    const std = @import("../std.zig");
    const mem = std.mem;

    /// Allocator that fails after N allocations, useful for making sure out of
    /// memory conditions are handled correctly.
    ///
    /// To use this, first initialize it and get an allocator with
    ///
    /// `const failing_allocator = &FailingAllocator.init(<allocator>,
    ///                                                   <fail_index>).allocator;`
    ///
    /// Then use `failing_allocator` anywhere you would have used a
    /// different allocator.
    pub const FailingAllocator = struct {
        index: usize,
        fail_index: usize,
        internal_allocator: mem.Allocator,
        allocated_bytes: usize,
        freed_bytes: usize,
        allocations: usize,
        deallocations: usize,
        stack_addresses: [num_stack_frames]usize,
        has_induced_failure: bool,

        const num_stack_frames = if (std.debug.sys_can_stack_trace) 16 else 0;

        /// `fail_index` is the number of successful allocations you can
        /// expect from this allocator. The next allocation will fail.
        /// For example, if this is called with `fail_index` equal to 2,
        /// the following test will pass:
        ///
        /// var a = try failing_alloc.create(i32);
        /// var b = try failing_alloc.create(i32);
        /// testing.expectError(error.OutOfMemory, failing_alloc.create(i32));
        pub fn init(internal_allocator: mem.Allocator, fail_index: usize) FailingAllocator {
            return FailingAllocator{
                .internal_allocator = internal_allocator,
                .fail_index = fail_index,
                .index = 0,
                .allocated_bytes = 0,
                .freed_bytes = 0,
                .allocations = 0,
                .deallocations = 0,
                .stack_addresses = undefined,
                .has_induced_failure = false,
            };
        }

        pub fn allocator(self: *FailingAllocator) mem.Allocator {
            return mem.Allocator.init(self, alloc, resize, free);
        }

        fn alloc(
            self: *FailingAllocator,
            len: usize,
            ptr_align: u29,
            len_align: u29,
            return_address: usize,
        ) error{OutOfMemory}![]u8 {
            if (self.index == self.fail_index) {
                if (!self.has_induced_failure) {
                    mem.set(usize, &self.stack_addresses, 0);
                    var stack_trace = std.builtin.StackTrace{
                        .instruction_addresses = &self.stack_addresses,
                        .index = 0,
                    };
                    std.debug.captureStackTrace(return_address, &stack_trace);
                    self.has_induced_failure = true;
                }
                return error.OutOfMemory;
            }
            const result = try self.internal_allocator.rawAlloc(len, ptr_align, len_align, return_address);
            self.allocated_bytes += result.len;
            self.allocations += 1;
            self.index += 1;
            return result;
        }

        fn resize(
            self: *FailingAllocator,
            old_mem: []u8,
            old_align: u29,
            new_len: usize,
            len_align: u29,
            ra: usize,
        ) ?usize {
            const r = self.internal_allocator.rawResize(old_mem, old_align, new_len, len_align, ra) orelse return null;
            if (r < old_mem.len) {
                self.freed_bytes += old_mem.len - r;
            } else {
                self.allocated_bytes += r - old_mem.len;
            }
            return r;
        }

        fn free(
            self: *FailingAllocator,
            old_mem: []u8,
            old_align: u29,
            ra: usize,
        ) void {
            self.internal_allocator.rawFree(old_mem, old_align, ra);
            self.deallocations += 1;
            self.freed_bytes += old_mem.len;
        }

        /// Only valid once `has_induced_failure == true`
        pub fn getStackTrace(self: *FailingAllocator) std.builtin.StackTrace {
            std.debug.assert(self.has_induced_failure);
            var len: usize = 0;
            while (len < self.stack_addresses.len and self.stack_addresses[len] != 0) {
                len += 1;
            }
            return .{
                .instruction_addresses = &self.stack_addresses,
                .index = len,
            };
        }
    };