remap

    fn remap(ctx: *anyopaque, mem: []u8, a: std.mem.Alignment, new_len: usize, _: usize) ?[*]u8 {
        const f: *FuzzAllocator = @ptrCast(@alignCast(ctx));
        assert(a == .@"4");
        assert(f.allocCount() == 1);

        const slot = f.memSlot(mem);
        if (new_len > f.bufs[slot].len or f.smith.value(bool)) return null;

        if (f.smith.value(bool)) {
            f.used_len[slot] = new_len;
            // remap in place
            return mem.ptr;
        } else {
            // moving remap
            const new_slot = ~slot;
            f.used_bitmap = ~f.used_bitmap;
            f.used_len[new_slot] = new_len;

            const new_buf = &f.bufs[new_slot];
            @memcpy(new_buf[0..mem.len], mem);
            return new_buf.ptr;
        }
    }