    const std = @import("std.zig");
    const builtin = @import("builtin");
    const root = @import("root");
    const assert = std.debug.assert;
    const testing = std.testing;
    const mem = std.mem;
    const os = std.os;
    const c = std.c;
    const maxInt = std.math.maxInt;
    const Allocator = std.mem.Allocator;

    pub const LoggingAllocator = @import("heap/logging_allocator.zig").LoggingAllocator;
    pub const loggingAllocator = @import("heap/logging_allocator.zig").loggingAllocator;
    pub const ScopedLoggingAllocator = @import("heap/logging_allocator.zig").ScopedLoggingAllocator;
    pub const LogToWriterAllocator = @import("heap/log_to_writer_allocator.zig").LogToWriterAllocator;
    pub const logToWriterAllocator = @import("heap/log_to_writer_allocator.zig").logToWriterAllocator;
    pub const ArenaAllocator = @import("heap/arena_allocator.zig").ArenaAllocator;
    pub const GeneralPurposeAllocator = @import("heap/general_purpose_allocator.zig").GeneralPurposeAllocator;
    pub const Check = @import("heap/general_purpose_allocator.zig").Check;
    pub const WasmAllocator = @import("heap/WasmAllocator.zig");
    pub const WasmPageAllocator = @import("heap/WasmPageAllocator.zig");
    pub const PageAllocator = @import("heap/PageAllocator.zig");
    pub const ThreadSafeAllocator = @import("heap/ThreadSafeAllocator.zig");
    pub const SbrkAllocator = @import("heap/sbrk_allocator.zig").SbrkAllocator;

    const memory_pool = @import("heap/memory_pool.zig");
    pub const MemoryPool = memory_pool.MemoryPool;
    pub const MemoryPoolAligned = memory_pool.MemoryPoolAligned;
    pub const MemoryPoolExtra = memory_pool.MemoryPoolExtra;
    pub const MemoryPoolOptions = memory_pool.Options;

    /// TODO Utilize this on Windows.
    pub var next_mmap_addr_hint: ?[*]align(mem.page_size) u8 = null;

    const CAllocator = struct {
        comptime {
            if (!builtin.link_libc) {
                @compileError("C allocator is only available when linking against libc");
            }
        }

        usingnamespace if (@hasDecl(c, "malloc_size"))
            struct {
                pub const supports_malloc_size = true;
                pub const malloc_size = c.malloc_size;
            }
        else if (@hasDecl(c, "malloc_usable_size"))
            struct {
                pub const supports_malloc_size = true;
                pub const malloc_size = c.malloc_usable_size;
            }
        else if (@hasDecl(c, "_msize"))
            struct {
                pub const supports_malloc_size = true;
                pub const malloc_size = c._msize;
            }
        else
            struct {
                pub const supports_malloc_size = false;
            };

        pub const supports_posix_memalign = @hasDecl(c, "posix_memalign");

        fn getHeader(ptr: [*]u8) *[*]u8 {
            return @as(*[*]u8, @ptrFromInt(@intFromPtr(ptr) - @sizeOf(usize)));
        }

        fn alignedAlloc(len: usize, log2_align: u8) ?[*]u8 {
            const alignment = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_align));
            if (supports_posix_memalign) {
                // The posix_memalign only accepts alignment values that are a

                // multiple of the pointer size

                const eff_alignment = @max(alignment, @sizeOf(usize));

                var aligned_ptr: ?*anyopaque = undefined;
                if (c.posix_memalign(&aligned_ptr, eff_alignment, len) != 0)
                    return null;

                return @as([*]u8, @ptrCast(aligned_ptr));
            }

            // Thin wrapper around regular malloc, overallocate to account for

            // alignment padding and store the original malloc()'ed pointer before

            // the aligned address.

            var unaligned_ptr = @as([*]u8, @ptrCast(c.malloc(len + alignment - 1 + @sizeOf(usize)) orelse return null));
            const unaligned_addr = @intFromPtr(unaligned_ptr);
            const aligned_addr = mem.alignForward(usize, unaligned_addr + @sizeOf(usize), alignment);
            var aligned_ptr = unaligned_ptr + (aligned_addr - unaligned_addr);
            getHeader(aligned_ptr).* = unaligned_ptr;

            return aligned_ptr;
        }

        fn alignedFree(ptr: [*]u8) void {
            if (supports_posix_memalign) {
                return c.free(ptr);
            }

            const unaligned_ptr = getHeader(ptr).*;
            c.free(unaligned_ptr);
        }

        fn alignedAllocSize(ptr: [*]u8) usize {
            if (supports_posix_memalign) {
                return CAllocator.malloc_size(ptr);
            }

            const unaligned_ptr = getHeader(ptr).*;
            const delta = @intFromPtr(ptr) - @intFromPtr(unaligned_ptr);
            return CAllocator.malloc_size(unaligned_ptr) - delta;
        }

        fn alloc(
            _: *anyopaque,
            len: usize,
            log2_align: u8,
            return_address: usize,
        ) ?[*]u8 {
            _ = return_address;
            assert(len > 0);
            return alignedAlloc(len, log2_align);
        }

        fn resize(
            _: *anyopaque,
            buf: []u8,
            log2_buf_align: u8,
            new_len: usize,
            return_address: usize,
        ) bool {
            _ = log2_buf_align;
            _ = return_address;
            if (new_len <= buf.len) {
                return true;
            }
            if (CAllocator.supports_malloc_size) {
                const full_len = alignedAllocSize(buf.ptr);
                if (new_len <= full_len) {
                    return true;
                }
            }
            return false;
        }

        fn free(
            _: *anyopaque,
            buf: []u8,
            log2_buf_align: u8,
            return_address: usize,
        ) void {
            _ = log2_buf_align;
            _ = return_address;
            alignedFree(buf.ptr);
        }
    };

    /// Supports the full Allocator interface, including alignment, and exploiting
    /// `malloc_usable_size` if available. For an allocator that directly calls
    /// `malloc`/`free`, see `raw_c_allocator`.
    pub const c_allocator = Allocator{
        .ptr = undefined,
        .vtable = &c_allocator_vtable,
    };
    const c_allocator_vtable = Allocator.VTable{
        .alloc = CAllocator.alloc,
        .resize = CAllocator.resize,
        .free = CAllocator.free,
    };

    /// Asserts allocations are within `@alignOf(std.c.max_align_t)` and directly calls
    /// `malloc`/`free`. Does not attempt to utilize `malloc_usable_size`.
    /// This allocator is safe to use as the backing allocator with
    /// `ArenaAllocator` for example and is more optimal in such a case
    /// than `c_allocator`.
    pub const raw_c_allocator = Allocator{
        .ptr = undefined,
        .vtable = &raw_c_allocator_vtable,
    };
    const raw_c_allocator_vtable = Allocator.VTable{
        .alloc = rawCAlloc,
        .resize = rawCResize,
        .free = rawCFree,
    };

    fn rawCAlloc(
        _: *anyopaque,
        len: usize,
        log2_ptr_align: u8,
        ret_addr: usize,
    ) ?[*]u8 {
        _ = ret_addr;
        assert(log2_ptr_align <= comptime std.math.log2_int(usize, @alignOf(std.c.max_align_t)));
        // Note that this pointer cannot be aligncasted to max_align_t because if

        // len is < max_align_t then the alignment can be smaller. For example, if

        // max_align_t is 16, but the user requests 8 bytes, there is no built-in

        // type in C that is size 8 and has 16 byte alignment, so the alignment may

        // be 8 bytes rather than 16. Similarly if only 1 byte is requested, malloc

        // is allowed to return a 1-byte aligned pointer.

        return @as(?[*]u8, @ptrCast(c.malloc(len)));
    }

    fn rawCResize(
        _: *anyopaque,
        buf: []u8,
        log2_old_align: u8,
        new_len: usize,
        ret_addr: usize,
    ) bool {
        _ = log2_old_align;
        _ = ret_addr;
        return new_len <= buf.len;
    }

    fn rawCFree(
        _: *anyopaque,
        buf: []u8,
        log2_old_align: u8,
        ret_addr: usize,
    ) void {
        _ = log2_old_align;
        _ = ret_addr;
        c.free(buf.ptr);
    }

    /// This allocator makes a syscall directly for every allocation and free.
    /// Thread-safe and lock-free.
    pub const page_allocator = if (builtin.target.isWasm())
        Allocator{
            .ptr = undefined,
            .vtable = &WasmPageAllocator.vtable,
        }
    else if (builtin.target.os.tag == .plan9)
        Allocator{
            .ptr = undefined,
            .vtable = &SbrkAllocator(std.os.plan9.sbrk).vtable,
        }
    else if (builtin.target.os.tag == .freestanding)
        root.os.heap.page_allocator
    else
        Allocator{
            .ptr = undefined,
            .vtable = &PageAllocator.vtable,
        };

    /// This allocator is fast, small, and specific to WebAssembly. In the future,
    /// this will be the implementation automatically selected by
    /// `GeneralPurposeAllocator` when compiling in `ReleaseSmall` mode for wasm32
    /// and wasm64 architectures.
    /// Until then, it is available here to play with.
    pub const wasm_allocator = Allocator{
        .ptr = undefined,
        .vtable = &std.heap.WasmAllocator.vtable,
    };

    /// Verifies that the adjusted length will still map to the full length
    pub fn alignPageAllocLen(full_len: usize, len: usize) usize {
        const aligned_len = mem.alignAllocLen(full_len, len);
        assert(mem.alignForward(usize, aligned_len, mem.page_size) == full_len);
        return aligned_len;
    }

    pub const HeapAllocator = switch (builtin.os.tag) {
        .windows => struct {
            heap_handle: ?HeapHandle,

            const HeapHandle = os.windows.HANDLE;

            pub fn init() HeapAllocator {
                return HeapAllocator{
                    .heap_handle = null,
                };
            }

            pub fn allocator(self: *HeapAllocator) Allocator {
                return .{
                    .ptr = self,
                    .vtable = &.{
                        .alloc = alloc,
                        .resize = resize,
                        .free = free,
                    },
                };
            }

            pub fn deinit(self: *HeapAllocator) void {
                if (self.heap_handle) |heap_handle| {
                    os.windows.HeapDestroy(heap_handle);
                }
            }

            fn getRecordPtr(buf: []u8) *align(1) usize {
                return @as(*align(1) usize, @ptrFromInt(@intFromPtr(buf.ptr) + buf.len));
            }

            fn alloc(
                ctx: *anyopaque,
                n: usize,
                log2_ptr_align: u8,
                return_address: usize,
            ) ?[*]u8 {
                _ = return_address;
                const self: *HeapAllocator = @ptrCast(@alignCast(ctx));

                const ptr_align = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_ptr_align));
                const amt = n + ptr_align - 1 + @sizeOf(usize);
                const optional_heap_handle = @atomicLoad(?HeapHandle, &self.heap_handle, .SeqCst);
                const heap_handle = optional_heap_handle orelse blk: {
                    const options = if (builtin.single_threaded) os.windows.HEAP_NO_SERIALIZE else 0;
                    const hh = os.windows.kernel32.HeapCreate(options, amt, 0) orelse return null;
                    const other_hh = @cmpxchgStrong(?HeapHandle, &self.heap_handle, null, hh, .SeqCst, .SeqCst) orelse break :blk hh;
                    os.windows.HeapDestroy(hh);
                    break :blk other_hh.?; // can't be null because of the cmpxchg

                };
                const ptr = os.windows.kernel32.HeapAlloc(heap_handle, 0, amt) orelse return null;
                const root_addr = @intFromPtr(ptr);
                const aligned_addr = mem.alignForward(usize, root_addr, ptr_align);
                const buf = @as([*]u8, @ptrFromInt(aligned_addr))[0..n];
                getRecordPtr(buf).* = root_addr;
                return buf.ptr;
            }

            fn resize(
                ctx: *anyopaque,
                buf: []u8,
                log2_buf_align: u8,
                new_size: usize,
                return_address: usize,
            ) bool {
                _ = log2_buf_align;
                _ = return_address;
                const self: *HeapAllocator = @ptrCast(@alignCast(ctx));

                const root_addr = getRecordPtr(buf).*;
                const align_offset = @intFromPtr(buf.ptr) - root_addr;
                const amt = align_offset + new_size + @sizeOf(usize);
                const new_ptr = os.windows.kernel32.HeapReAlloc(
                    self.heap_handle.?,
                    os.windows.HEAP_REALLOC_IN_PLACE_ONLY,
                    @as(*anyopaque, @ptrFromInt(root_addr)),
                    amt,
                ) orelse return false;
                assert(new_ptr == @as(*anyopaque, @ptrFromInt(root_addr)));
                getRecordPtr(buf.ptr[0..new_size]).* = root_addr;
                return true;
            }

            fn free(
                ctx: *anyopaque,
                buf: []u8,
                log2_buf_align: u8,
                return_address: usize,
            ) void {
                _ = log2_buf_align;
                _ = return_address;
                const self: *HeapAllocator = @ptrCast(@alignCast(ctx));
                os.windows.HeapFree(self.heap_handle.?, 0, @as(*anyopaque, @ptrFromInt(getRecordPtr(buf).*)));
            }
        },
        else => @compileError("Unsupported OS"),
    };

    fn sliceContainsPtr(container: []u8, ptr: [*]u8) bool {
        return @intFromPtr(ptr) >= @intFromPtr(container.ptr) and
            @intFromPtr(ptr) < (@intFromPtr(container.ptr) + container.len);
    }

    fn sliceContainsSlice(container: []u8, slice: []u8) bool {
        return @intFromPtr(slice.ptr) >= @intFromPtr(container.ptr) and
            (@intFromPtr(slice.ptr) + slice.len) <= (@intFromPtr(container.ptr) + container.len);
    }

    pub const FixedBufferAllocator = struct {
        end_index: usize,
        buffer: []u8,

        pub fn init(buffer: []u8) FixedBufferAllocator {
            return FixedBufferAllocator{
                .buffer = buffer,
                .end_index = 0,
            };
        }

        /// *WARNING* using this at the same time as the interface returned by `threadSafeAllocator` is not thread safe
        pub fn allocator(self: *FixedBufferAllocator) Allocator {
            return .{
                .ptr = self,
                .vtable = &.{
                    .alloc = alloc,
                    .resize = resize,
                    .free = free,
                },
            };
        }

        /// Provides a lock free thread safe `Allocator` interface to the underlying `FixedBufferAllocator`
        /// *WARNING* using this at the same time as the interface returned by `allocator` is not thread safe
        pub fn threadSafeAllocator(self: *FixedBufferAllocator) Allocator {
            return .{
                .ptr = self,
                .vtable = &.{
                    .alloc = threadSafeAlloc,
                    .resize = Allocator.noResize,
                    .free = Allocator.noFree,
                },
            };
        }

        pub fn ownsPtr(self: *FixedBufferAllocator, ptr: [*]u8) bool {
            return sliceContainsPtr(self.buffer, ptr);
        }

        pub fn ownsSlice(self: *FixedBufferAllocator, slice: []u8) bool {
            return sliceContainsSlice(self.buffer, slice);
        }

        /// NOTE: this will not work in all cases, if the last allocation had an adjusted_index
        ///       then we won't be able to determine what the last allocation was.  This is because
        ///       the alignForward operation done in alloc is not reversible.
        pub fn isLastAllocation(self: *FixedBufferAllocator, buf: []u8) bool {
            return buf.ptr + buf.len == self.buffer.ptr + self.end_index;
        }

        fn alloc(ctx: *anyopaque, n: usize, log2_ptr_align: u8, ra: usize) ?[*]u8 {
            const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
            _ = ra;
            const ptr_align = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_ptr_align));
            const adjust_off = mem.alignPointerOffset(self.buffer.ptr + self.end_index, ptr_align) orelse return null;
            const adjusted_index = self.end_index + adjust_off;
            const new_end_index = adjusted_index + n;
            if (new_end_index > self.buffer.len) return null;
            self.end_index = new_end_index;
            return self.buffer.ptr + adjusted_index;
        }

        fn resize(
            ctx: *anyopaque,
            buf: []u8,
            log2_buf_align: u8,
            new_size: usize,
            return_address: usize,
        ) bool {
            const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
            _ = log2_buf_align;
            _ = return_address;
            assert(@inComptime() or self.ownsSlice(buf));

            if (!self.isLastAllocation(buf)) {
                if (new_size > buf.len) return false;
                return true;
            }

            if (new_size <= buf.len) {
                const sub = buf.len - new_size;
                self.end_index -= sub;
                return true;
            }

            const add = new_size - buf.len;
            if (add + self.end_index > self.buffer.len) return false;

            self.end_index += add;
            return true;
        }

        fn free(
            ctx: *anyopaque,
            buf: []u8,
            log2_buf_align: u8,
            return_address: usize,
        ) void {
            const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
            _ = log2_buf_align;
            _ = return_address;
            assert(@inComptime() or self.ownsSlice(buf));

            if (self.isLastAllocation(buf)) {
                self.end_index -= buf.len;
            }
        }

        fn threadSafeAlloc(ctx: *anyopaque, n: usize, log2_ptr_align: u8, ra: usize) ?[*]u8 {
            const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
            _ = ra;
            const ptr_align = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_ptr_align));
            var end_index = @atomicLoad(usize, &self.end_index, .SeqCst);
            while (true) {
                const adjust_off = mem.alignPointerOffset(self.buffer.ptr + end_index, ptr_align) orelse return null;
                const adjusted_index = end_index + adjust_off;
                const new_end_index = adjusted_index + n;
                if (new_end_index > self.buffer.len) return null;
                end_index = @cmpxchgWeak(usize, &self.end_index, end_index, new_end_index, .SeqCst, .SeqCst) orelse
                    return self.buffer[adjusted_index..new_end_index].ptr;
            }
        }

        pub fn reset(self: *FixedBufferAllocator) void {
            self.end_index = 0;
        }
    };

    pub const ThreadSafeFixedBufferAllocator = @compileError("ThreadSafeFixedBufferAllocator has been replaced with `threadSafeAllocator` on FixedBufferAllocator");

    /// Returns a `StackFallbackAllocator` allocating using either a
    /// `FixedBufferAllocator` on an array of size `size` and falling back to
    /// `fallback_allocator` if that fails.
    pub fn stackFallback(comptime size: usize, fallback_allocator: Allocator) StackFallbackAllocator(size) {
        return StackFallbackAllocator(size){
            .buffer = undefined,
            .fallback_allocator = fallback_allocator,
            .fixed_buffer_allocator = undefined,
        };
    }

    /// An allocator that attempts to allocate using a
    /// `FixedBufferAllocator` using an array of size `size`. If the
    /// allocation fails, it will fall back to using
    /// `fallback_allocator`. Easily created with `stackFallback`.
    pub fn StackFallbackAllocator(comptime size: usize) type {
        return struct {
            const Self = @This();

            buffer: [size]u8,
            fallback_allocator: Allocator,
            fixed_buffer_allocator: FixedBufferAllocator,

            /// This function both fetches a `Allocator` interface to this
            /// allocator *and* resets the internal buffer allocator.
            pub fn get(self: *Self) Allocator {
                self.fixed_buffer_allocator = FixedBufferAllocator.init(self.buffer[0..]);
                return .{
                    .ptr = self,
                    .vtable = &.{
                        .alloc = alloc,
                        .resize = resize,
                        .free = free,
                    },
                };
            }

            fn alloc(
                ctx: *anyopaque,
                len: usize,
                log2_ptr_align: u8,
                ra: usize,
            ) ?[*]u8 {
                const self: *Self = @ptrCast(@alignCast(ctx));
                return FixedBufferAllocator.alloc(&self.fixed_buffer_allocator, len, log2_ptr_align, ra) orelse
                    return self.fallback_allocator.rawAlloc(len, log2_ptr_align, ra);
            }

            fn resize(
                ctx: *anyopaque,
                buf: []u8,
                log2_buf_align: u8,
                new_len: usize,
                ra: usize,
            ) bool {
                const self: *Self = @ptrCast(@alignCast(ctx));
                if (self.fixed_buffer_allocator.ownsPtr(buf.ptr)) {
                    return FixedBufferAllocator.resize(&self.fixed_buffer_allocator, buf, log2_buf_align, new_len, ra);
                } else {
                    return self.fallback_allocator.rawResize(buf, log2_buf_align, new_len, ra);
                }
            }

            fn free(
                ctx: *anyopaque,
                buf: []u8,
                log2_buf_align: u8,
                ra: usize,
            ) void {
                const self: *Self = @ptrCast(@alignCast(ctx));
                if (self.fixed_buffer_allocator.ownsPtr(buf.ptr)) {
                    return FixedBufferAllocator.free(&self.fixed_buffer_allocator, buf, log2_buf_align, ra);
                } else {
                    return self.fallback_allocator.rawFree(buf, log2_buf_align, ra);
                }
            }
        };
    }

    test "c_allocator" {
        if (builtin.link_libc) {
            try testAllocator(c_allocator);
            try testAllocatorAligned(c_allocator);
            try testAllocatorLargeAlignment(c_allocator);
            try testAllocatorAlignedShrink(c_allocator);
        }
    }

    test "raw_c_allocator" {
        if (builtin.link_libc) {
            try testAllocator(raw_c_allocator);
        }
    }

    test "PageAllocator" {
        const allocator = page_allocator;
        try testAllocator(allocator);
        try testAllocatorAligned(allocator);
        if (!builtin.target.isWasm()) {
            try testAllocatorLargeAlignment(allocator);
            try testAllocatorAlignedShrink(allocator);
        }

        if (builtin.os.tag == .windows) {
            const slice = try allocator.alignedAlloc(u8, mem.page_size, 128);
            slice[0] = 0x12;
            slice[127] = 0x34;
            allocator.free(slice);
        }
        {
            var buf = try allocator.alloc(u8, mem.page_size + 1);
            defer allocator.free(buf);
            buf = try allocator.realloc(buf, 1); // shrink past the page boundary

        }
    }

    test "HeapAllocator" {
        if (builtin.os.tag == .windows) {
            // https://github.com/ziglang/zig/issues/13702

            if (builtin.cpu.arch == .aarch64) return error.SkipZigTest;

            var heap_allocator = HeapAllocator.init();
            defer heap_allocator.deinit();
            const allocator = heap_allocator.allocator();

            try testAllocator(allocator);
            try testAllocatorAligned(allocator);
            try testAllocatorLargeAlignment(allocator);
            try testAllocatorAlignedShrink(allocator);
        }
    }

    test "ArenaAllocator" {
        var arena_allocator = ArenaAllocator.init(page_allocator);
        defer arena_allocator.deinit();
        const allocator = arena_allocator.allocator();

        try testAllocator(allocator);
        try testAllocatorAligned(allocator);
        try testAllocatorLargeAlignment(allocator);
        try testAllocatorAlignedShrink(allocator);
    }

    var test_fixed_buffer_allocator_memory: [800000 * @sizeOf(u64)]u8 = undefined;
    test "FixedBufferAllocator" {
        var fixed_buffer_allocator = mem.validationWrap(FixedBufferAllocator.init(test_fixed_buffer_allocator_memory[0..]));
        const allocator = fixed_buffer_allocator.allocator();

        try testAllocator(allocator);
        try testAllocatorAligned(allocator);
        try testAllocatorLargeAlignment(allocator);
        try testAllocatorAlignedShrink(allocator);
    }

    test "FixedBufferAllocator.reset" {
        var buf: [8]u8 align(@alignOf(u64)) = undefined;
        var fba = FixedBufferAllocator.init(buf[0..]);
        const allocator = fba.allocator();

        const X = 0xeeeeeeeeeeeeeeee;
        const Y = 0xffffffffffffffff;

        var x = try allocator.create(u64);
        x.* = X;
        try testing.expectError(error.OutOfMemory, allocator.create(u64));

        fba.reset();
        var y = try allocator.create(u64);
        y.* = Y;

        // we expect Y to have overwritten X.

        try testing.expect(x.* == y.*);
        try testing.expect(y.* == Y);
    }

    test "StackFallbackAllocator" {
        const fallback_allocator = page_allocator;
        var stack_allocator = stackFallback(4096, fallback_allocator);

        try testAllocator(stack_allocator.get());
        try testAllocatorAligned(stack_allocator.get());
        try testAllocatorLargeAlignment(stack_allocator.get());
        try testAllocatorAlignedShrink(stack_allocator.get());
    }

    test "FixedBufferAllocator Reuse memory on realloc" {
        var small_fixed_buffer: [10]u8 = undefined;
        // check if we re-use the memory

        {
            var fixed_buffer_allocator = FixedBufferAllocator.init(small_fixed_buffer[0..]);
            const allocator = fixed_buffer_allocator.allocator();

            var slice0 = try allocator.alloc(u8, 5);
            try testing.expect(slice0.len == 5);
            var slice1 = try allocator.realloc(slice0, 10);
            try testing.expect(slice1.ptr == slice0.ptr);
            try testing.expect(slice1.len == 10);
            try testing.expectError(error.OutOfMemory, allocator.realloc(slice1, 11));
        }
        // check that we don't re-use the memory if it's not the most recent block

        {
            var fixed_buffer_allocator = FixedBufferAllocator.init(small_fixed_buffer[0..]);
            const allocator = fixed_buffer_allocator.allocator();

            var slice0 = try allocator.alloc(u8, 2);
            slice0[0] = 1;
            slice0[1] = 2;
            var slice1 = try allocator.alloc(u8, 2);
            var slice2 = try allocator.realloc(slice0, 4);
            try testing.expect(slice0.ptr != slice2.ptr);
            try testing.expect(slice1.ptr != slice2.ptr);
            try testing.expect(slice2[0] == 1);
            try testing.expect(slice2[1] == 2);
        }
    }

    test "Thread safe FixedBufferAllocator" {
        var fixed_buffer_allocator = FixedBufferAllocator.init(test_fixed_buffer_allocator_memory[0..]);

        try testAllocator(fixed_buffer_allocator.threadSafeAllocator());
        try testAllocatorAligned(fixed_buffer_allocator.threadSafeAllocator());
        try testAllocatorLargeAlignment(fixed_buffer_allocator.threadSafeAllocator());
        try testAllocatorAlignedShrink(fixed_buffer_allocator.threadSafeAllocator());
    }

    /// This one should not try alignments that exceed what C malloc can handle.
    pub fn testAllocator(base_allocator: mem.Allocator) !void {
        var validationAllocator = mem.validationWrap(base_allocator);
        const allocator = validationAllocator.allocator();

        var slice = try allocator.alloc(*i32, 100);
        try testing.expect(slice.len == 100);
        for (slice, 0..) |*item, i| {
            item.* = try allocator.create(i32);
            item.*.* = @as(i32, @intCast(i));
        }

        slice = try allocator.realloc(slice, 20000);
        try testing.expect(slice.len == 20000);

        for (slice[0..100], 0..) |item, i| {
            try testing.expect(item.* == @as(i32, @intCast(i)));
            allocator.destroy(item);
        }

        if (allocator.resize(slice, 50)) {
            slice = slice[0..50];
            if (allocator.resize(slice, 25)) {
                slice = slice[0..25];
                try testing.expect(allocator.resize(slice, 0));
                slice = slice[0..0];
                slice = try allocator.realloc(slice, 10);
                try testing.expect(slice.len == 10);
            }
        }
        allocator.free(slice);

        // Zero-length allocation

        var empty = try allocator.alloc(u8, 0);
        allocator.free(empty);
        // Allocation with zero-sized types

        const zero_bit_ptr = try allocator.create(u0);
        zero_bit_ptr.* = 0;
        allocator.destroy(zero_bit_ptr);

        const oversize = try allocator.alignedAlloc(u32, null, 5);
        try testing.expect(oversize.len >= 5);
        for (oversize) |*item| {
            item.* = 0xDEADBEEF;
        }
        allocator.free(oversize);
    }

    pub fn testAllocatorAligned(base_allocator: mem.Allocator) !void {
        var validationAllocator = mem.validationWrap(base_allocator);
        const allocator = validationAllocator.allocator();

        // Test a few alignment values, smaller and bigger than the type's one

        inline for ([_]u29{ 1, 2, 4, 8, 16, 32, 64 }) |alignment| {
            // initial

            var slice = try allocator.alignedAlloc(u8, alignment, 10);
            try testing.expect(slice.len == 10);
            // grow

            slice = try allocator.realloc(slice, 100);
            try testing.expect(slice.len == 100);
            if (allocator.resize(slice, 10)) {
                slice = slice[0..10];
            }
            try testing.expect(allocator.resize(slice, 0));
            slice = slice[0..0];
            // realloc from zero

            slice = try allocator.realloc(slice, 100);
            try testing.expect(slice.len == 100);
            if (allocator.resize(slice, 10)) {
                slice = slice[0..10];
            }
            try testing.expect(allocator.resize(slice, 0));
        }
    }

    pub fn testAllocatorLargeAlignment(base_allocator: mem.Allocator) !void {
        var validationAllocator = mem.validationWrap(base_allocator);
        const allocator = validationAllocator.allocator();

        const large_align: usize = mem.page_size / 2;

        var align_mask: usize = undefined;
        align_mask = @shlWithOverflow(~@as(usize, 0), @as(Allocator.Log2Align, @ctz(large_align)))[0];

        var slice = try allocator.alignedAlloc(u8, large_align, 500);
        try testing.expect(@intFromPtr(slice.ptr) & align_mask == @intFromPtr(slice.ptr));

        if (allocator.resize(slice, 100)) {
            slice = slice[0..100];
        }

        slice = try allocator.realloc(slice, 5000);
        try testing.expect(@intFromPtr(slice.ptr) & align_mask == @intFromPtr(slice.ptr));

        if (allocator.resize(slice, 10)) {
            slice = slice[0..10];
        }

        slice = try allocator.realloc(slice, 20000);
        try testing.expect(@intFromPtr(slice.ptr) & align_mask == @intFromPtr(slice.ptr));

        allocator.free(slice);
    }

    pub fn testAllocatorAlignedShrink(base_allocator: mem.Allocator) !void {
        var validationAllocator = mem.validationWrap(base_allocator);
        const allocator = validationAllocator.allocator();

        var debug_buffer: [1000]u8 = undefined;
        var fib = FixedBufferAllocator.init(&debug_buffer);
        const debug_allocator = fib.allocator();

        const alloc_size = mem.page_size * 2 + 50;
        var slice = try allocator.alignedAlloc(u8, 16, alloc_size);
        defer allocator.free(slice);

        var stuff_to_free = std.ArrayList([]align(16) u8).init(debug_allocator);
        // On Windows, VirtualAlloc returns addresses aligned to a 64K boundary,

        // which is 16 pages, hence the 32. This test may require to increase

        // the size of the allocations feeding the `allocator` parameter if they

        // fail, because of this high over-alignment we want to have.

        while (@intFromPtr(slice.ptr) == mem.alignForward(usize, @intFromPtr(slice.ptr), mem.page_size * 32)) {
            try stuff_to_free.append(slice);
            slice = try allocator.alignedAlloc(u8, 16, alloc_size);
        }
        while (stuff_to_free.popOrNull()) |item| {
            allocator.free(item);
        }
        slice[0] = 0x12;
        slice[60] = 0x34;

        slice = try allocator.reallocAdvanced(slice, alloc_size / 2, 0);
        try testing.expect(slice[0] == 0x12);
        try testing.expect(slice[60] == 0x34);
    }

    test {
        _ = LoggingAllocator;
        _ = LogToWriterAllocator;
        _ = ScopedLoggingAllocator;
        _ = @import("heap/memory_pool.zig");
        _ = ArenaAllocator;
        _ = GeneralPurposeAllocator;
        if (comptime builtin.target.isWasm()) {
            _ = WasmAllocator;
            _ = WasmPageAllocator;
        }
    }