
    const ptr3 = try allocator.alloc(u8, 256);

    // ptr3 will be from a different size class, not coalesced from ptr1+ptr2
    const addr1 = @intFromPtr(ptr1.ptr);
    const addr3 = @intFromPtr(ptr3.ptr);

    // They should NOT be adjacent (different size classes)
    try testing.expect(addr3 < addr1 or addr3 >= addr1 + 256);

    allocator.free(ptr3);
}

test "slab allocator - reuse freed memory" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    const ptr1 = try allocator.alloc(u8, 64);
    const addr1 = @intFromPtr(ptr1.ptr);
    allocator.free(ptr1);

    // Allocate same size, should reuse from same slab
    const ptr2 = try allocator.alloc(u8, 64);
    const addr2 = @intFromPtr(ptr2.ptr);

    try testing.expectEqual(addr1, addr2);
    allocator.free(ptr2);
}

test "slab allocator - multiple size classes" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate various sizes - each creates a new slab
    var ptrs: [10][]u8 = undefined;
    const sizes = [_]usize{ 24, 40, 64, 88, 128, 144, 200, 256, 512, 1000 };

    for (&ptrs, sizes) |*ptr, size| {
        ptr.* = try allocator.alloc(u8, size);
        @memset(ptr.*, 0xFF);
    }

    // Should have created multiple slabs
    try testing.expect(slab_alloc.slabs.count() >= 10);

    // Free all
    for (ptrs) |ptr| {
        allocator.free(ptr);
    }
}

test "slab allocator - various sizes" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Test different sizes (not limited to powers of 2!)
    const sizes = [_]usize{ 8, 16, 24, 32, 40, 64, 88, 128, 144, 256 };

    for (sizes) |size| {
        const ptr = try allocator.alloc(u8, size);
        try testing.expect(ptr.len == size);
        @memset(ptr, @intCast(size & 0xFF));
        allocator.free(ptr);
    }
}

test "slab allocator - exact sizes (no rounding)" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Odd sizes stay exact (unlike buddy which rounds to power of 2)
    const ptr1 = try allocator.alloc(u8, 100);
    const ptr2 = try allocator.alloc(u8, 200);
    const ptr3 = try allocator.alloc(u8, 50);

    // Exact sizes!
    try testing.expect(ptr1.len == 100);
    try testing.expect(ptr2.len == 200);
    try testing.expect(ptr3.len == 50);

    allocator.free(ptr1);
    allocator.free(ptr2);
    allocator.free(ptr3);
}

test "slab allocator - chunk allocation" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate many items of same size to force multiple chunks
    var ptrs: [100][]u8 = undefined;
    for (&ptrs) |*ptr| {
        ptr.* = try allocator.alloc(u8, 64);
    }

    // Should have allocated multiple chunks (32 items per chunk)
    const slab = slab_alloc.slabs.getPtr(.{ .size = 64, .alignment = Alignment.@"1" }).?;
    try testing.expect(slab.chunks.items.len > 1);

    // Free all
    for (ptrs) |ptr| {
        allocator.free(ptr);
    }
}

test "slab allocator - reset with retain_capacity" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate some memory
    const ptr1 = try allocator.alloc(u8, 128);
    const ptr2 = try allocator.alloc(u8, 256);
    _ = ptr1;
    _ = ptr2;

    const slabs_before = slab_alloc.slabs.count();
    const slab_128 = slab_alloc.slabs.getPtr(.{ .size = 128, .alignment = Alignment.@"1" }).?;
    const chunks_before = slab_128.chunks.items.len;

    // Reset but keep chunks
    slab_alloc.reset(.retain_capacity);

    try testing.expectEqual(slabs_before, slab_alloc.slabs.count());
    try testing.expectEqual(chunks_before, slab_128.chunks.items.len);

    // Should be able to allocate again
    const ptr3 = try allocator.alloc(u8, 512);
    allocator.free(ptr3);
}

test "slab allocator - reset with clear" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate some memory
    const ptr1 = try allocator.alloc(u8, 128);
    _ = ptr1;

    try testing.expect(slab_alloc.slabs.count() > 0);

    // Reset and free everything
    slab_alloc.reset(.clear);

    try testing.expectEqual(@as(usize, 0), slab_alloc.slabs.count());

    // Should still work after reset
    const ptr2 = try allocator.alloc(u8, 256);
    allocator.free(ptr2);
}

test "slab allocator - stress test" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    var prng = std.Random.DefaultPrng.init(0);
    const random = prng.random();

    var ptrs: std.ArrayList([]u8) = .empty;

    defer {
        for (ptrs.items) |ptr| {
            allocator.free(ptr);
        }
        ptrs.deinit(allocator);
    }

    // Random allocations and frees
    var i: usize = 0;
    while (i < 100) : (i += 1) {
        if (random.boolean() and ptrs.items.len > 0) {
            // Free a random allocation
            const index = random.uintLessThan(usize, ptrs.items.len);
            allocator.free(ptrs.swapRemove(index));
        } else {
            // Allocate random size (8 to 512)
            const size = random.uintAtMost(usize, 504) + 8;
            const ptr = try allocator.alloc(u8, size);
            try ptrs.append(allocator, ptr);

            // Write to ensure it's valid
            @memset(ptr, @intCast(i & 0xFF));
        }
    }
}

test "slab allocator - alignment" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    const ptr1 = try allocator.create(u64);
    const ptr2 = try allocator.create(u32);
    const ptr3 = try allocator.create([100]u8);

    allocator.destroy(ptr1);
    allocator.destroy(ptr2);
    allocator.destroy(ptr3);
}

test "slab allocator - no resize support" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    const slice = try allocator.alloc(u8, 100);
    @memset(slice, 42);

    // Resize should fail (not supported)
    try testing.expect(!allocator.resize(slice, 90));
    try testing.expect(!allocator.resize(slice, 200));

    allocator.free(slice);
}

test "slab allocator - fragmentation pattern" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate 10 items
    var items: [10][]u8 = undefined;
    for (&items) |*item| {
        item.* = try allocator.alloc(u8, 64);
        @memset(item.*, 0xFF);
    }

    // Free every other one
    allocator.free(items[0]);
    allocator.free(items[2]);
    allocator.free(items[4]);
    allocator.free(items[6]);
    allocator.free(items[8]);

    // Allocate new items - should reuse freed slots
    const new1 = try allocator.alloc(u8, 64);
    const new2 = try allocator.alloc(u8, 64);
    const new3 = try allocator.alloc(u8, 64);

    // Should get some of the freed slots back
    const addrs = [_]usize{
        @intFromPtr(items[0].ptr),
        @intFromPtr(items[2].ptr),
        @intFromPtr(items[4].ptr),
        @intFromPtr(items[6].ptr),
        @intFromPtr(items[8].ptr),
    };

    const new1_addr = @intFromPtr(new1.ptr);
    var found = false;
    for (addrs) |addr| {
        if (new1_addr == addr) found = true;
    }
    try testing.expect(found);

    // Cleanup
    allocator.free(items[1]);
    allocator.free(items[3]);
    allocator.free(items[5]);
    allocator.free(items[7]);
    allocator.free(items[9]);
    allocator.free(new1);
    allocator.free(new2);
    allocator.free(new3);
}

test "slab allocator - many small allocations" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate 1000 small items
    var ptrs: std.ArrayList([]u8) = .empty;
    defer {
        for (ptrs.items) |ptr| {
            allocator.free(ptr);
        }
        ptrs.deinit(allocator);
    }

    var i: usize = 0;
    while (i < 1000) : (i += 1) {
        const ptr = try allocator.alloc(u8, 24);
        try ptrs.append(allocator, ptr);
    }

    // Should have created multiple chunks
    const slab = slab_alloc.slabs.getPtr(.{ .size = 24, .alignment = Alignment.@"1" }).?;
    try testing.expect(slab.chunks.items.len > 1);
}

test "slab allocator - zero waste for exact sizes" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // These sizes have zero internal fragmentation (unlike buddy)
    const sizes = [_]usize{ 24, 40, 56, 88, 144, 152, 184, 232, 648 };

    for (sizes) |size| {
        const ptr = try allocator.alloc(u8, size);

        // Exact size returned!
        try testing.expectEqual(size, ptr.len);

        @memset(ptr, 0xFF);
        allocator.free(ptr);
    }
}

test "slab allocator - different size classes don't interfere" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Allocate size 64
    const ptr_64 = try allocator.alloc(u8, 64);
    const addr_64 = @intFromPtr(ptr_64.ptr);
    allocator.free(ptr_64);

    // Allocate size 128 - should NOT reuse size-64 slot
    const ptr_128 = try allocator.alloc(u8, 128);
    const addr_128 = @intFromPtr(ptr_128.ptr);

    try testing.expect(addr_64 != addr_128);

    // Allocate size 64 again - SHOULD reuse original slot
    const ptr_64_again = try allocator.alloc(u8, 64);
    const addr_64_again = @intFromPtr(ptr_64_again.ptr);

    try testing.expectEqual(addr_64, addr_64_again);

    allocator.free(ptr_128);
    allocator.free(ptr_64_again);
}

test "slab allocator - 16-byte alignment" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    // Request 16-byte aligned memory
    const ptr = try allocator.alignedAlloc(u8, .@"16", 152);
    defer allocator.free(ptr);

    // Verify alignment
    const addr = @intFromPtr(ptr.ptr);
    try testing.expect(addr % 16 == 0);

    // Make sure we can use it
    @memset(ptr, 0xFF);
}

test "slab allocator - various alignments" {
    var slab_alloc = TestSlabAllocator.init(testing.allocator, 16);
    defer slab_alloc.deinit();

    const allocator = slab_alloc.allocator();

    const alignments = [_]std.mem.Alignment{ .@"1", .@"2", .@"4", .@"8", .@"16" };

    inline for (alignments) |alignment| {
        const ptr = try allocator.alignedAlloc(u8, alignment, 100);
        defer allocator.free(ptr);

        const addr = @intFromPtr(ptr.ptr);
        const align_value = alignment.toByteUnits();
        try testing.expect(addr % align_value == 0);
    }
}