zig/std/heap.zig

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

const Allocator = mem.Allocator;

pub const c_allocator = &c_allocator_state;
var c_allocator_state = Allocator{
    .reallocFn = cRealloc,
    .shrinkFn = cShrink,
};

fn cRealloc(self: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) ![]u8 {
    assert(new_align <= @alignOf(c_longdouble));
    const old_ptr = if (old_mem.len == 0) null else @ptrCast(*c_void, old_mem.ptr);
    const buf = c.realloc(old_ptr, new_size) orelse return error.OutOfMemory;
    return @ptrCast([*]u8, buf)[0..new_size];
}

fn cShrink(self: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) []u8 {
    const old_ptr = @ptrCast(*c_void, old_mem.ptr);
    const buf = c.realloc(old_ptr, new_size) orelse return old_mem[0..new_size];
    return @ptrCast([*]u8, buf)[0..new_size];
}

/// This allocator makes a syscall directly for every allocation and free.
/// Thread-safe and lock-free.
pub const DirectAllocator = struct {
    allocator: Allocator,
    heap_handle: ?HeapHandle,

    const HeapHandle = if (builtin.os == Os.windows) os.windows.HANDLE else void;

    pub fn init() DirectAllocator {
        return DirectAllocator{
            .allocator = Allocator{
                .reallocFn = realloc,
                .shrinkFn = shrink,
            },
            .heap_handle = if (builtin.os == Os.windows) null else {},
        };
    }

    pub fn deinit(self: *DirectAllocator) void {
        switch (builtin.os) {
            Os.windows => if (self.heap_handle) |heap_handle| {
                _ = os.windows.HeapDestroy(heap_handle);
            },
            else => {},
        }
    }

    fn alloc(allocator: *Allocator, n: usize, alignment: u29) error{OutOfMemory}![]u8 {
        const self = @fieldParentPtr(DirectAllocator, "allocator", allocator);

        switch (builtin.os) {
            Os.linux, Os.macosx, Os.ios, Os.freebsd, Os.netbsd => {
                const p = os.posix;
                const alloc_size = if (alignment <= os.page_size) n else n + alignment;
                const addr = p.mmap(null, alloc_size, p.PROT_READ | p.PROT_WRITE, p.MAP_PRIVATE | p.MAP_ANONYMOUS, -1, 0);
                if (addr == p.MAP_FAILED) return error.OutOfMemory;
                if (alloc_size == n) return @intToPtr([*]u8, addr)[0..n];

                const aligned_addr = mem.alignForward(addr, alignment);

                // Unmap the extra bytes that were only requested in order to guarantee
                // that the range of memory we were provided had a proper alignment in
                // it somewhere. The extra bytes could be at the beginning, or end, or both.
                const unused_start_len = aligned_addr - addr;
                if (unused_start_len != 0) {
                    const err = p.munmap(addr, unused_start_len);
                    assert(p.getErrno(err) == 0);
                }
                const aligned_end_addr = std.mem.alignForward(aligned_addr + n, os.page_size);
                const unused_end_len = addr + alloc_size - aligned_end_addr;
                if (unused_end_len != 0) {
                    const err = p.munmap(aligned_end_addr, unused_end_len);
                    assert(p.getErrno(err) == 0);
                }

                return @intToPtr([*]u8, aligned_addr)[0..n];
            },
            Os.windows => {
                const amt = n + alignment + @sizeOf(usize);
                const optional_heap_handle = @atomicLoad(?HeapHandle, &self.heap_handle, builtin.AtomicOrder.SeqCst);
                const heap_handle = optional_heap_handle orelse blk: {
                    const hh = os.windows.HeapCreate(0, amt, 0) orelse return error.OutOfMemory;
                    const other_hh = @cmpxchgStrong(?HeapHandle, &self.heap_handle, null, hh, builtin.AtomicOrder.SeqCst, builtin.AtomicOrder.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.HeapAlloc(heap_handle, 0, amt) orelse return error.OutOfMemory;
                const root_addr = @ptrToInt(ptr);
                const adjusted_addr = mem.alignForward(root_addr, alignment);
                const record_addr = adjusted_addr + n;
                @intToPtr(*align(1) usize, record_addr).* = root_addr;
                return @intToPtr([*]u8, adjusted_addr)[0..n];
            },
            else => @compileError("Unsupported OS"),
        }
    }

    fn shrink(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) []u8 {
        switch (builtin.os) {
            Os.linux, Os.macosx, Os.ios, Os.freebsd, Os.netbsd => {
                const base_addr = @ptrToInt(old_mem.ptr);
                const old_addr_end = base_addr + old_mem.len;
                const new_addr_end = base_addr + new_size;
                const new_addr_end_rounded = mem.alignForward(new_addr_end, os.page_size);
                if (old_addr_end > new_addr_end_rounded) {
                    _ = os.posix.munmap(new_addr_end_rounded, old_addr_end - new_addr_end_rounded);
                }
                return old_mem[0..new_size];
            },
            Os.windows => return realloc(allocator, old_mem, old_align, new_size, new_align) catch {
                const old_adjusted_addr = @ptrToInt(old_mem.ptr);
                const old_record_addr = old_adjusted_addr + old_mem.len;
                const root_addr = @intToPtr(*align(1) usize, old_record_addr).*;
                const old_ptr = @intToPtr(*c_void, root_addr);
                const new_record_addr = old_record_addr - new_size + old_mem.len;
                @intToPtr(*align(1) usize, new_record_addr).* = root_addr;
                return old_mem[0..new_size];
            },
            else => @compileError("Unsupported OS"),
        }
    }

    fn realloc(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) ![]u8 {
        switch (builtin.os) {
            Os.linux, Os.macosx, Os.ios, Os.freebsd, Os.netbsd => {
                if (new_size <= old_mem.len and new_align <= old_align) {
                    return shrink(allocator, old_mem, old_align, new_size, new_align);
                }
                const result = try alloc(allocator, new_size, new_align);
                if (result.len >= old_mem.len) {
                    mem.copy(u8, result, old_mem);
                } else {
                    @memcpy(result.ptr, old_mem.ptr, new_size);
                }
                _ = os.posix.munmap(@ptrToInt(old_mem.ptr), old_mem.len);
                return result;
            },
            Os.windows => {
                if (old_mem.len == 0) return alloc(allocator, new_size, new_align);

                const self = @fieldParentPtr(DirectAllocator, "allocator", allocator);
                const old_adjusted_addr = @ptrToInt(old_mem.ptr);
                const old_record_addr = old_adjusted_addr + old_mem.len;
                const root_addr = @intToPtr(*align(1) usize, old_record_addr).*;
                const old_ptr = @intToPtr(*c_void, root_addr);
                
                if(new_size == 0) {
                    if (os.windows.HeapFree(self.heap_handle.?, 0, old_ptr) == 0) unreachable;
                    return old_mem[0..0];
                }
                
                const amt = new_size + new_align + @sizeOf(usize);
                const new_ptr = os.windows.HeapReAlloc(
                    self.heap_handle.?,
                    0,
                    old_ptr,
                    amt,
                ) orelse return error.OutOfMemory;
                const offset = old_adjusted_addr - root_addr;
                const new_root_addr = @ptrToInt(new_ptr);
                var new_adjusted_addr = new_root_addr + offset;
                const offset_is_valid = new_adjusted_addr + new_size + @sizeOf(usize) <= new_root_addr + amt;
                const offset_is_aligned = new_adjusted_addr % new_align == 0;
                if (!offset_is_valid or !offset_is_aligned) {
                    // If HeapReAlloc didn't happen to move the memory to the new alignment,
                    // or the memory starting at the old offset would be outside of the new allocation,
                    // then we need to copy the memory to a valid aligned address and use that
                    const new_aligned_addr = mem.alignForward(new_root_addr, new_align);
                    @memcpy(
                        @intToPtr([*]u8, new_aligned_addr),
                        @intToPtr([*]u8, new_adjusted_addr),
                        std.math.min(old_mem.len, new_size),
                    );
                    new_adjusted_addr = new_aligned_addr;
                }
                const new_record_addr = new_adjusted_addr + new_size;
                @intToPtr(*align(1) usize, new_record_addr).* = new_root_addr;
                return @intToPtr([*]u8, new_adjusted_addr)[0..new_size];
            },
            else => @compileError("Unsupported OS"),
        }
    }
};

/// This allocator takes an existing allocator, wraps it, and provides an interface
/// where you can allocate without freeing, and then free it all together.
pub const ArenaAllocator = struct {
    pub allocator: Allocator,

    child_allocator: *Allocator,
    buffer_list: std.LinkedList([]u8),
    end_index: usize,

    const BufNode = std.LinkedList([]u8).Node;

    pub fn init(child_allocator: *Allocator) ArenaAllocator {
        return ArenaAllocator{
            .allocator = Allocator{
                .reallocFn = realloc,
                .shrinkFn = shrink,
            },
            .child_allocator = child_allocator,
            .buffer_list = std.LinkedList([]u8).init(),
            .end_index = 0,
        };
    }

    pub fn deinit(self: *ArenaAllocator) void {
        var it = self.buffer_list.first;
        while (it) |node| {
            // this has to occur before the free because the free frees node
            it = node.next;

            self.child_allocator.free(node.data);
        }
    }

    fn createNode(self: *ArenaAllocator, prev_len: usize, minimum_size: usize) !*BufNode {
        const actual_min_size = minimum_size + @sizeOf(BufNode);
        var len = prev_len;
        while (true) {
            len += len / 2;
            len += os.page_size - @rem(len, os.page_size);
            if (len >= actual_min_size) break;
        }
        const buf = try self.child_allocator.alignedAlloc(u8, @alignOf(BufNode), len);
        const buf_node_slice = @bytesToSlice(BufNode, buf[0..@sizeOf(BufNode)]);
        const buf_node = &buf_node_slice[0];
        buf_node.* = BufNode{
            .data = buf,
            .prev = null,
            .next = null,
        };
        self.buffer_list.append(buf_node);
        self.end_index = 0;
        return buf_node;
    }

    fn alloc(allocator: *Allocator, n: usize, alignment: u29) ![]u8 {
        const self = @fieldParentPtr(ArenaAllocator, "allocator", allocator);

        var cur_node = if (self.buffer_list.last) |last_node| last_node else try self.createNode(0, n + alignment);
        while (true) {
            const cur_buf = cur_node.data[@sizeOf(BufNode)..];
            const addr = @ptrToInt(cur_buf.ptr) + self.end_index;
            const adjusted_addr = mem.alignForward(addr, alignment);
            const adjusted_index = self.end_index + (adjusted_addr - addr);
            const new_end_index = adjusted_index + n;
            if (new_end_index > cur_buf.len) {
                cur_node = try self.createNode(cur_buf.len, n + alignment);
                continue;
            }
            const result = cur_buf[adjusted_index..new_end_index];
            self.end_index = new_end_index;
            return result;
        }
    }

    fn realloc(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) ![]u8 {
        if (new_size <= old_mem.len and new_align <= new_size) {
            // We can't do anything with the memory, so tell the client to keep it.
            return error.OutOfMemory;
        } else {
            const result = try alloc(allocator, new_size, new_align);
            if (result.len >= old_mem.len) {
                mem.copy(u8, result, old_mem);
            } else {
                @memcpy(result.ptr, old_mem.ptr, new_size);
            }
            return result;
        }
    }

    fn shrink(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) []u8 {
        return old_mem[0..new_size];
    }
};

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

    pub fn init(buffer: []u8) FixedBufferAllocator {
        return FixedBufferAllocator{
            .allocator = Allocator{
                .reallocFn = realloc,
                .shrinkFn = shrink,
            },
            .buffer = buffer,
            .end_index = 0,
        };
    }

    fn alloc(allocator: *Allocator, n: usize, alignment: u29) ![]u8 {
        const self = @fieldParentPtr(FixedBufferAllocator, "allocator", allocator);
        const addr = @ptrToInt(self.buffer.ptr) + self.end_index;
        const adjusted_addr = mem.alignForward(addr, alignment);
        const adjusted_index = self.end_index + (adjusted_addr - addr);
        const new_end_index = adjusted_index + n;
        if (new_end_index > self.buffer.len) {
            return error.OutOfMemory;
        }
        const result = self.buffer[adjusted_index..new_end_index];
        self.end_index = new_end_index;

        return result;
    }

    fn realloc(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) ![]u8 {
        const self = @fieldParentPtr(FixedBufferAllocator, "allocator", allocator);
        assert(old_mem.len <= self.end_index);
        if (old_mem.ptr == self.buffer.ptr + self.end_index - old_mem.len and
            mem.alignForward(@ptrToInt(old_mem.ptr), new_align) == @ptrToInt(old_mem.ptr))
        {
            const start_index = self.end_index - old_mem.len;
            const new_end_index = start_index + new_size;
            if (new_end_index > self.buffer.len) return error.OutOfMemory;
            const result = self.buffer[start_index..new_end_index];
            self.end_index = new_end_index;
            return result;
        } else if (new_size <= old_mem.len and new_align <= old_align) {
            // We can't do anything with the memory, so tell the client to keep it.
            return error.OutOfMemory;
        } else {
            const result = try alloc(allocator, new_size, new_align);
            if (result.len >= old_mem.len) {
                mem.copy(u8, result, old_mem);
            } else {
                @memcpy(result.ptr, old_mem.ptr, new_size);
            }
            return result;
        }
    }

    fn shrink(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) []u8 {
        return old_mem[0..new_size];
    }
};

// FIXME: Exposed LLVM intrinsics is a bug
// See: https://github.com/ziglang/zig/issues/2291
extern fn @"llvm.wasm.memory.size.i32"(u32) u32;
extern fn @"llvm.wasm.memory.grow.i32"(u32, u32) i32;

pub const wasm_allocator = &wasm_allocator_state.allocator;
var wasm_allocator_state = WasmAllocator{
    .allocator = Allocator{
        .reallocFn = WasmAllocator.realloc,
        .shrinkFn = WasmAllocator.shrink,
    },
    .start_ptr = undefined,
    .num_pages = 0,
    .end_index = 0,
};

const WasmAllocator = struct {
    allocator: Allocator,
    start_ptr: [*]u8,
    num_pages: usize,
    end_index: usize,

    fn alloc(allocator: *Allocator, size: usize, alignment: u29) ![]u8 {
        const self = @fieldParentPtr(WasmAllocator, "allocator", allocator);

        const addr = @ptrToInt(self.start_ptr) + self.end_index;
        const adjusted_addr = mem.alignForward(addr, alignment);
        const adjusted_index = self.end_index + (adjusted_addr - addr);
        const new_end_index = adjusted_index + size;

        if (new_end_index > self.num_pages * os.page_size) {
            const required_memory = new_end_index - (self.num_pages * os.page_size);

            var num_pages: u32 = required_memory / os.page_size;
            if (required_memory % os.page_size != 0) {
                num_pages += 1;
            }

            const prev_page = @"llvm.wasm.memory.grow.i32"(0, num_pages);
            if (prev_page == -1) {
                return error.OutOfMemory;
            }

            self.num_pages += num_pages;
        }

        const result = self.start_ptr[adjusted_index..new_end_index];
        self.end_index = new_end_index;

        return result;
    }

    // Check if memory is the last "item" and is aligned correctly
    fn is_last_item(allocator: *Allocator, memory: []u8, alignment: u29) bool {
        const self = @fieldParentPtr(WasmAllocator, "allocator", allocator);
        return memory.ptr == self.start_ptr + self.end_index - memory.len and mem.alignForward(@ptrToInt(memory.ptr), alignment) == @ptrToInt(memory.ptr);
    }

    fn realloc(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) ![]u8 {
        const self = @fieldParentPtr(WasmAllocator, "allocator", allocator);

        // Initialize start_ptr at the first realloc
        if (self.num_pages == 0) {
            self.start_ptr = @intToPtr([*]u8, @intCast(usize, @"llvm.wasm.memory.size.i32"(0)) * os.page_size);
        }

        if (is_last_item(allocator, old_mem, new_align)) {
            const start_index = self.end_index - old_mem.len;
            const new_end_index = start_index + new_size;

            if (new_end_index > self.num_pages * os.page_size) {
                _ = try alloc(allocator, new_end_index - self.end_index, new_align);
            }
            const result = self.start_ptr[start_index..new_end_index];

            self.end_index = new_end_index;
            return result;
        } else if (new_size <= old_mem.len and new_align <= old_align) {
            return error.OutOfMemory;
        } else {
            const result = try alloc(allocator, new_size, new_align);
            mem.copy(u8, result, old_mem);
            return result;
        }
    }

    fn shrink(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) []u8 {
        return old_mem[0..new_size];
    }
};

pub const ThreadSafeFixedBufferAllocator = blk: {
    if (builtin.single_threaded) {
        break :blk FixedBufferAllocator;
    } else {
        // lock free
        break :blk struct {
            allocator: Allocator,
            end_index: usize,
            buffer: []u8,

            pub fn init(buffer: []u8) ThreadSafeFixedBufferAllocator {
                return ThreadSafeFixedBufferAllocator{
                    .allocator = Allocator{
                        .reallocFn = realloc,
                        .shrinkFn = shrink,
                    },
                    .buffer = buffer,
                    .end_index = 0,
                };
            }

            fn alloc(allocator: *Allocator, n: usize, alignment: u29) ![]u8 {
                const self = @fieldParentPtr(ThreadSafeFixedBufferAllocator, "allocator", allocator);
                var end_index = @atomicLoad(usize, &self.end_index, builtin.AtomicOrder.SeqCst);
                while (true) {
                    const addr = @ptrToInt(self.buffer.ptr) + end_index;
                    const adjusted_addr = mem.alignForward(addr, alignment);
                    const adjusted_index = end_index + (adjusted_addr - addr);
                    const new_end_index = adjusted_index + n;
                    if (new_end_index > self.buffer.len) {
                        return error.OutOfMemory;
                    }
                    end_index = @cmpxchgWeak(usize, &self.end_index, end_index, new_end_index, builtin.AtomicOrder.SeqCst, builtin.AtomicOrder.SeqCst) orelse return self.buffer[adjusted_index..new_end_index];
                }
            }

            fn realloc(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) ![]u8 {
                if (new_size <= old_mem.len and new_align <= old_align) {
                    // We can't do anything useful with the memory, tell the client to keep it.
                    return error.OutOfMemory;
                } else {
                    const result = try alloc(allocator, new_size, new_align);
                    if (result.len >= old_mem.len) {
                        mem.copy(u8, result, old_mem);
                    } else {
                        @memcpy(result.ptr, old_mem.ptr, new_size);
                    }
                    return result;
                }
            }

            fn shrink(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) []u8 {
                return old_mem[0..new_size];
            }
        };
    }
};

pub fn stackFallback(comptime size: usize, fallback_allocator: *Allocator) StackFallbackAllocator(size) {
    return StackFallbackAllocator(size){
        .buffer = undefined,
        .fallback_allocator = fallback_allocator,
        .fixed_buffer_allocator = undefined,
        .allocator = Allocator{
            .reallocFn = StackFallbackAllocator(size).realloc,
            .shrinkFn = StackFallbackAllocator(size).shrink,
        },
    };
}

pub fn StackFallbackAllocator(comptime size: usize) type {
    return struct {
        const Self = @This();

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

        pub fn get(self: *Self) *Allocator {
            self.fixed_buffer_allocator = FixedBufferAllocator.init(self.buffer[0..]);
            return &self.allocator;
        }

        fn realloc(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) ![]u8 {
            const self = @fieldParentPtr(Self, "allocator", allocator);
            const in_buffer = @ptrToInt(old_mem.ptr) >= @ptrToInt(&self.buffer) and
                @ptrToInt(old_mem.ptr) < @ptrToInt(&self.buffer) + self.buffer.len;
            if (in_buffer) {
                return FixedBufferAllocator.realloc(
                    &self.fixed_buffer_allocator.allocator,
                    old_mem,
                    old_align,
                    new_size,
                    new_align,
                ) catch {
                    const result = try self.fallback_allocator.reallocFn(
                        self.fallback_allocator,
                        ([*]u8)(undefined)[0..0],
                        undefined,
                        new_size,
                        new_align,
                    );
                    mem.copy(u8, result, old_mem);
                    return result;
                };
            }
            return self.fallback_allocator.reallocFn(
                self.fallback_allocator,
                old_mem,
                old_align,
                new_size,
                new_align,
            );
        }

        fn shrink(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) []u8 {
            const self = @fieldParentPtr(Self, "allocator", allocator);
            const in_buffer = @ptrToInt(old_mem.ptr) >= @ptrToInt(&self.buffer) and
                @ptrToInt(old_mem.ptr) < @ptrToInt(&self.buffer) + self.buffer.len;
            if (in_buffer) {
                return FixedBufferAllocator.shrink(
                    &self.fixed_buffer_allocator.allocator,
                    old_mem,
                    old_align,
                    new_size,
                    new_align,
                );
            }
            return self.fallback_allocator.shrinkFn(
                self.fallback_allocator,
                old_mem,
                old_align,
                new_size,
                new_align,
            );
        }
    };
}

test "c_allocator" {
    if (builtin.link_libc) {
        var slice = try c_allocator.alloc(u8, 50);
        defer c_allocator.free(slice);
        slice = try c_allocator.realloc(slice, 100);
    }
}

test "DirectAllocator" {
    var direct_allocator = DirectAllocator.init();
    defer direct_allocator.deinit();

    const allocator = &direct_allocator.allocator;
    try testAllocator(allocator);
    try testAllocatorAligned(allocator, 16);
    try testAllocatorLargeAlignment(allocator);
    try testAllocatorAlignedShrink(allocator);
}

test "ArenaAllocator" {
    var direct_allocator = DirectAllocator.init();
    defer direct_allocator.deinit();

    var arena_allocator = ArenaAllocator.init(&direct_allocator.allocator);
    defer arena_allocator.deinit();

    try testAllocator(&arena_allocator.allocator);
    try testAllocatorAligned(&arena_allocator.allocator, 16);
    try testAllocatorLargeAlignment(&arena_allocator.allocator);
    try testAllocatorAlignedShrink(&arena_allocator.allocator);
}

var test_fixed_buffer_allocator_memory: [40000 * @sizeOf(usize)]u8 = undefined;
test "FixedBufferAllocator" {
    var fixed_buffer_allocator = FixedBufferAllocator.init(test_fixed_buffer_allocator_memory[0..]);

    try testAllocator(&fixed_buffer_allocator.allocator);
    try testAllocatorAligned(&fixed_buffer_allocator.allocator, 16);
    try testAllocatorLargeAlignment(&fixed_buffer_allocator.allocator);
    try testAllocatorAlignedShrink(&fixed_buffer_allocator.allocator);
}

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..]);

        var slice0 = try fixed_buffer_allocator.allocator.alloc(u8, 5);
        testing.expect(slice0.len == 5);
        var slice1 = try fixed_buffer_allocator.allocator.realloc(slice0, 10);
        testing.expect(slice1.ptr == slice0.ptr);
        testing.expect(slice1.len == 10);
        testing.expectError(error.OutOfMemory, fixed_buffer_allocator.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..]);

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

test "ThreadSafeFixedBufferAllocator" {
    var fixed_buffer_allocator = ThreadSafeFixedBufferAllocator.init(test_fixed_buffer_allocator_memory[0..]);

    try testAllocator(&fixed_buffer_allocator.allocator);
    try testAllocatorAligned(&fixed_buffer_allocator.allocator, 16);
    try testAllocatorLargeAlignment(&fixed_buffer_allocator.allocator);
    try testAllocatorAlignedShrink(&fixed_buffer_allocator.allocator);
}

fn testAllocator(allocator: *mem.Allocator) !void {
    var slice = try allocator.alloc(*i32, 100);
    testing.expect(slice.len == 100);
    for (slice) |*item, i| {
        item.* = try allocator.create(i32);
        item.*.* = @intCast(i32, i);
    }

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

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

    slice = allocator.shrink(slice, 50);
    testing.expect(slice.len == 50);
    slice = allocator.shrink(slice, 25);
    testing.expect(slice.len == 25);
    slice = allocator.shrink(slice, 0);
    testing.expect(slice.len == 0);
    slice = try allocator.realloc(slice, 10);
    testing.expect(slice.len == 10);

    allocator.free(slice);
}

fn testAllocatorAligned(allocator: *mem.Allocator, comptime alignment: u29) !void {
    // initial
    var slice = try allocator.alignedAlloc(u8, alignment, 10);
    testing.expect(slice.len == 10);
    // grow
    slice = try allocator.realloc(slice, 100);
    testing.expect(slice.len == 100);
    // shrink
    slice = allocator.shrink(slice, 10);
    testing.expect(slice.len == 10);
    // go to zero
    slice = allocator.shrink(slice, 0);
    testing.expect(slice.len == 0);
    // realloc from zero
    slice = try allocator.realloc(slice, 100);
    testing.expect(slice.len == 100);
    // shrink with shrink
    slice = allocator.shrink(slice, 10);
    testing.expect(slice.len == 10);
    // shrink to zero
    slice = allocator.shrink(slice, 0);
    testing.expect(slice.len == 0);
}

fn testAllocatorLargeAlignment(allocator: *mem.Allocator) mem.Allocator.Error!void {
    //Maybe a platform's page_size is actually the same as or
    //  very near usize?
    if (os.page_size << 2 > maxInt(usize)) return;

    const USizeShift = @IntType(false, std.math.log2(usize.bit_count));
    const large_align = u29(os.page_size << 2);

    var align_mask: usize = undefined;
    _ = @shlWithOverflow(usize, ~usize(0), USizeShift(@ctz(large_align)), &align_mask);

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

    slice = allocator.shrink(slice, 100);
    testing.expect(@ptrToInt(slice.ptr) & align_mask == @ptrToInt(slice.ptr));

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

    slice = allocator.shrink(slice, 10);
    testing.expect(@ptrToInt(slice.ptr) & align_mask == @ptrToInt(slice.ptr));

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

    allocator.free(slice);
}

fn testAllocatorAlignedShrink(allocator: *mem.Allocator) mem.Allocator.Error!void {
    var debug_buffer: [1000]u8 = undefined;
    const debug_allocator = &FixedBufferAllocator.init(&debug_buffer).allocator;

    const alloc_size = os.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);
    while (@ptrToInt(slice.ptr) == mem.alignForward(@ptrToInt(slice.ptr), os.page_size * 2)) {
        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;

    // realloc to a smaller size but with a larger alignment
    slice = try allocator.alignedRealloc(slice, os.page_size * 2, alloc_size / 2);
    testing.expect(slice[0] == 0x12);
    testing.expect(slice[60] == 0x34);
}